Category Archives: Boat AC Topics

Power Quality

1/20/2022 – Initial post
1/25/2022 – major addition: “IS ANY OF THIS STUFF REAL?”
3/8/2023 – editorial “clean-up”

This article discusses “Power Quality.” Throughout North America, the standard AC voltage waveform is a “sine wave” at a frequency of 60hZ.  A sine wave is an S-shaped waveform defined by the mathematical function y = sin x.  “Power Quality” is a term that refers to imperfections in the shape of the AC voltage waveform in an AC utility power distribution system.  AC voltages are ideally 1) free of distortion and 2) within the rated voltage and frequency specified for the host system. When the waveform is less than “perfect,” end user electrical equipment can be negatively affected; particularly, equipment that contains digital control circuitry.

I often remind boaters that boat electrical systems are different from residential systems. Because of several differences related to commercial utility power distribution systems, the power delivered to residential premises is “cleaner” (fewer “noise” and “distortion” components) than the power that boats might receive at marinas. And, this is not about the difference between 120V/240V and 120V/208V power; this topic has to do with distortion of, and noise on, the AC waveform. Waveform distortion on boats occurs more commonly when running on generators (and inverters) than shore power.

Figure 1 shows a conceptual portrayal of some common forms of voltage events that can lead to customer premises equipment failures:

Figure 1: Some “Typical” Power Quality Events that Occur in Utility Power Distribution Systems

Fortunately, many of these situations are fairly easy to identify and observe through simple measurement means. But, those labeled “Miscellaneous Waveform Distortions” can be quite technical and very difficult to diagnose, pinpoint and confirm without expensive, professional measuring tools. Two of those are:

  • Common Mode Currents and
  • Harmonic Distortion

Much of the content of this article is likely new to most people, including many electricians and many marine electrical technicians. The reason boat owner/operators should be aware of these concepts is that when they occur, they can and do affect the performance of electrical equipment onboard boats, and often with intermittent and obscure symptoms.  “Honey, it never did THAT before!”  Symptoms of these issues DOES NOT mean that affected equipment, itself, is necessarily faulty or failing.

Figure 2 expands upon the conceptual depictions of Figure 1. Here we see the waveform detail that characterizes and defines “Power Quality” problems. These waveform distortion “anomalies” can occur as amplitude and wave shape distortions, and can be present one-at-a-time or in random combination of several-at-a-time.  All voltage waveform anomalies can cause customer premises equipment to fail to operate correctly or to fail to operate at all.

Figure 2: AC Voltage Waveforms Associated with Select Power Quality Issues

When, where and how can this appear to boaters?

  • anywhere, any time of day, any season of the year, randomly…
  • as random equipment shutdowns on boats in a marina…
  • as random equipment malfunctions on genset power but not on shore power…
  • as random HVAC equipment power errors…
  • as random refrigeration system shut down…
  • as random TV picture distortion anomalies…

What can the cause(s) be…

  • equipment not designed or intended for use in “mobile” (marine) applications…
  • ungrounded or improperly grounded equipment on boat…
  • faulty equipment on other boats at a dock
  • faulty premises equipment in a boatyard or commercial facility…
  • anomalies in the campus power delivered to a dock by the facility power distribution system, including inadequate dock wiring…
  • anomalies in the power provided to the facility by the local electric utility at it’s “Point of Common Connection” (PCC) to the grid…

In order to understand “Power Quality” issues, some introduction to technical concepts is helpful and necessary.  The following is written to my dad (banking and finance), my dock neighbor (911 dispatcher) and my best friend (printer) in discussing these topics. In other words, to those with little or no electrical background. The next three topics are building blocks for understanding  and becoming familiar with the larger issues that cause problems for boaters.


While much of what follows is new, most readers will (I assume) have heard of “Ohm’s Law.” In my era, Ohm’s Law was a high school science topic. Ohm’s Law is a fundamental law of electro-physics, that describes the inter-relationships of resistance (Ohms), voltage (Volts) and current (Amps).  Ohm’s Law recognizes that electrical circuit components all have the property of “resistance” (“impedance” in AC circuits). In a circuit, as one quantity changes, the others follow in direct linear or direct inverse proportion.  In our homes and on our boats, we expect the incoming electrical voltage (120V/240V) to stay mostly constant, so as the resistance of the circuit changes, current follows.  For example, in a 120V/240V residential system:

  • turn lights “on” throughout the house, total current used goes up;
  • temperature satisfied in water heater, water heater shuts “off,” current use goes down;
  • thermostat tells Air Conditioner to turn “on,” current use goes up;
  • toaster completes your breakfast muffins, current use goes down;
  • induction fry pan turned “on” to make hash-browns, current use goes up.

Figure 3: Linear AC Loads – Typical

Ohm’s Law describes the relationship between current and voltage as “Linear.”  As shown in Figure 3, the current waveform follows voltage waveform in a perfectly proportional and aligned manner.  In technical literature, electrical loads that elicit this behavior are characterized as “linear loads.”

There are many kinds of electrical equipment and appliances that are not linear in behavior. Electrical circuits where current does not follow voltage are characterized as a “non-linear;” simply stated, they do not adhere to the simple proportionality of Ohm’s Law.  Figure 4 shows an example of a non-linear load, where current behaves quite independently of voltage.

Figure 4: Non-Linear AC Load – Typical

In homes and on boats, we find many examples of both linear and non-linear loads. Water heaters, clothes dryers, cook tops, crock pots and “old fashioned” incandescent lamps are “purely resistive” linear devices.  HVAC and refrigeration compressors, florescent lighting ballasts, “new fangled” LED lighting fed by AC power bricks, microwaves, inverter/chargers, DC-to-DC converters, engine alternator Voltage Regulators and Switched-Mode Isolation Transformers are non-linear devices.


A “Switched Mode Power Supply” (“SMPS”) is now by far the most common type of power supply found in modern consumer electronics, especially digital electronics, both AC and DC. As shown below in Figure 5, an SMPS is a “non-linear device” that utilizes solid state switching devices (IGBT – Insulated Gate Bipolar Transistor) to continuously switch power “on” and “off” at very high frequencies (more on that in the section on “Pulse Width Modulation”).

Figure 5: Capacitor Voltage and Current in a Power Supply Application

A sidebar of “geek speak” follows, for those interested, to illustrate the cause of the non-linear current. Others can “skip it.”

In an SMPS, incoming power is fed to “energy storing components” (capacitors and inductors). The energy storing devices smooth DC voltage and supply power to the circuit during the non-conduction state of the switching transistors.

The basic SMPS design variations are categorized based on input and output voltage type. The four principle groups are:

  • AC to DC – DC power supplies as found in many end-user devices;
  • DC to DC – Converter to change or regulate DC voltage;
  • DC to AC – Inverter;
  • AC to AC – Cyclo-converter (“frequency changer;” i.e., 60Hz AC to 50 Hz AC or vice versa).

​SMPS Advantages:

  • More compact and use smaller transformers; smaller size and lighter weight is an advantage for electronic devices with limited space and in mobile applications;
  • Regulated and reliable voltage outputs regardless of variations in input supply voltage;
  • High efficiency: 70% to 90% vs 45% for traditional power supplies.

SMPS Disadvantages:

  • Generate Electro-Magnetic Interference (EMI/EMC) and electrical waveform noise/distortion;
  • Complex electrical designs;
  • More components resulting in greater expense vs traditional linear supplies.

The main internal components of an SMPS are:

  • Input rectifier and filter;
  • Inverter (consisting of a high frequency signal and switching devices);
  • Power transformer;
  • Output rectifier and filter;
  • Feedback system and circuit controller.

Figure 6 is a very highly conceptualized block diagram showing power flow through a SMPS. This example is typical of a DC power supply in a TV, VCR, computer/printer/copier power brick, all kinds of battery chargers and other electronic equipment. This example uses 120V AC wall input and produces clean, highly regulated DC Output.

Versions of this same technology can use DC input power to generate Pure Sine Wave AC, and can be used to change AC line frequencies (50hz to 60Hz, or vice versa). And, versions of this same technology are used extensively in DC-to-DC applications, like the Balmar external alternator voltage regulator, Victron solar DC-to-DC controllers or Sterling Power DC-to-DC Converters used in battery charging, voltage doubling or voltage halving applications. These power supply designs are also used in DC navigation equipment, VHF radio equipment, and other DC equipment since the internal “high speed switch” is, electrically, an inverter.

Figure 6 shows an “InAC” Input and an “InDC” input; likewise, an “OutAC” and “OutDC” output.  All of these input and output types do not usually appear in the same device, but different mix-’n-match combinations of AC input and output, and DC input and output designs are manufacturer’s-choice product alternatives. Because of the PWM signal generator, the “AC” output is an AC “Pure Sine Wave,” such as what is found in 12V/24V PSW inverters on boats. The “DC” output is a highly-regulated DC voltage, such as found in a DC-to-DC Converter, or in the power supply inside sensitive made-for-purpose navigation electronics.

Figure 6: Block Diagram of a “Typical” SMPS Stand-Alone DC Power Supply

Pulse Width Modulation in a SMPS:

Figure 6 shows a PWM Signal Generator (the signal in the red oval).  This is both “the heart of the magic” and the source of some of its problems. A “PWM Signal Generator” produces a DC Square Wave, where the individual pulses have varying widths. A DC Square Wave is technically also an AC waveform, so it can be fed into a transformer just like any other AC waveform.

PWM Signal Generators use very high internal DC square-wave signal frequencies (50kHz).  This enables the use of smaller, lighter transformers in power supply applications, and greatly simplifies filtering of the DC output voltage. A significant potential penalty of this technology is electrical noise and waveform distortion reflected backwards into the local dockside electrical system, as well as local RF interference, which is very common with LED lighting that isn’t filtered well enough.

Another sidebar of “geek speak” here, for those interested, to frame the operation of “Pulse Width Modulation.” Others can “skip it.”

In a SMPS circuit, a PWM signal is generated by feeding a reference signal and a carrier signal through a comparator. The output signal is based on the difference between the two inputs. In an inverter application, the reference is a sinusoidal wave at the frequency of the desired output signal. The carrier wave is a triangular, or “sawtooth,” waveform which operates at a frequency significantly greater than the reference. During times when the carrier signal voltage exceeds the reference signal voltage, the output square wave is in one state, and at times when the reference voltage exceeds the carrier signal voltage, the output square wave is in the opposite state. Figure 7 shows the signals, with the carrier signal in blue, the reference wave in red, and the PWM DC output square wave in green.

Figure 7: DC “Square Wave” Pulse Width Modulation Signal


Two electrical concepts with which I would expect most laymen to be unfamiliar are “Differential Mode” and “Common Mode” voltages and currents in a circuit. Common Mode currents are usually noise; that is, an AC disturbance between one or more signal or power conductors and an external conduction path, such as an earth or chassis ground or miscellaneous conductive material not intended to conduct the power or signals (including ground fault current). Even to power engineers, this is arcane stuff. Arcane, that is, outside the marine environment. On a boat, it can rear its ugly head as power quality issues in onboard electrical equipment.

Pictures will make this discussion much easier to follow, so the next several drawings are a progressive sequence of views of the same thing, each building on the previous one, to help understand Differential Mode currents and Common Mode currents.

Figure 8: Typical Components and Circuits in a SWPS

Figure 8 is the starting point; the same basic electrical circuit shown in Figure 6, but this time, with some of the internal circuit details shown. The voltage waveforms in the various parts of the circuit are as shown earlier, and have the same meanings here.

Figure 9: Diagram as above, Showing Electrically Isolated Metallic Case

In Figure 9, the very same circuit diagram is repeated, but here, the metallic equipment case of the device is portrayed as a grey box in the background of the diagram.

Notice (lower left) that the equipment case is grounded to the incoming AC power source, but the logic circuit itself is electrically isolated from the case. The electrical isolation from the case of the unit is to help minimize the presence of Common Mode signals and other types of electrical noise.

Figure 10: Sources of Common Mode Voltages/Currents in SWPS


in Figure 10, we begin to see the emergence of the “noise problem” (or “magic,” as some might see this).

Capacitors are electrical components that block DC but pass AC. It’s actually much more complex than that, but that’s enough for now.

A Switch Mode Power Supply (SMPS) develops high frequency signals that also vary in frequency, cyclically over fixed time intervals, in normal operation. These high frequency AC and AC-like DC signals couple through internal parasitic capacitances (stray capacitance between electrical circuit components) directly to the equipment ground, and also couple through the inverter circuit via magnetic field coupling.  This generates undesirable noise currents which find their way back to the external power supplying source.  Here, the little red capacitors show the parasitic capacitive connections between the darker grey component heat sinks and component metal part content and the metallic case of the equipment.

And following the electrical path of noise currents from the parasitic capacitances shown in Figure 11, we see the noise currents reaching the power source’s “safety ground” conductor.

Figure 11: Common Mode Noise Currents Find Their Way Back Into the Host Power Distribution Network


For simplicity, the Earth connection appears on this drawing, but remember from many previous discussions that the actual earth connection is back at the “derived source” in the facility’s infrastructure.

Finally now, we can see, and point to, the distinctions between Differential Mode signals (voltages and currents) and Common Mode signals in a Power Distribution System on a dock.

Figure 12 shows both kinds of signals in the distribution system and attached equipment. In this case, Differential Mode signals are the desirable, wanted voltages and currents that make attached equipment work. They are portrayed in blue. They originate at the facility power source, travel to loads on the line conductor, and return from loads on the neutral conductors.

I want to emphasize that Differential Mode currents and voltages are the same old AC currents that we know and love and have always talked about.  They are the currents flowing from the source to the load in the Line conductors (L1 and L2) and returning from the load to the source in the Neutral (N) conductor.  We have just never needed to talk about them as “Differential Mode Currents” (or “Differential Mode Voltages”) before. It’s not language that’s commonly found in the ordinary course of “electricity” discussions, because we have never needed to differentiate these normal currents from anything abnormal; until now.

Common Mode currents are undesirable and engineers work hard to minimize and eliminate them. They are portrayed in red in Figure 12.  The electrical “source” of Common Mode signals is in the SMPS of the premises equipment.  In most marine environments, there are many, many, many of these devices on any given dock. AC non-linear loads produce undesirable Common Mode signals that
require suppression by complex and costly circuits designed specifically for noise filtering and suppression.


Figure 12: Differential Mode Currents in Blue, Common Mode Currents in Red


Again in Figure 12, the high frequency Common Mode Noise Currents ORIGINATE in the Rectifier and Inverter sections of the SMPS, capacitively couple to the equipment ground, and flow along the ground conductor into the external system’s power source. From there, they can flow as electrical noise in many directions. Above, they are shown flowing in phase with each other (which is the technical definition of Common Mode Currents) back to the SMPS in which they originated (Rule 1).  They are flowing in the same direction on BOTH the Line and Neutral conductors of the device (Rule 2).  But, since the line, neutral and ground conductors are shared in parallel across many, many end-user circuits, that noise will ALSO flow on those parallel paths (Rule 2).  And since the device ground is connected to earth ground, Common Mode currents can also flow through the earth/water to impact other parts of the common, shared system.  And so, a noise-producing fault on one boat can and will propagate to other nearby boats.

And folks, that’s the reason to care about any of this stuff in the first place!

Figure 13: Typical Oscilloscope Traces of “Electrical Noise”

What does “electrical noise” look like? Figure 13 shows screenshots of electrical noise from articles I’ve found online.

Instead of a nice, clean waveform, it’s a distorted jumble of spikes, ripples and gaps.


In systems with Common Mode Currents causing electrical noise, both the line conductors and the neutral conductor will be oscillating up and down at the noise frequency, so Differential Mode Currents can appear almost normal.  But excessive Common Mode noise can cause equipment circuits to malfunction. Lots of money is spent to design power supplies to recognize and suppress these undesirable noise components. That works; to a point. But, sometimes under some conditions in some places, the noise components become too large to be fully suppressed by “standard” means, and then end-user equipment may fail. Noise filtering adds cost to components, and so may not be found in all equipment. Buyer beware.

Equipment manufacturers do know about this kind of noise and its causes, and try to design filtering circuits to minimize it. Let’s look quickly at another source of Common Mode noise that is found on many, many cruising boats.

Refrigerators with the ever-so-common BD35, BD50 Danfoss/Secop compressors are advertised as 12V and/or 24V DC appliances. And as far as the power supplied to the refrigerator is concerned, that’s true. However, the compressor motor itself IS NOT a 12VDC or 24VDC motor. That little compressor motor is a 3-phase, variable frequency motor that runs at nominal 277VAC.


Indeed, I did say that. Cowabunga, dude!

The Danfoss/Secop power module (101N0510) in my Vitrifrigo fridge accepts either 12VDC/24VDC or 120VAC and converts that input voltage into 3Ø, 277VAC to run the little compressor. The conversion is via a 3Ø AC SMPS. Yes, that makes the fridge a non-linear device to incoming 120V AC. Figure 14 is a highly conceptualized diagram of the 101N0510 SMPS:

Figure 14: Three-Phase SMPS with DC Input


The 3Ø SMPS is a Variable Frequency Drive application which spins the compressor at slower speeds when cooling demands are low and higher speeds when cooling demands are high, all while regulating the AC Voltage required by the motor.

For those familiar with three-phase systems, the Danfoss/Secop compressor motor is a Wye-connected 3Ø circuit with a totally isolated, floating neutral.  The 3Ø electricity is created by 6 IGBT switches, so as the phases rotate, the motor’s neutral star-point DOES NOT stay at 0 v with respect to frame ground.  Great pains are taken to avoid capacitive coupling from the motor’s star point neutral to the frame of the motor, because that becomes the source of a Common Mode Current. But yes, there is capacitive coupling from that neutral to the frame of the fridge… And. Insulation does break down as it ages and goes through thousands of heating/cooling cycles…

Here’s a link to an 11-minute Danfoss video, explaining the above, that readers may find interesting:


“Harmonic Distortion” is another true AC sine wave malformation. The math describing Harmonic Distortion is called “Fourier Analysis,” or a “Fourier Transform.” No, we’re not going to look at Fourier math in this article! The concept is, if a waveform is a true sine wave, it is made up of one, and only one, fundamental frequency. So, ANY sine wave that is not “prefect,” is actually not a “sine wave.” That waveform contains, by definition, “harmonic frequency components.” Harmonic frequencies are even and odd multiples of the fundamental sine wave frequency. For a 60-Hz sine wave, the 1st harmonic is 120 Hz, the 2nd harmonic is 180 Hz, the 3rd harmonic is 240 Hz, the 4th harmonic is 300 Hz, the 5th harmonic 360 Hz, etc, etc, etc. Fourier analysis will tell engineers exactly what harmonics are present, as well as their real and relative amplitudes. There is expensive handheld test equipment, such as the Fluke 40, 41 and 345, AEMC 8336, and others, that can identify harmonics for electrical technicians working on Power Quality problems at customer premises.

Harmonic waveform voltages combine with (add to and subtract from) the fundamental waveform voltage to produce the observed “apparent waveform.” OK. Time for a picture:

Figure 15: Waveform Distortion Caused by the Presence of Harmonic Frequency Components

Figure 15 shows a 60Hz fundamental frequency waveform (solid black line) and for simplicity, only 3rd and 5th harmonics (dotted lines) of the fundamental frequency. Instantaneous voltages of the fundamental and harmonic waveform voltages “add up” to produce the resulting voltage waveform that is actually experienced by equipment in the system.

Depending on the specific mix of harmonics and their amplitudes, many variations of malformed voltage wave shapes are possible. Two additional, “typical” malformed AC mains power waves are shown in green and blue in Figure 16. The red wave shape above is called “Flat Topping,” for obvious reasons. Flat topping is characteristic of harmonics injected into the mains power by SMPS power supplies. Other malformed waveforms are characteristic of inadequately filtered Variable Frequency Drives (VFD). There are many more harmonics than are shown in this highly simplified chart.

Figure 16: Infinite Malformed Wave Shapes Are Possible, Depending on Details of Specific Harmonic Content

All electrical equipment is rated to several industry quality standards by its manufacturer for its tolerance of Harmonic Distortion and its contribution to back-feeding distortion into the AC service mains and onto the grid in the neighborhood. The applicable utility company Power Quality standards (IEC 1000-3-2 or EN61000-3-2 and IEEE-519) are to enhance the quality, reliability and stability of the electrical power grid and it’s infrastructure.

The issue of Harmonic Distortion can become symptomatic on boats when digitally controlled equipment is running on generators vs when running on shore power. The reason traces back to Ohm’s Law. ALL ELECTRICAL CIRCUITS have the property of electrical resistance. In AC circuits, its called “Impedance,” but it works the same way as resistance in Ohm’s Law math.

Another sidebar of “geek speak” follows, but please don’t skip this one, because the punch line may be worth the price of reading through it.

Figure 17: Effects of Linear and Non-Linear Loads Fed by a Weak Grid

Figure 17, left side, represents “source impedance” as a combination of the source’s electrical resistance, Rg, and inductance, Lg. Ohm’s Law predicts there will be a voltage drop, Vdrop, across that source’s impedance as a result of current, Ig, flowing from the source into attached loads. The effects of that current flow is reflected in the waveform corruption seen on the drawing.  In the case of commercial utility distribution systems, the subscript “g” means “grid.” In the case of a boat’s onboard genset, the subscript “g” means “genset.” For utility systems, the heavy vertical line in the center of the drawing is the “Point of Common Connection” between the utility grid and a collection of shared loads, such as residential neighborhoods or commercial facilities. On boats, the PCC is where the genset connects to the onboard electrical panel; it’s easy to think of that point as being the Generator Transfer Switch. The effective impedance of the AC shore power utility grid system is proportionally much smaller than the effective impedance of an onboard genset. Per Ohm’s Law, the effect of harmonic currents is therefore much greater and more significant within the proportionally larger impedance of the proportionally smaller capacity generator power source. Especially when that small source is running above 60%-70% capacity, where magnetic saturation issues begin to come into play. The result of all of this is that onboard AC equipment may run just fine when on shore power, but experiences intermittent failures when running on the genset.


I’m sure some readers ask themselves that question from time to time.  Well, how’s about I show you some oscilloscope waveforms and you decide for yourself?

Aboard Sanctuary, we have a Dometic model DTU16-410A, 16kBTU, 120V, 1∅, mfg. no. 205160160,  heat pump installed.  User input is provided by a Dometic model SMX II remote control unit.  The SMX II stages loads “on” in order to more evenly distribute inrush currents and avoid tripping the HVAC branch circuit breaker.  When calling for heating or cooling, first the fan come “on,” then after short delay, the raw water circulator stages “on,” and again after a short delay, the compressor stages “on.”  These units are staged “off” in reverse order once the thermostat becomes satisfied.  Aboard Sanctuary, this heat pump provides for our main heating and cooling needs.  Many readers will have this unit, or very similar and equivalent units, aboard their boats.

The blower in this heat pump is a variable speed drive motor that speeds up when heating or cooling demand is high and then slows when the thermostat is satisfied.  Figure 18 show the 120V voltage waveform (yellow) and the current waveform (blue) with the blower-only running, on its “slow speed:”

Figure 18: AC Current Waveform – Blower only

Clearly here, the current drawn by this blower IS NOT a sine wave, with somewhat more than 50% of the leading portion of the waveform missing.  This is reminiscent of what we saw above in Figure 4.  This motor is a typical example of a non-linear load.  Because it’s a non-linear load, it contains many harmonic frequencies.

Figure 19 shows the current waveform (blue) when the raw water circulator stages “on,”  in this case, in a “heating” cycle example:

Figure 19: Current Waveform for Fan and Raw Water Circulator.

In this waveform, we can see the current drawn by the non-linear blower superimposed on top of the current drawn by the linear raw water circulator pump.  The pump motor’s waveform is a sine wave, and this composite waveform shows the “bump” of the fan’s non-linear load atop the motor’s linear sine wave.  This resultant waveform is non-linear, itself with harmonics, but different harmonics than were found in the fan waveform by itself.

Finally, Figure 20 shows the current waveform when the fan, raw water circulator pump and compressor are all “on,” running, at the same time.

Figure 20: Current Waveform: Blower plus Raw Water Circulator plus Compressor

This waveform is the composite result of the current drawn by the Blower, the current drawn by the raw water pump and the current drawn by the compressor motor.  This waveform is still not a sine wave, distorted as it is by the blower’s non-linear components.  And, as can easily be seen here, the top of the wave is “flattened” by the fan component.  And again, there are lots of harmonics here.

My oscilloscope has a built-in Fast Fourier Transform (FFT) math function to analyze the harmonics present in waveforms under analysis.  Figure 21 shows the FFT Analysis for the composite waveform of the Fan, the circulator pump and the compressor:

Figure 21: Fast Fourier Transform (FFT) of the Overall Load Drawn by the Heat Pump in its “Heat” Mode.

This FFT waveform shows the 60 Hz “fundamental frequency” as the highest peak on the left side of the screen.  Then, the peaks show several succeeding harmonics, each with their characteristic descending magnitude.

ALL OF THESE HARMONICS contribute to the shape of final waveform shown in Figure 20 and experienced by the equipment on this particular power inlet to our boat.  This (and one other, smaller, heat pump) is the only equipment on this 120V line into our boat, so what happens here is exactly what is drawn from the dock feeder by our boat.

The day I took these screen shots was “chilly” and rainy (downright “cold” for La Florida), and many live-aboard boats on the dock feeder probably had heating systems in various stages of “working.”  Notice that there is minimal distortion of the voltage (yellow) sinusoid.  That’s because the dock infrastructure is capable of providing power to the loads attached to the dock feeder; in electrical speak, the feeder has a “low internal impedance.”

One final waveform is shown in Figure 22.  I have not described this at all in the preceding text because it is generally not an issue for end users of electric power, but it does contribute to waveform distortion if/when aggregated into the electric utility.

Figure 22: Power Factor, Showing Current Waveform Lagging the Voltage Waveform for this Device

In this screenshot, I adjusted the amplitude of the current waveform (blue) to show it visually to the same scale as the voltage waveform (yellow).  Notice the obvious non-sinusoidal shape (distortion) of the current wave form; proof positive of the presence of Harmonic Distortion.  But also notice that the current waveform lags the voltage waveform in time; that is, these two waveforms are slightly out-of-phase with one another.  The voltage waveform makes its “zero crossing” before the current waveform makes its “zero crossing.”  This phenomena is an electrical “characteristic result” of “inductive loads” (motors, transformers) in AC systems, and it’s called “power factor.”  It doesn’t mean much to boaters, and there is nothing end users of electric power can do about it, which is why I haven’t discussed it in more detail, but it can and does mean a lot to the utility supplying the docking facility with electric power.  It means the electric feeder cables need to be bigger – sometimes significantly bigger – than they otherwise would in order to carry the total load of the facility.


What was the impetus for this article? Consider the question, “what is an acceptable limit for neutral-to-ground voltage?” The National Electric Code says 2.0 VAC is the max, and that’s based on resistive voltage drops that can occur in the neutral conductor in long cable runs as found on boat docks. But, since some of these Power Quality signals can look like Differential Mode currents between neutral and ground, it is possible in unusual circumstances to experience very high neutral to ground voltages. Anything higher than 2.0VAC should be treated as suspicious, and should be investigated. These waveform distortion faults are not simply aggravating and frustrating, they can be economically significant.”  They degrade wire insulation and the mechanical parts of equipment (especially, motor bearings), so they can have real economic impacts to equipment service life.


So, to wrap it up, there are several power quality issues that can occur on boats, and particularly when running on relatively “small” power sources (compared to facility shore power), like inverters and generators. Power Quality issues can cause attached equipment to experience intermittent or continuous failures. These problems can appear alone or in combination with other types of electrical
disturbances. Some of these disturbances are intermittent and very complex and arcane. When they happen, they can be enormously frustrating, time consuming and expensive.

In our “modern” “throw it away” vs “fix it” culture, it’s easy to fall into a “replace it” trap without identifying or realizing the actual root cause of the problem. Since the equipment itself may not be the cause of the fault symptom(s), replacing equipment with new, like-for-like equipment is just as likely to result in continued exposure to the same old performance issues with the new, replacement equipment. So I bring this to your attention for awareness. I haven’t talked much about how to identify these issues, because that is definitely an advanced skill requiring expensive, time-interval and data recording test equipment. But simply being aware of these issues can enable boaters to ask the right questions and get to qualified service technicians to reach a good, and probably permanent, problem correction.



Earthing and Grounding

11/6/2020: Initial Post
11/16/2020: Text and Graphics added
2/9/2021: Graphics added; minor edits


This is an introductory article, written to provide a basic understanding of a complex aspect of AC electric systems to an audience with little or no prior background in electricity. This subject is fundamental to AC system wiring in buildings and on boats, and is a prominent underlying part of the discussion in many other articles about AC Shore Power found on this website.

The concepts around “earthing” and “grounding” are at the very core of making electrical systems as safe as possible to people, pets, farm animals and wildlife. But, “earthing” and “grounding” may or may not mean the same thing when used in conversations and when used without context. These subtle concepts and the terminology they involve can be new and confusing to people without prior electrical backgrounds, and are among the most important to electrical safety. “Grounds” and “grounding” are topics that embrace multiple related ideas. “Earthing” and “Grounding” have different implications in residential single-family house settings than they do on boats, and residential electricians often are not aware of issues that apply to electrical safety on boats. Context is very important to understanding these issues, and as always in electricity, there are many “language shortcuts” that occur in group discussions on docks. Boaters will benefit from an understanding of these topics.

Static Electricity/Lightening

In nature, there is a form of electricity called “static electricity.” A major characteristic of static electricity is that it “flows” outside of wired electrical circuits, through the air. Its flow is intermittent and spontaneous. Static electricity is caused by the friction of two surfaces moving across one another. Static electricity results from the accumulation of electrons on one object (“negative charge”) and a deficit of electrons on another object (“positive charge”). It occurs where friction between surfaces creates a negative charge on the surface with excess electrons and a positive charge on the surface from which electrons were taken.

Residents of low-humidity, cold climates are familiar with static electricity. Little static shocks result from walking across a carpeted room in wool socks, petting the cat or dog, putting on a sweater or overcoat, and then contacting a doorknob, another person, or a car door (or any number of similar life activities).

“St. Elmo’s Fire” is a visible ionized corona; a static electricity “charge” that occurs when conditions are still and humid. In St. Elmo’s Fire, a sphere of blue or purple ionized plasma forms at the sharp points of outdoor structures, such as electric utility towers, church spires, chimney’s, masts, spars, flag poles, weather vanes, etc.  In the far upper atmosphere, a similar phenomena is responsible for the “Northern Lights.”

In clouds, warm, rising water droplets collide with cold, descending ice crystals, causing static charge to accumulate and eventually result in lightening.  The “discharge” of static electricity is a visible flash – an “electric arc” – composed of electrons flowing through ionized air. Electrons “flowing” between two points is the definition of an “electric current.” With static electricity, a voltage difference (electric charge) between the two poles of the static system becomes instantaneously great enough that the insulating characteristic of the normally nonconductive air gap breaks down and conducts. In household situations, the arc is mainly a nuisance, although it can damage modern semiconductor electronics and the “shock,” together with an occasionally audible “snap,” can scare/surprise its animal and human victims.

Lightening is by far the most impressive static electricity discharge phenomena with which we are all familiar. Lightening is static electricity with a massive visible arc composed of many, many thousands of amps. That arc current creates many thousands of degrees of instantaneous temperature rise in the surrounding air, resulting in thunder. Lightening releases massive amounts of energy (mega-joules) and often results in severe damage at its earth contact point. Lightening is more than capable of killing animals and people.

The arc of a static discharge “neutralizes” the accumulated positive and negative atomic charge of the oppositely charged poles of the static “system.” Protecting building electrical systems from being damaged by the discharge arc of lightening involves creating a means to get the arc current to flow AROUND, rather than THROUGH, the electrical system of the building, or its structural components. To protect a building, metallic “air terminals” are placed high, on roofs. A network of heavy electrical conductors connect the air terminals to rods driven into the earth. Large communications towers, bridges and high rise buildings often utilize their own metallic structure as a safe path for guiding discharge currents into the earth. Farm structures (barns, grain elevators, windmill pumps, etc), industrial sites (refineries, chemical plants, chimneys, etc, etc), and hospitals are protected with air terminals and metallic paths to earth ground. These protective devices are apparently considered unsightly and undesirable in suburbia, because they are rarely found on single-family residential buildings. When we lived in Indiana, our neighbor across the street had the chimney blown off his house by a lightening strike to that unprotected structure.

Lightening protection for boats is a separate and complex study; inexact, expensive to install, and impossible to properly retrofit if not built into the initial design at the construction phase of the boat’s life. Boats struck by lightening almost always experience severe electrical system damage and extensive damage to electronic equipment aboard. Lightening can literally blow a hole in a boat’s hull on its way to earth ground.

See my article on “Faraday Cages” for ways to protect sensitive electronic gadgets from lightening; for example, hand-held VHF radios, hand-held GPS, computers, back-up hard drives and cellular telephones.

Residential Electric Circuit “Wire” Naming and Identification

All operational electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”

In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and can also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the DC color code was changed to “yellow” to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (almost always) not technically correct, since “ground” wires are not intended to carry current in normally operating systems (reasons explained later).

In AC circuits in buildings, the power on the conductors is alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work. In single phase 120V circuits in North America, the two conductors are named for their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained later), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

In electrical engineering, “earth” is the single reference point in an electrical system from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the National Electric Code for AC distribution circuits in buildings has required “Equipment Bonding Conductors” and an “Equipment Grounding Conductor.” In the NEC, Article 250 is the standard for “grounding and bonding.” Each individual conductor that is an individual component that comprises the network of conductors that make up the “ground system” has its own specific name. For the purpose of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”

The NEC, Article 100, defines an “Effective Ground-Fault Path” as an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origination point of a ground-fault in a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault sensors. The purpose of the safety ground is to create an “effective ground-fault path.” That low resistance path is intended to function as a fault-clearing path; for that single “emergency use” only. “Fault-clearing” means that the circuit breaker feeding that faulting circuit will trip to remove power from the circuit. Under normal conditions in a properly wired electrical system, the safety ground conductors (including the bonding system network on a boat) DO NOT/MUST NOT carry current in normal, routine operation. The safety ground conductors are intended to ONLY carry current when there is a “fault” in the system. In buildings on land, the ground conductor is typically bare copper wire. On boats and in appliances, the ground wire is insulated, and solid green in color, or green with a yellow stripe.

Subtle take-away: the DC “negative” conductor has the same role in a DC circuit that the AC “neutral” conductor has in a residential/boat AC circuit. That is, the DC “Negative” conductor returns current from the load to the power source (battery). In a “grounded DC electrical system” (which is uncommon), there is a third conductor that is part of the DC circuit, just as there is a safety ground in a 120V AC system. The B- conductor is a “Current-Carrying, Grounded Conductor,” and is entirely separate from the actual ground conductor.

NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new and repair work, always install the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape. The NEC does not allow green safety ground wiring to be changed. Safety ground wiring must be green, and green must not be used for any other purpose.

Note: on some but not all boats built overseas, AC wire colors may be different than the North American standard (NEC and ABYC) colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white. This difference is also common on boats built overseas, because they follow the European color standards. If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine how that wiring is used to ensure equipment, fire and personnel safety.

However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.

Electrical Circuits

Core concept: opposite to the situation with static electricity, in man-made electrical circuits, the electricity originates at a point that is know to be its “source.” This can be a battery, a solar cell, a fuel cell, a generator, or a point-of-connection to the electrical grid. An electric “circuit” is said to exist when an electric “current” has a path that enables electrons to flow out of the source on a conductor, travel through a load to do useful work, and then return to its source on another conductor. The “source” can be DC or AC. Whether DC or AC, a “voltage” can appear at the output terminals of a source (like a battery or generator), but a “circuit” does not exist unless electrons can flow out of the source, through a load, and back into the source. A “circuit” consists of is a round trip of continuous conductive wiring for current flow out of a source and back into the source. A “switch” is any electrical device that “opens” a circuit to prevent electron flow as a matter of convenience and/or function; a relay is a device that interrupts current flow in the circuit that it controls; a “fuse” or “circuit breaker” is a device that “opens” a circuit to protect conductor insulation or remove power as a matter of fire prevention and/or personnel safety; and, a “severed” (“broken”) wire is a “malfunction” (“fault”) that “opens” a circuit so that there is, in effect, no round-trip circuit for electrons.

Fundamental Physics of Electric Circuits

Rule 1: Electric currents MUST RETURN TO THEIR OWN SOURCE.
Rule 2: Electric current will return to its source on ALL AVAILABLE PATHS. Corollary: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path.
Rule 3: An electric “circuit” does not exist UNLESS current has a continuous conductive path on which to flow from source back to source.

Readers will come back to these fundamental rules of electrical behavior over-and-over again when dealing with electrical systems and the concept of electrical faults. The more complex the electrical system, the more numerous and complex the issues, but electrical safety always comes back to the physics that underlies the behavior of electric currents.

The National Electric Code (NEC) and the American Boat and Yacht Council (ABYC) electrical standard, E-11, provide design and installation requirements that define system controls that manage how voltages will be safely removed and currents will be safely stopped (disconnected) in response to faults of various kinds that may occur in an electrical system. It is actually quite easy “to get something to work.” It is much more complicated and much more important to control electricity when something isn’t right. Disconnecting power, and disconnecting power safely, is the only way to prevent fires and electric shock risks to personnel.

Electric Code Grounding Categories

Finally, we get to “earthing” and “grounding.” There are two contexts for electrical “grounding” as required by the NEC.

  1. System Grounding
  2. Equipment Grounding (Bonding)

Residential System Grounding

“Ground” is the standard reference point for measurement of voltages. The NEC, Article 100, defines the crust of our beloved home planet as “Ground.” Ergo, Sir Knight, the electrical potential (natural voltage) of the earth’s “soil” is defined to be “zero volts.” All voltages are measured from an earth ground reference point.

The crust of the earth is electrically conductive. The earth’s crust contains many minerals and mineral salts which provide “free electrons.” In response to an impressed voltage, electrons will flow from point-to-point around and within the earth’s crust. An important corollary is that currents flowing in the crust of the earth follow the fundamental rules of electro-physics, including “Ohm’s Law” and “Kirchhoff’s Law.” In order to create a residential electrical system connection to “earth ground,” one or more interconnected metallic rods (often copper) are driven into the earth.

In the North American residential AC system model, three conductors arise from the utility power transformer at the street. All three are “Current Carrying Conductors.” Two of those conductors are considered, by conventional agreement, to be “energized” (“L1” and “L2”) and one is the neutral line (“N”). This is known as a “Single Phase, Center Tapped, Three-Pole,” system. The “Neutral is the transformer’s center-tap connection. As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V is present between “L1” and “N” and between “L2” and “N.” Note, however, that at the street, these voltages “float” with respect to their external environmental surroundings. They are not connected to anything. This situation is referred to as a “floating neutral system,” and in a “floating neutral system,” the voltage between the neutral and earth ground is unlikely to be “zero.”

If these three lines were connected to a distribution panel in a residence, all electrical appliances would work correctly. All of the necessary operating voltages inside the building would be correct. But, measured against a ground reference, it’s entirely likely the neutral would be at some perhaps large voltage difference with respect to the metal sink where food is prepared, or the metal bathtub when the baby gets bathed, or the metal faucets in the family shower. Clearly, a shock hazard would exist. To eliminate that hazard, the “Neutral” is electrically “tied” (connected) to an earth-ground reference point.

To create a system referenced to a known. zero-volt earth ground, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. This connection results in an earth-ground, “grounded neutral” system, throughout the premises. In a grounded neutral system, the voltage between the neutral conductor and the safety ground conductor is “zero,” or should be very close to “zero.”

While it’s true that the earth is electrically conductive, the earth is not a good conductor. Even at its best, “dirt” is not as good at conducting electricity as aluminum and copper wire (and also not as good as salt water). But rest assured, Ohm’s Law is a fixed “law” of physics, and it does apply to currents flowing in the earth. So while “dirt” may not be a great conductor, it is a very large-diameter conductor, with an infinite number of parallel paths, and with virtually unlimited ampacity. Just how well any local parcel of “dirt” conducts electricity depends on many things, including mineral and moisture content. The NEC requires that ground rods have a minimum contact resistance of 25Ω to earth. Sometimes, that can be achieved with a single 10′ rod driven into the soil; sometimes it requires a long rod driven 40′ – 50′ into the ground; and, sometimes it requires an entire network of long ground rods, all driven deep, and all connected together in parallel.

The essential point here is that “earth ground” is a universal reference point for all terrestrial power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the absence of any voltage. This works well because in a properly functioning, properly wired system, no current flows on the grounding system. Since no current flows, the voltage at the contact point with the copper grounding rods stays reliably at zero volts (as predicted by Ohm’s Law). Electrical faults (discussed later) create vastly different, sometime dangerous conditions.

Important to realize in this discussion, the earth ground alone DOES NOT protect against electric shock. It is merely a reference point against which system voltages are stabilized at “zero.” Earth ground IS NOT a reference for protective devices (fuses, circuit breakers) to trip to remove power when an electrical fault condition occurs. The Earth IS NOT the “source” for any DC or AC electrical energy. Remember Rule 1: “All electric currents MUST RETURN TO THEIR OWN SOURCE.” Electrical currents in residential and boat electrical systems DO NOT originate in the earth, and so, do not return to the earth. However, under some kinds of fault conditions, current can and does return to its source by traveling through the “dirt;” or, through the water in which a boat is floating!

Well then, why do we have the “Earthing” connection? Well, “Earthing/Grounding” in this context is a single-point-of-connection (one point and ONLY ONE POINT) to the earth for the purposes of mitigating:

  1. Static build-up (wind induced),
  2. System voltage instability, including:
    ▪ Unintentional physical contact with a higher voltage system (automobile accident or severe weather incident involving “hot” utility services),
    ▪ Repetitive intermittent short circuits (dispatched to first responders as “trees on wires, burning!”), and
    ▪ Utility switchyard and distribution system switching surges (spikes).
  3. Nearby vicinity lightning splash, and
  4. Transient interference (from static discharge and local RF emissions).

Not all of these exceptional conditions apply equally to all residential premises systems, but because some do apply in all areas, the National Electric Code treats all alike.


Consider a building’s main electrical service panel as the “source” of AC power (volts and amps) for the building and all of its branch circuits. In a household AC electrical system, current from that source emerges from a wall outlet on one appliance conductor and returns to the wall outlet on the other appliance conductor. Refer to Rule 1: “Electric currents MUST RETURN TO THEIR OWN SOURCE.”

Now consider a hot water heater, washing machine, trash compactor, dish washer, garbage disposal, microwave or toaster oven, each constructed with a metal exterior cabinet. The appliance is an electrical “load.” Electricity is provided to it from the wall and returns from it to the wall. With just two conductors (supply and return), the appliance can work normally. But, what happens if there is a frayed or cut wire inside the cabinet, and in physical contact with the metal cabinet of the appliance? In that event, the cabinet will have a non-zero “touch potential” (voltage) on it’s metal enclosure, and that voltage could easily be a shock hazard to residents. This is exactly how houses were wired before the 1950s, and many people reading this will remember the “two prong” duplex outlets of that time. In those days, people did get shocks from household appliances, fans and table lamps. Sometimes even from the iron. (Did Granny’s iron have fraying cotton insulation at the plug end? Does anyone actually iron anymore?) And sometimes, the shocks were serious. These shocks were the results of “faults” in the circuit.

A “fault” is said to exist when:
1) an electric current does not flow when it should, or
2) an electric current flows in an unintended path to get back to its source.

Clearly, an electric shock – which is a path through a person’s body – is an unintended path. To avoid shocking experiences like this, a third electrical conductor (safety ground) was added to electrical systems in homes, garages, barns, workshops, supermarkets, retail stores, office buildings, malls, commercial offices, workplaces, etc. That is, anywhere people might come into contact with electricity.

Grounding Conductor

This brings us to the next major category of “grounds” and “grounding.” Not to the earth itself, although it is connected to the earth, but rather to a common point in the building’s main electric panel. In this context, the word “ground” is a useful – but misleading – concept, because the ground conductor does not live in the ground and it does not send fault current into the ground. The ground conductor is connected to the ground rods at the service entrance, so it is REFERENCED to ground. That way, that ground conductor is held at “zero” volts with respect to all other components in the electrical system.

The “equipment grounding conductor” (a/k/a the “safety ground”) in residential and boat electrical systems is designed and intended to cause fuses or circuit breakers to trip in order to DISCONNECT POWER in case of a fault. It is, in the words of the NEC, an “Effective Ground-Fault Current Path;” that is, an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origin point of a ground fault in a wiring system to the electrical supply’s source and that facilitates the operation of the overcurrent protective device or ground-fault sensors.

Disconnecting power is the ONLY WAY to protect against fire and personal injury caused by ground faults in an electrical system. Equipment grounding is the intentional (in fact, NEC Article 250.2 mandatory) act of providing a network of conductors that interconnects the metallic cases of all electrical equipment attached to an electrical distribution panel. The bare copper or green-insulated “grounding conductor” discussed earlier is connected to the metallic cabinets of all modern appliances, and to the round ground pin of North American 15A and 20A household electrical utility outlets. The wires that make up the network of grounding conductors in a home have several names, but “safety ground” is representative for this discussion. On a boat, this network of green grounds is called the “bonding system,” of which the AC Safety Ground is a key part.

Residential dwelling units in North America range from tiny houses to single family homes to compounds with outbuildings to multi-family buildings of all kinds. A “ground buss” is always located in the main service panel of a dwelling unit, and in any sub-panels that may be supplied from that main service panel. Ground conductors from all branch circuits in the panel are connected together at the panel’s “ground buss.” Sub-panel grounds are in turn brought back to the ground buss in the main service panel. Boats are wired as sub-panels, not as main service panels.

At ONE PLACE in the main electrical service panel of the building, the “Grounding Conductor” is electrically connected (bonded) to the “Neutral” “Current Carrying Conductor.” By code, there is ONLY ONE “Neutral-to-Ground” bond in a residential electrical system, and it is placed at the Main Service Panel – never in sub-panels. A boat is wired as a sub-panel, so there should NEVER be a neutral-to-ground bond aboard a boat connected to, and operating on, shore power. This mistake in wiring on a boat is a very common cause of boats tripping shore power ground fault sensors on docks.

Now consider the fault case where an internal fault of some amount tries to put a touch potential voltage on the metal cabinet of an appliance. Rule 2 applies; “electricity will return to the source on all available paths.” Since the grounding conductor is attached to that metal cabinet, the Grounding Conductor does two things. First, it holds the voltage of the appliance cabinet at zero volts (because it’s “grounded” at the main service panel to the network of ground rods), which protects people and pets from shock. Second, it provides a very low-resistance path back to the service panel, via the neutral-to-ground connection, which instantaneously draws a very large spike of current through the circuit breaker (or fuse). That instantaneous large overload trips the circuit breaker to REMOVE POWER from the faulting circuit. Removing power is how the system protects buildings against fire and protects people from electric shock.

Ground Faults

The earth’s crust is electrically conductive, so that creates two electrical system design and code issues.

Rule 1 again: Electric currents MUST RETURN TO THEIR OWN SOURCE; and
Rule 2 again: Electric current will return to its source on ALL AVAILABLE PATHS;

Enter, our Corollary to Rule 2:: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path. This law of physics is called Kirchhoff’s Law, which states that when there are multiple parallel paths back to the source, current will divide and some portion of the total will take each available path back to its source.

In both home appliances and boat appliances, the two most common causes of “ground faults” are aging water heater elements and aging motor/transformer windings. In a water heater, power can leak through the water in the heater between the energized heating element and the metallic case of the water heater. In a motor, over time, dust and other airborne contaminants build up in motor windings, and at the same time, heating and cooling cycles cause the winding’s insulation to break down and develop micro-pores. In these cases, the fault current isn’t enough to trip a circuit breaker, but small amounts of power can leak to the Grounding Conductor, and then back to their source at the main service entrance panel. This is a ground fault by definition, because ANY current flowing on the safety ground is flowing on an unintended path. In this case, the fault current flows back to the source on the Safety ground’s conductor. More in a couple of paragraphs, but first, some illustrations.

Here’s a homeowner scenario… Dad’s gonna trim up the lawn, trim some plants, and wash the car (he’s young and energetic, unlike myself). He runs a 100′ extension cord in order to power an electric hedge trimmer, grass trimmer, circular saw, reciprocating saw, radio, charcoal fire starter, polisher/buffer, whatever. The extension cord has a ground wire, but the “tools” attached to it by multi-outlet adapter either have only two wires, or the ground pin has been cut off as a “matter of portability convenience.” Tools that aren’t actively in use are lying on the ground, where they and their cords are in contact with the ground. Now there is a path for power to get back to its source through the soil, to the ground rod(s) serving the main electric panel, and back to the neutral in the main service panel. That is a ” ground fault” because it is clearly an unintended and unwanted electrical path through the soil (ground). And at some point in this scenario, Dad will pick up his tools and possibly have a shocking experience. Possibly even, a lethal shocking experience. Without a continuous “effective fault-clearing path,” there is no way to shut off the power to save Dad from a shocking experience

OK, here’s another scenario with which my daughter and I have direct, personal experience. One Halloween “Hell Night,” Kate came home in need of a shower to remove 17 cans of different brands of shaving creme and lord-only knows what else she had encountered while “out with friends.” She went off to the shower, whereupon Peg and I laughed at her state of dishevelment! Note here, one of our sons had just finished his shower from his night “out with friends.” After just a couple of minutes, there arouse a righteous and shrill scream from the upper reaches:

“Daddy! Turn the water back on!”

In my total, complete and absolute innocence, I grunted at Peg: “Huh?”

The house water pressure had disappeared to a dribble while Kate was all lathered up. Mid-shower! Springing into action, Mom was “off to the rescue,” and Dad was “off to the basement.” In the basement, all seemed OK, but alas, there was no house water pressure.

Plumbing leaks? No water on the floor!
Pressure in the well tank? No! Gauge reading “zero.”
Pump Circuit Breaker “on?” Yes; and not tripped.
Pump relay OK? Yes, relay “picked.”

“Uh oh!” “Darn it!” (or words to that effect)! “Must be the well pump!”

Our homestead in the Catskill Mountains – and all of our neighbors – had a private deep-well that supplied our drinking water.  Our well was 100′ deep, and the pump lived at the 90′ level (not very deep). As the pump started and stopped over many years, it twisted (torquing) on the end of 90′ of semi-flexible PVC hose. The wires running to the pump abraded against the earth and rock walls of the well, and eventually the wire’s insulation wore through. This created a ground fault connection from the exposed bare wire directly to the earth about 70′ down.

Deep well pumps are usually two-wire, 240V circuits. One conductor of ours was in direct contact with the wall of the well. If the point-of-contact had been within the cast iron portion of well casing, it’s likely the circuit breaker would have tripped, because that metal casing did have an equipment grounding conductor. But in our case the point-of-contact was with sediment or rock, the 240V circuit breaker indeed did not trip. That did, however, create a significant ground fault. The pump was trying to start, but didn’t get enough voltage to overcome the weight of a 90′ column of water. Power was flowing into the earth, but not enough to overload and trip the pump’s circuit breaker. Power divided where the bare wire touched the well’s wall. Some of the power going down that hole got to the pump and returned on the other current carrying conductor, but some of the power going down that hole flowed back to the panel through the earth, to our home’s ground rods, and back to the service panel’s neutral.

In these situations, a newly-installed (since 2002 or so) residential service panel would have been fit with “Ground Fault Circuit Interrupter” (GFCI) to remove power and terminate the ground fault condition. In the case of yard tools creating a shock hazard at the end of an extension cord, GFCI could literally save Dad’s life. In the case of the deep well fault, GFCI could have saved equipment from damage. Our deep-well pump got burned out by the prolonged stall created by the low supply voltage. Relate this to boats on docks with pedestals fit with 30mA “Equipment Protective Devices.” This is a case where a 30mA EPD on the well supply would have saved the well pump from damage, and would have provided a clear hint to the location and nature of the fault.

GFCIs and EPDs work by monitoring the outgoing and returning current on the two Current Carrying Conductors. The currents should balance equally between the two conductors. If not, there is a ground fault and the GFCI device trips power off. What happens if there is no GFCI, as was our case at that time? Well then, the ground fault condition continues, because power flows out from the source, but has multiple parallel return paths, one through the returning current carrying conductor and the other through the earth to the ground rods at the main service panel at the same time.

See my article on causes of ground faults on boats for information specific to that topic.

See my article on GFCIs for more detail on how these devices work.

Ground faults on boats behave in the same manner, but are very dangerous, because instead of flowing through dirt, which is largely inaccessible to people, pets and wildlife, ground faults on boats can and do flow through the water. People – especially children – pets and wildlife are sometimes found in the water.

See my article on “Electric Shock Drowning” to read about ground faults in the water.

Ground faults on land can be quite dangerous in another, subtly different way. Suppose a 240V mercury arc exterior driveway light has a ground fault at the pole base that is not large enough to trip an over-current circuit breaker. We all now know from my well scenario, above, that 240V in direct contact with the earth will probably not trip a circuit breaker. But in that condition, the soil surrounding the point-of-contact between the energized conductor and the soil itself is electrically “hot.” This condition sets up a “voltage gradient” on the surface soil surrounding the point-of-contact. Using 240V in this example, at the point-of-contact with the voltage, the voltage in the soil is the same as the supply voltage, so there is no DIFFERENCE in the pole voltage and the soil voltage. But Ohm’s Law applies here, and however much current is flowing into the ground and back to the service entrance panel is creating a voltage drop along the surface of the soil (or driveway). So, the resistance of the local soil matters. One electrical standard1 assumes that 25% of the total voltage drop due to path resistance will be found in the first foot of distance away from the point-of-contact. One foot away from the point-of-contact, the soil is at 163V of shock “step potential.” Three feet from the point-of-contact, the soil is at 202V. Five feet from the point-of-contact, the soil is at 206V. As you can see, straddling the voltage gradient of the surface soil can create dangerous “step potentials” in the soil. Imagine the potential for what could happen when Rover comes over to “mark his spot” at that light pole.

The same sort of voltage gradient forms in the water around the prop and rudder or a boat if there is an AC ground fault on the boat. That gradient is quite enough to get a diver’s undivided attention. If the fault itself is in a heat pump, and the diver is working on the boat when the heat pump cycles “on,” … Well, that diver would quickly know how Rover felt…

See my article on “Electric Shock Drowning” to read about ground fault voltage gradients in the water.

Ground faults can be very dangerous!

Do not defeat safety devices.

Install GFCI and ELCI on boats.


  1. ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1]

Inverters On Boats

7/20/2020: Initial Post

The ABYC definition of an inverter is “an electronic device, powered by batteries, designed primarily to provide AC current at a required voltage and frequency.”  In North America, inverters produce 120V AC (or 240V AC) at 60 Hz from energy stored in 12V or 24V batteries.  On boating forums that I follow, there have recently been many questions about selecting and installing inverters on boats, so in this article, the topic is “Inverters on Boats.”

There are two types of inverter installations found on boats.  The first case is the stand-alone inverter.  These are usually smaller inverters used for charging cell phone batteries or powering portable computers.  Larger stand-alone inverters can be installed alongside, but separate and isolated from, the built-in AC system of the host boat.  Stand-alone inverters are  limited in features, requiring manual intervention each time they are needed.  They are turned “on” manually when needed and turned “off” manually when no longer needed.  Their un-shared outlets are often mounted on the unit itself.

The second case is inverters installed within the host AC power system of a boat.  When installed fully-integrated within a boat’s AC power system, inverters offer boat owners a whole-boat “Uninterruptible Power Supply” (UPS), and commonly function as battery chargers while external AC power is available.  Inverters installed within the host electrical system must comply with cUL/UL-458 per the ABYC Electrical Standards E-11 and A-31.

In 2020, most inverters sold for installation on boats are Pure Sine Wave (PSW) devices.  Older inverters were Modified Sine Wave (MSW) devices.  Some 120V household devices did not work well, sometimes not at all, on MSW inverters.  Generally, PSW devices are to be preferred for overall compatibility with consumer electronics in household equipment and appliances.

Figure 1 shows a stand alone inverter.  Inverters in operation can demand a great deal of DC current from batteries. Regardless of stand-alone or fully integrated installation, the B+ and B- cables from the batteries to the inverter must be sized for the maximum current the inverter can draw from the battery.  The B+ feed must be fused to protect the cables, and should have a disconnect switch rated for continuous use at or exceeding the maximum demand of the inverter.  The device itself must be “grounded” to the grounding buss of the host boat.  Unfortunately, I too often see stand-alone inverters that do not meet these ABYC electrical standard requirements, which apply to all DC devices.

The ABYC electrical standard, E-11, “AC And DC Electrical Systems On Boats,” July, 2018, treats stand-alone inverters in the same way it treats any other DC device (windlass, winch, thruster, water pump, instruments, auto-pilot).  The AC output of a stand-alone inverter is entirely separate and isolated from the boat’s host AC power system.  Thus, there are no specific ABYC requirements for the AC output of a stand-alone inverter.  These devices are easy to install, relatively inexpensive, and can meet basic AC power needs.  Some stand-alone inverters do not comply with North American residential electrical system requirements (grounded-neutral).  Stand-alone inverters enable bad user practices, such as extension cords running across the floor of a boat, and wiring that is too small for the loads.  A common “operator error” is to forget to turn the stand-alone inverter “off” after use, which can damage or destroy batteries.  These “owner errors” are common as fire and personal safety concerns.

Figure 2 is a “simplified view” of a typical 120V AC shore power system as found on many cruising boats.  I have taken a shortcut to also show that this boat has a generator installed.

The ABYC E-11 electrical standard does apply to this AC system.  In a previous article, I discussed the E-11 Standard as it correlates to Sanctuary’s AC system.

There is an important US National Electric Code/Canadian Standards Association “rule” to remember about all end-user AC power systems in North America.  For fire and shock safety, AC power sources are grounded at their source.  The result is called a “grounded-neutral” system.  The neutral conductor itself is a current-carrying conductor that returns current from the load to its source.  To automatically disconnect electrical faults, the neutral conductor is held at zero volts by a connection between the neutral conductor and the facility’s ground conductor.  The connection is called the “neutral-to-ground bond,” or “System Bonding Jumper.”  So in Figure 2, the shore power neutral conductor is “bonded to” the shore power ground conductor before these conductors come onto the boat, in the electrical infrastructure of the marina/boatyard.  The neutral of the boat’s onboard generator is “bonded to” the boat’s AC safety ground network at the metal frame of the generator.

The “grounded-neutral” requirement is the reason the “energized” (“hot”) Line conductor AND the “grounded” Neutral conductor must BOTH be switched by the Generator Transfer Switch (GTS).  When the GTS is in the “Shore” position, the neutral-to-ground bond comes onto the boat from the shore facility, via the shore power cord.  When the GTS is in the “Generator” position, the neutral-to-ground bond is at the generator, as shown in Figure 2.  To eliminate a ground fault path, the generator’s neutral-to-ground bond CANNOT also be in the active circuit when shore power is feeding the boat.  So, it is switched “out” of the active circuit by the GTS, which switches both the hot and neutral conductors.

Figure 3 shows the case of an inverter that is fully-integrated into the host AC system of the boat.  In this case, the inverter is not stand-alone, as in Figure 1, but is installed within the host AC system, between any other AC power source(s) and the boat’s AC distribution panel.  Here, it can be operated manually, or it can operate automatically, changing modes as incoming AC power comes and goes.  Automatic operation is helpful when commercial power fails, or when a dock neighbor inadvertently turns “off” the pedestal breaker of another boat.

As shown in Figure 3, power from either shore or the onboard generator is supplied to the inverter’s AC input.  This cUL/UL-458 compliant design operates in one of two modes.

STANDBY mode – passes power that originates upstream of the inverter through to attached downstream loads (“passthru”); in Figure 3, all of the boat’s AC loads are fed via the inverter.

INVERT mode – draws energy from the onboard batteries in order to create AC output at the rated voltage (120V, 240V) and frequency (60Hz) to feed downstream loads.

Figure 4 shows a similar system, but here some loads are powered via the inverter and other loads are powered only by upstream AC sources.  On Sanctuary, our onboard utility outlets are powered via our inverter, but our hot water heater, genset battery charger and fridge only receive AC power from upstream sources.  That arrangement greatly conserves our available battery capacity.

Note that Figures 3 and 4 refer to the Underwriter’s Laboratory’s UL-458 Standard, which is entitled, “Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts.”  Recall that all AC power sources in North America must be grounded at the source (grounded-neutral), and so shore power is grounded in the facility infrastructure and the generator is grounded at the generator.  To accomplish automatic ground switching, inverters intended for use on mobile platforms (ambulances, trucks, airplanes, RVs and boats) MUST comply with cUL/UL-458.

This is a good time to digress for a moment to look at the ABYC portfolio of electrical safety standards.  These standards fall broadly into two categories.  The first is standards that apply to the design and construction of individual electrical components, such as:

    • A-16 Electric Navigation Lights
    • A-27 Alternating Current (AC) Generator Sets
    • A-28 Galvanic Isolators
    • A-31 Battery Chargers And Inverters
    • A-32 AC Power Conversion Equipment And Systems
    • E-10 Storage Batteries

The second is standards which apply to joining individual component parts together to work within a unified boat system, such as:

    • E-11 AC And DC Electrical Systems On Boats
    • E-30 Electric Propulsion Systems
    • H-22 Electric Bilge Pump Systems
    • TE-4 Lightening Protection
    • TE-12 Three Phase Electrical Systems On Boats

All of these standards make reference to other Industry Standard sources for detailed specification of performance requirements.  Typical outside references are to established by  industry standards organizations including IEEE, IEC, ISO, cUL/UL and eTL.

So as applies to inverters, there is an ABYC standard (A-31) that is specific to the design of the unit itself, and a second ABYC standard (E-11) governing the system into which the unit is installed.  For inverters, the design reference is UL-458 in the US (and CSA C22.2#107.1 in Canada).

When a UL-458 compliant inverter is in “Invert” mode, a relay inside the inverter automatically creates the inverter’s neutral-to-ground bond.  When the inverter is in “Standby” mode, that same relay automatically removes the inverter’s internal neutral-to-ground bond so both AC power and the source’s neutral-to-ground bond are “passed through” the inverter to the boat’s AC power panel.  Functionally, this is what a GTS does in the case of a generator; i.e., when the GTS is set to “Shore Power,” the neutral-to-ground bond at the generator is switched out of the system.  The GTS transfers both hot and neutral, and transferring the origin of the neutral is what changes the origin location of the Neutral-to-Ground bond.

Figure 5 shows a simplified drawing of a UL-458 inverter in “Standby” mode.  AC power passes through (“passthru”) the inverter from an external AC power source, whether that be shore power or generator.  The relay shown in the red circle is “energized” (“picked”) by the presence of external AC power, so it connects the incoming power hot and neutral conductors to the output load circuits.  The green circle shows the inverter’s ground connection, but since external power is present, the relay is “picked,” so the neutral-to-ground bond that is located at the incoming source is “passed through” the inverter to protect downstream branch circuits.

Figure 6 shows the same inverter operating in INVERT mode.  In this case, incoming AC power is absent, so the inverter’s internal relay (red circle) is de-energized (“down”).  Because the relay is “down,” AC output from the inverter is created by the inverter’s electronics from energy stored in the boat’s battery bank.

The green circle highlights the inverter’s internal neutral-to-ground bond, which in this mode is connected via the relay.  That connection is required because the inverter, in INVERT mode, is the actual “source” of the AC power being delivered to the boat.

Following in Figure 7 is a complete circuit diagram of the AC system aboard Sanctuary.  Our 120V, 30A, two inlet AC System is fairly common on boats of our size class, and consists of eight AC branch circuits serving the equipment on the boat.  Other than completeness, our system is just like the simplified view portrayed in Figure 4.  Boats with 240V, 50A shore power service (3-pole, 4-wire cords) will look slightly different on the front end, but 120V inverter installations will be the same as shown here.

Sanctuarys generator is in the upper-right corner of the drawing.  Note the generator’s neutral-to-ground bond, highlighted there in green.

Our fully-automatic, fully-integrated inverter/charger is in the lower left-center of the drawing, in the small red circle.

On the right middle, in the dotted red circle, is our house, “Shore 1,” AC distribution panel, containing the eight branch circuits.  The top four branch circuits are fed only from either shore or generator power, whichever is selected by the GTS.  The bottom four branch circuits are fed via “Invert” or “Standby (passthru)” power via the inverter.  Our inverter is always part of our outlet distribution circuit, 24x7x365-1/4.

In Sanctuary’s system, at inverter installation-time, the hot buss feeding the branch circuit breakers on the AC power panel had to be divided into two parts (blue ellipses) in order to accept two separate feeds from 1) external power and 2) the inverter.  Dividing the hot buss required modification of the OEM electrical panel.  Also at installation-time, the neutral buss (red ellipses) had to be divided in order to separate the neutrals of circuits that are not fed via the inverter from the neutrals of circuits that are fed via the inverter.

The need to separate the neutrals stems from the requirements of the 2011 NEC and 2012 ABYC E-11 standard, adopted in coordination to reduce/eliminate dangerous ground fault currents flowing into the water from docks and boats.  (See the article on Electric Shock Drowning for more information.)  If the neutrals are not separated, an unintended ground fault leakage path can be present.  The day the boat arrives at a marina or boatyard where pedestals are fit with ground fault sensing shore power breakers is the day that boat may trip the shore power breaker, and will not be able to get shore power.  The dock attendant will tell the unhappy boat owner that “there is an electrical problem on your boat.”  The unhappy boat owner will think, “but it’s been working for many years!”  Both statements are correct.  It had worked for many years, but there is “an electrical problem on the boat!”

A fundamental rule of all electricity is, current will flow on all available paths to get back to it’s source.  If the neutrals from one AC circuit on the boat are cross-connected to the neutrals of another AC circuit on the boat, power will divide at the cross-connection (neutral buss) and flow back to the source via all available paths.  That situation is, by definition, a ground fault.

Following are two relevant and important excerpts from ABYC E-11, July, 2018: Isolation of Sources – Individual circuits shall not be capable of being energized by more than one source of electrical power at a time.  Each shore power inlet, generator, or inverter is considered a separate source of power. Transfer of Power – The transfer of power to a circuit from one source to another shall be made by a means that opens all current-carrying conductors, including neutrals, before closing the alternate source circuit, to maintain isolation of power sources.

Ordinarily we think of cross-connected neutrals as a situation that affects boats fit with two 120V shore power inlets; indeed, the neutrals from those two inlet circuits must not be cross-connected on the boat.  But more subtly, the separation requirement also applies to distribution circuits fed from generators and inverters.  UL-458 is the design standard that specifies that the needed neutral-to-ground bond in an inverter be “established” and “removed” based on operating mode.  If the inverter neutrals and non-inverter neutrals are cross-connected (as, for example, all sharing a common neutral buss on the boat), the terms of may not be met, resulting in a short ground fault condition.  In that case, there can be a duplicate path, if only momentarily, for shore power to use to return to the pedestal.  The following events happen in a fraction of a second.  Just “milliseconds (mS).”

At the instant (time=0.000) shore power is applied to the boat, any AC current that comes onto the boat via the hot conductor should also return to the pedestal on the shore power neutral conductor, and ONLY the neutral conductor.  Period!  Full stop!  Fundamental rule!

But…   At the instant shore power is applied to the boat (t=0.000), the inverter is in “Invert” mode with its internal neutral-to-ground bond still in place.  For the time it takes the inverter to respond to shore power and transfer its internal relay from “Invert” mode to “Standby” mode, there are two paths for the newly applied shore power to take to get back to its source at the pedestal.  The first path is via the shore power neutral, as intended.  But with unseparated neutrals, there is also a second effective (ground fault) return path.  The ground fault path starts at the neutral buss, where the returning current divides.  Some current will return as intended, on the shore power neutral conductor, but some will divert to the shore power cord’s ground conductor, through the inverter’s as yet unbroken neutral-to-ground connection.  That diversion path is a true ground fault.  One half of the total current will flow in each path.  The pedestal ground fault sensor expects the outgoing and returning currents to balance (within 30mA), but in this case, that sensor will see much less current returning on the neutral conductor than what was delivered on the hot conductor.   The pedestal breaker will want to trip.  How fast will it take for the trip to happen?  Usually between 30mS (t=0.030) and 50mS (t=0.050), but in all cases, less than 100mS (t=0.100), the maximum specified for the pedestal circuit breaker to trip.

In any case, we now have a “race” condition.  The race “contestants” are 1) the inverter relay against 2) the ground fault sensor.  The intent is for the inverter relay to “win.”  My inverter’s spec for transfer time is 18mS (t=0.018).  But, if the time it takes for the shore power Ground Fault sensor to trip is less than the time it takes the inverter’s relay to transfer into “Standby” mode, the pedestal breaker will indeed trip.  Furthermore, turning the inverter “off” will not eliminate that ground fault condition because the inverter’s internal relay would still be de-energized (“down”), and therefore, even with the inverter set “off,” its internal neutral-to-ground bond would still be present, creating the ground fault path.  Regardless, if the neutrals are separated, no cross-connection, so “no problem!”  So yes, it really is necessary to separate the branch circuit neutrals of the inverter-fed circuits from the neutrals of circuits that are not fed from the inverter. Elimination of the cross-connection of these neutrals is what eliminates the unintended, unwanted ground fault path.

Although I have not implemented an Inverter Bypass Switch aboard Sanctuary, I have drawn up a circuit diagram for such a switch, for those interested.  In Figure 8, the bypass switch is shown in the “Bypass” position.

When in “Bypass,” the switch’s external AC “power in” (red lines) comes from the hot and neutral lines that also feed external AC to the inverter.  Note that the “hot” feed for the bypass switch is upstream of the inverter’s power switch on the “Shore 1” AC panel.  This arrangement allows for bypassing the inverter while at the same time enabling a service technician to apply AC power to the inverter for diagnostic testing and repair verification.

When planning for the installation of an inverter, two pre-purchase considerations are, 1) what branch circuits will be powered from the inverter, and 2) what does the capacity of the inverter need to be in order to support the load of those circuits?  Aboard Sanctuary, we determined that we wanted to have AC power in the galley and at other utility outlets while underway.  That allows us to use our coffee maker, microwave, toaster and crockpot (not all at the same time), keep our DVR and AC lighting active, and occasionally charge utility batteries for my power tools.  We selected a 2kW inverter/charger to do that, which provides a maximum continuous AC output of 15A, shared by our four utility branch circuits.  That has served us well for 12 years.

Following is a “cut ‘n paste” from my “project plan” for the installation of our UL-458 compliant inverter/inverter-charger into Sanctuary’s DC and AC electrical systems, and timeframes based on my personal DIY-install timeline.  My need to “reconfigure” the B+ and B- DC busses on Sanctuary was because I consolidated the batteries from two separate banks (“house” and “start”) into a single bank at the same time, and updated the battery monitor from a stand-alone Xantrex monitor to a Magnum BMK. Combining banks greatly simplified battery charging from both the inverter/charger and the engine alternator.  Those steps are not specifically necessary for the inverter installation, but I like the consolidated battery bank.  Click to see my article describing that change.

Harmonic Distortion of AC Power

Initial post: 6/7/202
Minor edits: 6/8/2020

I’m posting this here because it came up on a boating club Forum that I follow.  As I have said often, my “target audience” is people in boating that do not have much prior background in matters of electricity.  This topic is a bit arcane, and does tend to be an advanced topic.  But at the same time, it does show up as a symptom that affects some boaters in some situations, so I offer it here for awareness.

Here is the question that started the discussion:

– – – – – – – – – – – – – – – – – – – QUOTE – – – – – – – – – – – – – – – – – – – –

“I would like to elicit opinions from the electrically minded of us regarding the following.  When running my NL 9Kw gen at anchor my Dometic/Cruisair heat pumps (240V, 16000 btu) work fine with just one of my Magnum Energy MS2812 (2800W, 125A charger) active to charge the batteries. But, when the 2nd charger is activated (now balanced loads on the gen legs), the heat pump compressors stop active function (no heating/cooling), fan drops to minimum level, but, amp load is unchanged. The above occurs whether 1 or all 3 Dometic units are running (this is not about trying to start one of the compressor motors with the gen loaded).  I have not noted this interference when the battery charging load is minimal.  The gen amp output at 100% is 37.5/240V.  Max charger demand is 17A both legs.  All 3 heat pumps together draw 13-14A. There is no problem if the water heater is run (240V/10A) with the heat pumps on and just one charger (brief test – 40A on one leg).

“It seems as though there must be some type of electrical interference that is occurring when the 2nd charger is added to the circuit affecting the heat pump compressor motor function. Any ideas as to what this might be and how it can be tested for? Emails were sent to NL and Dometic with no response. Thanks!”

– – – – – – – – – – – – – – – – – – – END QUOTE – – – – – – – – – – – – – – – – – – – –

Here is my response to this question, edited for completeness, which I offer to others who may be experiencing similar intermittent, “weird” symptoms:

– – – – – – – – – – – – – – – – – – – START – – – – – – – – – – – – – – – – – – – –

What you are describing sounds like a somewhat out-of-the-ordinary (but not “extraordinary”) problem called Harmonic Distortion.  Here’s the electrical theory of HD in four sentences: A pure resistance – water heater heating element, light bulb, running motor – draws current in linear proportion to its impedance (according to Ohm’s Law).  Electronic devices do not follow Ohm’s Law;  they can and do draw current in short bursts within the AC sine wave voltage cycle.  These electronic devices are called “non-linear” loads.  Since in non-linear loads, current does not follow Ohm’s Law against voltage, the apparent internal impedance of the source can cause the waveshape of the AC voltage to distort (dip, flatten at the top and bottom), rather than be or remain a pure sine wave, clean as the driven snow.

So in your situation, the inverters are AC loads being used for battery charging, but the battery charger’s internal DC circuits are non-linear, “switch-mode” devices.  That creates non-linear current demand on the input AC waveform that is reflected back into the source.  The system doesn’t fail on shore power because the apparent impedance of the shore power source is many, many, many times less than the apparent impedance of the genset.  That doesn’t mean the phenomena isn’t there on shore power.  It just means the source is big enough to overcome the magnitude of the non-linear load component.  On shore power, the ratio of load impedance to source impedance is sort of analogous to David-on-Goliath.   But with the much smaller capacity of the genset, the aggregate effect of the switch-mode current demand can affect the shape of the genset’s output voltage sine wave.  Here, the ratio of load impedance to source impedance is definitely David-on-David.  What tends to happen with Harmonic Distortion is that the positive and negative peaks of the AC sine wave flatten, although more complex distortion is possible in extreme cases, even to the point of approaching a square wave with a flat top and very low peak voltage.

You mentioned in your post that you have a 9kW NL genset.  Nine kilowatts is somewhat under-sized for a 250V, 50A boat.  The power that can be absorbed by a 240V, 50A load is 12000 Watts, or 12 kW.  What you have is NOT “bad” from the perspective of genset loading or the perspective that you rarely need the entire capacity of the generator anyway.  But, if what you have is a symptom related to Harmonic Distortion, the smaller genset will have a higher apparent impedance than a larger genset would have.  The higher the apparent impedance of the source, the more likely it is that Harmonic Distortion would present itself as a noticeable and annoying symptom.

My conjecture that this is Harmonic Distortion is easily confirmed with an oscilloscope.  In the old days, that was the only way to see it.  But today, you can confirm it easily it if you have a means to read TRUE RMS voltage and a means to measure the TRUE PEAK voltage.  The peak of a 60Hz sine wave should be 1.414 times the RMS value.  I use an Ideal SureTest 61-164 or 61-165 circuit tester for this task.

So let’s assume you have a stable 60Hz voltage at 118V when running on the genset.  And we must also assume you have a stable 60hZ frequency, ±2 hZ, when running on the generator.  Multiply the 118 x 1.414, and the peak of the voltage waveform should be 167V.  If you then measure the actual peak, and it’s – let’s say – 156V, then you know you have Harmonic Distortion taking place, and the wave form isn’t a pure sine wave.

Now, the tolerance of the inverter/charger(s), the SMX Controller electronics and the blower drive electronics of the heat pump to AC voltage waveform shape, for which they, themselves, are responsible for distorting in the first place, may not be favorable.  That is a vicious circle.  It’s creating something that it, itself, can’t live with.  Since the genset is also feeding the Dometic SMX heat pump control unit and the blower and compressor control electronics of the heat pumps, those circuit boards can also be impacted by distortion of the voltage waveform.  Symptoms across the onboard system can be unpredictable, and can vary from attachment to attachment.  Pure resistance loads will not be affected, but electronic devices can be to varying extents.

Harmonic Distortion and Power Factor are two of the most challenging problems power utility companies have to manage.  A distorted AC voltage sine wave waveform is called “dirty power,” and it costs utilities a lot of money to manage.  Buildings with banks of computers and servers cause huge HD problems on the power grid, often affecting their neighbors and neighborhood.  Virtually all electronic devices cause Harmonic Distortion, right down to the family flat screen TV and stereo.  Power quality is a huge problem at the level of commercial power utilities serving residential neighborhoods.

And by the way, from the perspective of the 9kW NL generator itself, the higher apparent impedance and distorted wave shape will cause additional heat in the windings of the genset.  That heat is not related to useful work done by the generated power.  It amounts to excessive waste heat of which the genset’s cooling system has to dispose.  This can be worse than having unbalanced 120V loads on each side of the genset.

The fix?  You’d need a bigger capacity generator; i.e., one with lesser internal impedance.  With a lower reflected impedance, the genset would maintain the shape of the waveform for equivalent non-linear loads.  Or, your can just choose to live with it…

I have not written about Harmonic Distortion or Power Factor for my website because it’s definitely not a beginner’s/layman’s topic.  (Well, I have now, haven’t I?)  And even if you have HD, there’s little that can be practically done.  But if you want to read more about HD, click here for a fairly readable and reasonably good explanation from Pacific Gas & Electric; and click here for a better explanation of non-linear loads.  Start on page 3, at the heading called “ELECTRICAL HARMONICS.”  Skip the math; you don’t need it to understand the concepts.

Hope this helps.  And of course, this is only a guess on my part…   Cough, cough, choke, choke…

I wish I could recommend something practical that would make this better, but in the current system configuration, I think it’s a permanent restriction.

– – – – – – – – – – – – – – – – – – –  END  – – – – – – – – – – – – – – – – – – – –

Understanding Harmonic Distortion is complex and it’s definitely an advanced problem in an electrical distribution system.  What I’ve written above is just the very tip of the the technical iceberg.  But, although relatively rare, HD can produce observable symptoms related to the performance of boat AC electrical attachments.  It can affect the quality of sound from an entertainment system or produce what looks like interference (snow, lines) on a TV.  And, it can affect the operation of other types of equipment, like network routers, DVRs and printers.  If you have these symptoms and all else has been ruled out, consider Harmonic Distortion as a possible cause.  If you have these symptoms, it will be necessary to call in a skilled professional electrical technician to troubleshoot and confirm the diagnosis.  The tools that are necessary are expensive, and the skills to appreciate and understand the causes are advanced.  This is not a job for a residential electrician.

Electrical Behavior of a 208V/240V Boat

This article discusses the electrical behavior of the two 120V AC circuits on a boat that is natively wired for 125V/250V, 50A shore power service.  Topics include current flow (Amps) in the different appliance loads, power limitations when connected through a “Smart Splitter,” and the constraints and limitations encountered with the use of certain shore power transformers when powered from 208V dock utility voltages.

Use Case 1: a boat wired with a 125V/250V, 50A shore power cord, but not fit with 240V appliance loads.

Figure 1 is a generic wiring diagram illustrating this use case.  The system includes a genset and a Galvanic Isolator.  In Figure 1, the dock power source is on the far left and the boat’s appliance loads are on the far right.  Dockside 50A circuit breakers are omitted for simplicity.  The 50A shore power cord is highlighted in the red oval.  One 120V load (the heat pump) is highlighted in red.  Other 120V loads (house loads) are shown in black.  This boat DOES NOT have 240V loads.  This use case is a very common “50A” boat configuration.

Use Case 2: a boat wired with a 125V/250V, 50A shore power cord adapted to two 120V, 30A pedestal outlets to obtain limited 208V/240V power.

Figure 2 is a generic wiring diagram illustrating this use case.  Most commonly, a “Smart Wye” splitter adapter is used (ref: Appendix 1).  A “Smart Wye” splitter has two 30A twistlock plugs (NEMA L5-30P) and one 50A receptacle (NEMA SS2).  The two 30A receptacles (NEMA L5-30R) are on the dock pedestal.  The splitter and the 3-pole, 4-wire, 50A power cord are shown in the red ovals.  The rest of this system is identical to Figure 1.

Figure 3 applies to both Use Case 1 and Use Case 2 configurations.  Figure 3 shows logical blocks instead of actual circuit detail in order to make it easier to visualize the electrical behavior in this AC system.  In Figure 3, incoming power is shown as being derived from “any suitable 240V source.”  Electrically, we really don’t care how we get shore power as long as it’s “3-pole, 4-wire” of the right voltages.  In Figures 1 and 2, the loads were shown as they are wired, but Figure 3 shows them as they are logically arranged in the overall electrical circuit.  As the drawing shows, the red-highlighted 125V, L2 heat pump load is connected in series with the black-highlighted 125V, L1 appliance loads.  These two load groups share a common “Neutral” conductor.  The Neutral conductor anchors and maintains the midpoint voltage of the series connection under varying demand conditions.

Visualizing this electrical configuration in the mind’s eye as two 120V loads connected in series across a 240V source is the first key concept in this article.

Having identified the electrical arrangement of the two 120V appliance load groups of this 240V system, further analysis is on a) the voltages present, b) current flows, and c) power available to do work.

Figure 4 shows the two series load components of this boat’s 240V boat system, each with 120V across them.  The L2 load group is comprised of the boat’s heat pump(s) and raw water circulator.  The L1 load group is comprised of the hot water heater, fridge, battery charger(s) and multiple utility outlets.  Measuring across the L2 load between points A and B, there are 120V.  Measuring across the L1 load between points B and C, there are 120V.  The series pair receive the 240V mains supply voltage measured between points A and C.

Next, consider the electrical currents (measured in Amps) flowing through the two series load groups in a variety of specific but different load circumstances.  Understand that in the following analyses, different specific devices are “on” and others are “off” at any specific point in time.  Assume the following scenario: the boat’s owners have been away from their boat for a mid-summer week.  Upon late day arrival at the boat, outside air temperatures are in the mid-to-high 80s with 85% relative humidity.  Our boat owners will turn on some space lighting, and will immediately turn on the heat pump for air conditioning.  They will turn on the hot water heater and battery charger, stow fresh veggies, ice cream and adult beverages into the fridge, and perhaps turn on the DVR/TV.

Electrically, assume the heat pump draws 20A.  Also assume that house loads (hot water heater, battery charger, fridge, space lighting, computers and DVR/TV) add up to drawing 20A.

In Figure 5, the heavy red line represents this 20A flow of current (Amps).  This example is a special case called a “balanced-load” condition; that is, both of the 120V loads just happen to draw the same amount of current (20A).  The Amps flow from the dock pedestal into the loads on one of the energized line legs (L1), and flow back to the pedestal on the other energized line leg (L2).  In this balanced-load condition, no current flows in the neutral conductor (N).

Very importantly, notice that no more than 20A is flowing anywhere in this system. A double-pole 30A circuit breaker that serves the boat via a Smart Splitter at the dock pedestal sees 20A on both legs, L1 and L2.  Since there is no place in the system carrying more than 20A, the 30A pedestal circuit breaker is perfectly happy.  The second extremely key concept to take from this article is that the 20A flowing to power the heat pump circuit is the same 20A that flows through the House circuits to power the water heater, battery charger, fridge and utility outlets.

The word “power” is highlighted above to make the point that the same 20A flowing in the two 120V loads does useful work in both 120V load groups.  The basic formula for “Power” is P = Volts x Amps.  So in the heat pump load group, we have 120V * 20A = 2400 Watts.  In the house appliance load group, we also have 120V * 20A = 2400 Watts of power doing useful work.  In total, we have 4800 Watts of work being done at this time, in this system.

Up to 30A is available from a 30A shore power pedestal without exceeding the capacity of the circuit breakers.  The maximum power possible for each load is 120 * 30 = 3600 Watts.  Because the two load groups are in series, the maximum work that can be done by 30A, in total, is 7200 Watts.  If the boat had access to its design maximum of 125V/240V, 50A shore power, there would be the potential for 240 * 50 = 12000 Watts, total.  It quickly becomes clear why careful load management is necessary when running with two 30A cords feeding a 50A boat through a 30A Smart Splitter.

Following from our earlier scenario, after an hour or so, the hot water heater has done its water heating work, the fridge has done its cooling/freezing work, and the batteries are fully charged.  But, the heat pumps are still running to cool the boat.  Now, although we have 20A flowing in the heat pump load, current on the house side has dropped to 4A for the DVR/TV and space lighting.  Figure 6 shows what happens electrically.

The heavy red line represents the 20A needed by the heat pump.  But this time, there are only 4A needed by the house, represented by the thin red line continuing through the House circuit.  There is no longer a balanced-load.  The arithmetic difference between the heat pump demand and the house demand is 16A.  That 16A returns to the pedestal in the system’s neutral (N) conductor.  In this example, as before, there are 120 * 20 = 2400 Watts of work being done in the heat pump load group, and 120 * 4 = 480 Watts of work being done in the House load group.  There are never more than 20A flowing in any part of this system.  Neither the shore power pedestal breakers nor the Neutral conductor are overloaded.  All is safe and well within specifications.

At the end of the evening, when our sample boaters retire to bed, assume they turn off all of the house loads.  The hot water heater is satisfied, the battery charger is satisfied, the fridge is satisfied, the TV is “off,” the laptop and iGadget batteries are charged (and the screens have gone “dark”), and the space lighting is “off.”  Now, there is no current at all flowing in the House loads.  Ah, yes, but the air conditioning is still needed.

Figure 7 represents the electrical status in this case.  Since the heat pumps are still running, there are 20A flowing in the heat pump circuit.  Since there is nothing “on” in the House load group, the arithmetic difference is 20 amps, which returns on the neutral (N) conductor.  Again, no part of the circuit carries more than a total of 20A.




Use Case 3: a boat wired with a 125V/250V, 50A shore power cord, but fit with 240V appliance loads aboard.

Figure 8 shows the addition of pure 240V loads at the far right of the drawing.  Boats with 125V/250V, 50A shore power service which have both 120V and 240V appliance loads (hot water heater, cooktop, electric dryer, heat pump compressor) are electrically very similar to those without 240V appliances.  Very few “240V appliances” are “pure” 240V devices.  The only ones that come to mind are 2-pole, 240V deep well pumps and 2-pole, 240V hot water heaters.  Appliances like heat pumps, cook tops, ovens, clothes dryers and watermakers, are usually “hybrid devices;” ie, they need both 120V and 240V to operate.  The control circuits in hybrid appliances are generally 120V circuits.  In a dryer, for example, the heating elements are 240V but the motor that turns the drum and the clock timer circuit both require 120V.  Hot water heaters can be pure 240V-only loads which do not need or have a neutral conductor.

In Figure 8, the pure 240V appliance loads are electrically in parallel with the two 120V series loads, and the 240V loads add to the amps drawn in the 120V supply mains, L1 and L2.  So, if we had the 20A L2 load running a 120V heat pump, as has been the example throughout this article, and in addition, a 240V hot water heater simultaneously calling for 12A, the result would be a 32A total Amps in L2.  Attached to a 50A pedestal, all would be OK, but attached to a 30A splitter, the result would be a tripped 30A pedestal circuit breaker.  So again for emphasis, it is up to the boat owner/operator to understand load management and ensure that pedestal breaker capacity is not exceeded.

Potential Power Issues with Certain Shore Power Transformers

The utility power on docks can originate from two kinds of public utility sources.  “Single phase” sources will appear as conventional 120V/240V.  “Three phase” sources will appear as 120V/208V.  Because this electrical fact is a well-understood, and very common in boating, UL Marine certified electrical appliances are designed to accommodate the difference between 240V and 208V.  Residential appliances MAY NOT have have that same flexibility.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for  125V/250V, 50A applications   are manufactured in three “flavors:”

  1. Basic, single input, single output, 240V transformer; least expensive flavor.
  2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input to achieve a constant 240V output.
  3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.   While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.   Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”   The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.    The output of this transformer (the secondary) provides single phase, 3-pole, 4-wire power to the boat. In English, that means there is a conventional black, red, white and green output.    If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.   But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances.   With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances.  Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.  Microwaves will run but will take slightly longer to cook.   Coffee pots will perk, but will take slightly longer to perk.    Electric blankets will keep sleepers warm and cozy.   Water Heaters will heat water, but take slightly longer to reach target temperature.   Stovetop burners will heat, but not get as hot at the same setting.  Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage.  Marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.   One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V should be designed to detect the condition, put up a power warning fault light, and self-disconnect.    Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer marine heat pump designs do that.

WARNING:  if there is a 240V shore power supply voltage applied to a manual transformer set to a 208V input voltage, then the AC voltages aboard can get high enough to “damage” appliances.

Article Summary:

  1. When operating a 125V/250V, 50A boat which does not have 240V loads, total loads of up to 2 * 3600 Watts can be supported with two conventional 30A pedestal outlets.  In this case, neither the energized (hot) conductors nor the Neutral conductor are ever overloaded.  No individual circuit conductor ever conducts more than 30A.
  2. When operating a boat with pure 240V loads, the Amps required by the 240V loads add to the Amps needed in the 120V loads.  The owner/operator must monitor total amps drawn/power used to keep total power consumed below 3600 Watts per side.
  3. Some shore power circuit breakers are housed in inaccessible, locked locations ashore.  If a boater accidentally trips a shore power circuit breaker, particularly after hours, it may not be possible to gain access to it in order to reset it
  4. It is necessary for boat owner’s to closely monitor power usage and limit the amount of  current used to prevent tripping shore power circuit breakers.  Care must be exercised to not run high amp draw appliances (coffee pots, microwave ovens, inductive cookware, hair dryers, clothes washer/dryers and similar devices) at the same time.  Boats with multiple heat pumps will probably be unable to run all of them at the same time on 30A services.
  5. The examples in this article assume that the heat pump circuit is on one 30A load leg and house loads are on the other leg.  Obviously, some boats are wired differently. Systems with heat pumps and house loads distributed across both incoming energized 120V legs will have to monitor loads and current draws in the same manner, but the electrical principles discussed above remain the same.
  6. The specific balance of currents in the load one group and the load two group changes constantly.  L1, L2 and Neutral current (Amps) never exceeds 30A.

Appendix 1:

To the right is the electrical diagram of a typical “Smart Wye” splitter.  This Figure represents the electrical circuit detail of the splitter shown in Figure 2 in the earlier text.  Note that the splitter contains a relay – labeled “K” in the drawing.  The relay requires 208V or 240V to close.  Without at least 208V, the relay will not close and the splitter will not pass any power through to the boat.

Following is a link to my article describing Smart Splitters, and the receptacles required for their successful operation.

AC Electricity Fundamentals – Part 2: The Boat AC Electric System

Article posted: April 20, 2019
Added content: Shore Power Transformers; July 22, 2019
Added content: DC Electric Circuit “Wire” Naming and Identification; October 6, 2020;
Minor edits: October 6, 2020
Minor edits: February 16, 2021

Updated Diagrams: April 18, 2021

About this article

The AC Electricity Fundamentals – Part 1 article precedes this article and discusses 1) the concepts, terminology, components and layout of National Power Grid generating equipment, 2) delivery of AC power into residential neighborhoods, and 3) the configuration of AC electrical systems within a residential building, all at an introductory level. Boats attach to marina shore power systems.  The shore power system is the “host system,” and boats attach as “appliances.” An understanding of residential AC power systems is foundational to an understanding of AC power systems on boats.

The Part 1 article concluded by showing that the AC shore power system on boats is equivalent to a sub-panel in a terrestrial building. In the NEC architecture of terrestrial AC building systems, sub-panels are subordinate to the main service entrance panel of the building. In the same way, boats are subordinate to the shore power AC electrical infrastructure of a terrestrial facility.

This article focuses on the overall AC electric “platform” aboard cruising boats. On boats, shore power is only one component of a typical AC system “platform,” which can also include a mix of onboard generator(s), inverter(s) and in some cases, shore power transformer(s). This introductory Part 2 article will answer some questions, and will undoubtedly raise others. The goal of this article is to help readers understand AC electrical concepts and topics to be able to discuss questions, concerns, symptoms and options with marine-certified, professional electrical technicians.

Personal Safety

Virtually all electricity can be dangerous to property and life. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard. This is especially true of inverter-chargers. The large batteries found on boats can produce explosive gasses and store enough energy to start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! Anyone working in noisy environments, with running engines or other loud machinery, MUST WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!


Electrocution is a biological insult arising from an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death. Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electric shock or electrocution, in any locale around boats or water, there are several things that need to be done immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM! You can not save someone else if you panic!
  3. SCREAM FOR HELP! ATTRACT ATTENTION! Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again! Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate. CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.

Boat Electrical System – Scope

Viewing the shore power AC system of a boat as a residential sub-panel in a single family residence is simple and technically accurate, but the AC electrical system of a typical cruising boat is more than just shore power. A simple block diagram of a boat electrical platform can show the relationships among the various components of the boat’s AC (and DC) electrical systems. Sanctuary’s energy flow diagram is shown in Figure 1. This diagram shows Sanctuary as she is today. When we bought her, she did not have a genset, she did not have an inverter-charger, and she was fit with two inappropriate battery banks. She was less complex yet poorly designed for our intended use.

Figure 1

Figure 1 shows Sanctuary’s AC and DC electrical systems as a complete and integrated operational “platform.” From the platform perspective, owners can evaluate the impacts of contemplated alterations and upgrades. The diagram shows energy flow, not wiring detail.   Its simplicity allows one to visualize and understand both individual components and how components feed and are fed by one another. It is all too easy to add, remove and change system components without fully appreciating the impact(s) to the overall host electrical platform.

Sanctuary’s OEM factory configuration consisted of two 8D batteries, one dedicated to engine starting, and the other dedicated to modest OEM space and navigation lighting loads. An 8D was excessive and poorly utilized for engine starting, and inadequate for our house needs. The energy flow diagram gave us the ability to visualize the impacts of consolidating the two separate battery banks into a single bank.

Sanctuary’s OEM factory battery charger was an obsolete technology single-stage unit. We wanted AC power aboard without having to run our genset. We decided to change the battery charger to a fully automatic inverter-charger. This was a major upgrade that affected both the AC and DC electrical systems aboard. Mechanically, the upgrade was simple, but modification of branch circuit wiring to comply with the ABYC electrical standard was a big impact to our host AC electrical system.

Adding a new genset to a boat includes adding a Generator Transfer Switch and reworking the distribution wiring of the existing shore power circuits. Replacing an old-iron 60Hz AC genset with a new 60Hz AC genset would be relatively easy and non-disruptive. Converting to a DC genset (a diesel-driven DC battery charger) has very different implications. Cost, technical complexity and value of the alternatives can be compared and evaluated.

The energy flow diagram shows that Sanctuary is now fit with a single battery bank for both “engine start” and “house” support. Adding a wind generator or adding solar panels are energy management solutions that would have technical impacts to the existing system. Each can be evaluated from the perspective of our energy flow diagram. Boat owners are strongly encouraged to take a “platform view” as opposed to a “component view” of the electrical systems on their boats.

High Complexity Aboard Boats – Power Sources

Screen Shot 2021-04-18 at 09.21.04

What is immediately clear from Sanctuary’s energy flow diagram is that there are three entirely independent AC sources that can feed power to our onboard AC loads:
1. shore power (source ashore),
2. generator (source aboard),
3. inverter, (source aboard) and on some boats,
4. shore power transformers (source aboard) (not installed aboard Sanctuary.

DC Electric Circuit “Wire” Naming and Identification

In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the ABYC color code for DC wiring was changed to yellow to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (often) not technically correct, since “ground” wires are not intended to carry current. 

Key points from the Part 1 article to keep in mind on boats: In AC circuits, the conductors are alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work.  In single phase 120V circuits in North America, the two conductors are named for on their role in the circuit.  The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor.  By code and convention in North America, “L1” is black in color.  The other conductor in a 120V circuit is the return conductor.  It is called the “Grounded Conductor,” or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.  

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.”  They are commonly called “Line 1” and “Line 2.”  “L1” is black, and “L2” is red.  In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

Subtle take-away: the DC “negative” conductor has the same role in a DC circuit the the AC “neutral” conductor has in a residential/boat AC circuit.  That is, the DC “Negative” conductor returns current to the source.  In a “grounded DC electrical system” (very rare) the B- conductor is a “Current-Carrying”, “Grounded Conductor.”

Note: on some but not all boats built overseas, AC wire colors may be different than the ABYC Standard colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white.  If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine what the colors mean to ensure ongoing equipment, fire and personal safety.  

Key Electrical Concepts For AC Services aboard Boats

Key points from the Part 1 article to keep in mind on boats:

  1. “Shore power” arises from the electrical system of a terrestrial facility, ashore, while AC power from a “generator,” “inverter,” and/or “shore power transformer” arises from equipment installed aboard the boat.
  2. The residential AC power standard in North America is a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system. This definition broadly applies to all terrestrial buildings with which people interact, and includes boats.
  3. State/Province, county and municipal jurisdictions across North America adopt local statutes and codes-of-regulations that originate with the NEC/CSA to govern terrestrial building electrical installations.
  4. There are no statutory electrical codes for boats. The American Boat and Yacht Council (ABYC) provides voluntary standards to boat builders. ABYC electrical standards are fully compatible with NEC shore power, assuring safe, reliable inter-operability between terrestrial and boat-resident AC systems.

As discussed in the Part 1 article, an essential safety requirement of all of these standards and codes is that single phase electrical systems be “grounded” at their “derived source.” This brings us face-to-face with some core ABYC “recommendations” that govern switching of AC wiring for equipment installations on boats:

  1. only one source is allowed to power loads aboard boats at any one time,
  2. sources must be thoroughly and completely isolated from one another,
  3. a “grounded neutral system” is required:
    • when on shore power, the neutral-to-ground connection is provided to the boat through the shore power cord, (i.e., the neutral-to-ground connection is in the shore power infrastructure), and
    • when on generator or inverter power, or when shore power is received through an onboard shore power transformer, the neutral-to-ground connection is made at the onboard source.

High Complexity Aboard Boats – Ground

In a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system, what does “grounded neutral” mean? Recall in the residential AC system model that three conductors arise from the utility power transformer at the street; two energized lines (“L1” and “L2”) and one neutral line (“N”). As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V between “L1” and “N” and between “L2” and “N,” but these voltages “float” with respect to their external environmental surroundings (recall the discussion of birds and squirrels on wires from Part 1). This situation is referred to as a “floating neutral.” To create a safe, known zero-volt system reference, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. The result is an earth-ground “grounded neutral” system.  The ground rod(s) reference the building neutral conductor to a very large mass at a zero-volt electrical potential (the Earth).  This Earth Ground IS NOT the same as “circuit ground.”  It would be the exception for a direct connection between L1 or L2 and the building’s ground rods to cause a circuit breaker to trip.  That is the function of “Equipment Bonding Conductors,” NOT the ground connection.  See my article on “Earthing and Grounding” on this website for more detail.

Grounding the neutral is very straight-forward at buildings. Since there is only one place where utility power enters the building from the utility company’s electric meter, it’s easy to understand and visualize that entrance location as the “derived source” of the power. Electrician’s working in terrestrial buildings learn to mix neutral and safety ground conductors on the same buss bars in the main service panel. In one of the examples I showed in the Part 1 article, we saw that some main service panels are built with only one buss bar which serves to collect both neutrals and grounds.

Boats are different!  In the architecture of the North American power framework, boats are sub-panels to the shore power infrastructure, not main service panels. Furthermore, it is common to have more than one AC power source for the AC system platform on a boat, including as we saw in Figure 2, AC Shore Power connections, onboard generators, inverters or inverter-chargers, and maybe shore power transformers (isolation transformer, polarization transformer). All of these sources are AC “derived sources” within the definitions of the ABYC Electrical Standard, E-11.

The NEC requires the neutral-to-ground bond to be at the “newly derived source” of the terrestrial shore power system. The ABYC electrical standard complements and supports the NEC requirement for boats operating on shore power. For boats operating on shore power, neutral-to-ground connections are not permitted aboard the boat. Why? Follow this scenario:

  1. Start with the NEC-required neutral-to-ground bond being correctly installed at the terrestrial facility’s main service panel (derived source) .
  2. The shore power neutral-to-ground bond is carried aboard the boat via the shore power cord, per ABYC E-11,
  3. The intent of the safety ground is to provide a low resistance electrical path to disconnect power as close as possible to the source in an electrical emergency:
    • in a normal AC system, no power flows in the safety ground conductor,
    • but in the case of an electrical fault, current flows in the safety ground for the purpose of removing power (fault removal) by tripping the circuit breaker that feeds the faulting circuit),
  4. Because there is a neutral-to-ground bond in the shore power main service panel, if there were also a neutral-to-ground bond aboard the boat, the neutral and ground conductors between the shore infrastructure and the boat would be electrically in parallel with each other, enabling power to flow in the safety ground (by definition, a ground fault). This results in two issues for boaters:
    • constantly trips a dockside ground fault sensing circuit breaker, and
    • the AC safety ground would, itself, be energized, thus providing a path to the underwater metals of the boat, thus enabling AC power to escape the boat’s electrical system into the water.
  5. The above consequences of paralleling the neutral and the safety ground pose a shock and electrocution threat to people, pets and wildlife in the water.

So, now we understand why a neutral-to-ground bond is not permitted aboard the boat when connected to shore power. But, we also know that ABYC does require a neutral-to-ground bond for onboard generators, inverters operating in “invert” mode and shore power transformers; that is, ABYC requires a grounded-neutral AC system throughout the boat regardless of the source of AC power.

Summarizing the above:  Shore power can’t have a neutral-to-ground bond aboard the boat, but generators and inverters must have neutral-to-ground bonds at the respective equipment aboard the boat. Isn’t this an irreconcilable “Catch-22?” In a word, “no!” It is a complex wiring situation that does not occur in terrestrial buildings where only one power source is present. (It does apply in terrestrial buildings if an outdoor emergency generator installed, and it also occurs in terrestrial off-grid solar applications.)

The technical solution that allows compliance with these apparently self-contradictory ABYC configuration requirements involves complex switching solutions. When connected to shore power, onboard neutral-to-ground bond connections must be “switched out.” When running on an onboard generator or an inverter in “invert” mode, the neutral-to-ground bond connection must be “switched in.”

High Complexity Aboard Boats – Switching

Marine-certified AC disconnect circuit breakers are readily available in a variety of form factors to fit different power panels of different companies found on different boats. With 120VAC, 30A inlet circuits, “Double Pole” breakers disconnect the “L1” and “N” lines. With 240VAC, 50A inlet circuits, “Double Pole” breakers disconnect “L1 and “L2,” but not “N”. It is up to the installing electrical technician to ensure that the correct disconnect breakers are used in the correct application to maintain compliance with the ABYC electrical standard and compatibility with the shore power infrastructure.

Looking at Sanctuary’s energy flow diagram, Figure 1, we can see that the boat’s Generator Transfer Switch (GTS) is used to transfer the “load” (the “load” in this case is the boat’s entire AC electrical system) between one of two AC power sources (either shore power or the onboard generator). The GTS must be constructed in a way that it simultaneously transfers the load’s “hot” lines (“L1 and L2”) and the load’s “neutrals” “N.” Figure 3 shows the electrical diagram of Sanctuary’s physical GTS. “Source 1” and “Source 2” are our 120V, 30A shore power inlets. “Source 3” is our 240V, 50A generator input (happens to be the way our generator is configured). In order to comply with the neutral-to-ground bonding requirements of the NEC and ABYC, the GTS is built to switch the neutrals as well as the “hot” lines. In this way, the required neutral-to-ground bond can be installed at the generator, aboard the boat, and the entire platform remains compliant with the ABYC electrical standard and compatible with the NEC for shore power.

About Shore Power Transformers

Shore power transformers are expensive, large, heavy and require significant physical space surrounded by free-flowing air for ventilation. These transformers can suppress spikes and electrical noise from entering the boat from the shore power grid. Some transformer designs can automatically compensate for “low” dock voltage (shore power “brownout,” normal 208VAC). There are two shore power transformer wiring configurations: an “isolation configuration” and a “polarization configuration.” In both cases, the transformer is installed aboard the boat. The secondary winding of the shore power transformer is defined to be the “derived source” of AC power aboard the boat.

For a 30A, 120V isolation transformer, the primary requires a double pole breaker, preferably fit with ELCI, which breaks both “L1” and the neutral, “N.” The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V branch circuits.

For a 50A, 240V isolation transformer, the “L1” and “L2” hot lines are brought aboard through a double pole disconnect breaker, preferably fit with ELCI. The pedestal neutral, N, is not brought aboard at all. The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V/240V branch circuits.

The difference between “isolation” and “polarization” is the wiring configuration of the safety ground. With isolation transformers, the safety ground in the shore power cord terminates at a shield in the transformer. With polarization transformers, the safety ground of the shore power cord is connected to the boat’s safety ground buss, and is brought back to the shore power pedestal. With a polarization transformer, it is best practice to also install a Galvanic Isolator in the safety ground wire.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for 125V/250V, 50A and 125V/250V, 100A applications  are manufactured in three “flavors:”

1. Basic, single input, single output, 240V transformer; least expensive flavor.
2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input for 240V output.
3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.  While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.  Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”  The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.   The output of this transformer (the secondary) produces single phase, 3-pole, 4-wire output which powers the boat.   In English, that means there is a conventional black, red, white and green output.   If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.  But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances. With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a “low” utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances. Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.   Microwaves will run but will take longer to cook.  Coffee pots will perk, but will take longer to do their thing.   Electric blankets will keep sleepers warm and cozy.  Water Heaters will heat water, but take longer to reach target temperature.   Stovetop burners will heat, but will take longer to get as hot. Heat pump compressors and fans should all run, but some “non-marinized” motors may overheat and cut out to protect themselves from damage; marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.  One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V “low voltage” should be designed to detect the condition, put up a power warning fault light, and self-disconnect.   Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer 120V marine heat pumps do that.

About Generators

An AC generator is a mechanical machine consisting of a propulsion engine that drives an alternator. The machine must spin at a constant rotational speed to maintain the 60Hz output frequency. The waveform from a rotating genset is a pure sine wave. Although gensets are rarely actually run at their full load capacity, AC gensets must be rated for the largest electrical load they will ever have to support. Mechanical speed controls in these machines add to the requirement for a relatively great deal of preventive and corrective maintenance. Replacement parts are expensive and heavy. An AC generator can power all normal household appliances including heat pumps. Considering capital expense and lifetime fuel and maintenance costs, AC gensets are inherently expensive, per kW-h, to produce AC electricity on a boat.

A DC generator can be a practical alternative to an AC genset for most cruising boats.    DC gensets such as those made by Alten®, Hamilton-Ferris®, PolarPower® and ZRD® are essentially used aboard as “motor-driven battery chargers.”  The AC power used aboard the boat arises from the battery bank via inverter(s).  Multiple smaller inverters can provide for staged comfort and convenience options as well as systems redundancy.   Because batteries can supplement total power demand (Kirchhoff’s Laws), DC gensets do not have to be rated for max demand, as do AC gensets.  When onboard loads are light, the DC genset provides enough energy to power both the inverter(s) (for conversion to AC) and the battery bank (for battery charging).  When demand exceeds the generator output capacity, the batteries themselves make up the difference.  This means DC gensets can be of smaller capacity and can adjust to light loads more efficiently than AC gensets. Since DC gensets charge batteries, they do not need to spin at a regulated speed and are mechanically less complex.  AC generators are sensitive to rotational speed to keep the AC output frequency at 60Hz, +/- two Hz.  The DC machine has no such restriction, and so are much more fuel efficient. Boaters faced with installing a net new genset or replacing an old genset would do well to consider the DC genset option.

High Complexity Aboard Boats – Inverter

From the perspective of “electrical standards,” boats are a sub-class of a larger category of “mobile platforms.” Inverter and inverter-charger devices can be installed in many types of mobile platforms, including cars, trucks, ambulances, emergency vehicles, RVs and airplanes. All classes of “mobile platform” have identical shore power interface compatibility requirements, and very similar user safety requirements. Inverters installed in host AC systems on boats carry significant complexity.

About Inverter-Chargers

An “inverter-charger” is an electronic device that converts DC from batteries into 120V/240V, 50Hz/60Hz AC and ALSO uses 120V/240VAC, when available from external sources, to re-charge battery banks. The shape of the AC waveform from inverters can be a “modified sine wave” (MSW) or a “pure sine wave” (PSW). PSW devices dominate in the marketplace in 2019, and since some electronic appliances do not tolerate MSW well, are to be preferred aboard boats.

There are two installation use cases that apply to any discussion of inverter or inverter-charger installations on boats.

Use case one:  consists of a stand-alone inverter that powers dedicated AC utility outlets that are separate and apart from the wiring and outlets of the host boat’s main AC electrical system. To have AC power at those outlets, the inverter must be turned “on.” When the Inverter is turned “off,” AC power is “off.” The AC wiring attached to this inverter would be expected to comply to the normal requirements for all AC wiring aboard. There is no automatic power transfer switching. Ideally, an inverter used in this way would feed a distribution panel that would provide overload protection to branch circuit wiring. The manual nature of this use case is not considered “desirable” by boat designers and builders. Specific standards for this use case are not enumerated in the ABYC E-11 standard, AC and DC Electrical Systems on Boats.

Use case two: an inverter or inverter/charger that is fully integrated into, and functions as a part of, the host boat’s AC electrical system.   There are no separate or isolated utility outlets. All powered utility outlets are overload-protected by the host system’s branch circuit distribution panel. Branch circuit utility outlets and appliances either 1) receive externally-provided AC power “passed through” the inverter or 2) receive AC power from the inverter via the energy stored in the boat’s batteries.   The inverter senses loss of external power automatically, and switching from external power to battery power is likewise automatic. User safety and convenience is maximized. This use case is covered in detail by ABYC E-11, AC and DC Electrical Systems on Boats, and ABYC A-31, Battery Chargers and Inverters. ABYC specifies device compliance with UL458 to maintain compatibility with neutral-to-ground switching aboard the boat.
As shown in Figure 4, when either shore power or generator power is available, the inverter automatically switches to “standby mode.” In “standby mode,” the internal transfer relay is energized by the external power source. The internal transfer relay has two functions. One is to pass the external power through the inverter (“passthru”) to the boat’s power distribution panel (red arrow), and the other is to simultaneously remove the device’s internal neutral-to-ground bond (red oval). This second function maintains compliance with the ABYC neutral-to-ground bonding requirements for shore power.Later, when external power is no longer present, the device automatically switches from “standby mode” to “invert mode.” As shown in Figure 5, the internal transfer relay drops, and the inverter begins to generate AC power by drawing energy from the boat’s batteries (red arrow). When the transfer relay drops, it simultaneously establishes the required neutral-to-ground bond (red oval). Since the inverter in “invert mode” is now the “derived source” of AC power, grounding the neutral via the internal relay maintains compliance with the ABYC electrical standard, E-11.

Inverters – Installation Impacts

Referring again to Figure 1, the “energy flow diagram” for Sanctuary, it is apparent that the 120V feed of branch circuits 1 – 3 and 4 are powered from either shore power or generator power through the Generator Transfer Switch. When either shore power or generator power is present, the inverter operates in “standby mode,” and AC for branch circuits 5 – 8 “passes through” the power transfer relay of the inverter-charger to feed AC from the respective source to those circuits. When the boat is under way, and an external AC power source is not present, the inverter switches to “invert mode.” In that case, branch circuits 5 – 8 are powered by the inverter-charger.

What is not obvious in the energy flow diagram is that, because the “hot” lines for circuits 5 – 8 originate at the inverter, the neutrals for circuits 5-8 must be separated from the neutrals of circuits 1-4. This is a manufacturer’s installation requirement for the inverter-charger device which has its origins in ABYC Standard, A-31, Battery Chargers and Inverters.

Inverters – Advanced Feature(s)

In 2019 in worldwide boating markets, Victron Energy B.V.® manufactures a series of inverter-chargers carrying the MultiPlus™ and Quattro™ brand names that have an advanced feature Victron® calls “Power Assist.” With this feature, the inverter is capable of “piggybacking” on top of a limited shore power source to boost the total amount of power available to power loads aboard the boat. Batteries are charged during periods of low demand, and support the inverter during periods of higher demand. Across a day of use, users must monitor the system to assure batteries are adequately charged.

A typical “Power Assist” scenario: assume a boat fit with one of these inverters visits a private residential dock, a public wall, or any similar location where only very limited AC shore power is available from a single 125V, 15A/20A residential outlet. Generally, 15A is not sufficient for powering boat loads by itself. That said, if the demand on the 15A circuit can be held below a level that causes the shore power overload circuit breaker to trip, convenience aboard the boat can be enhanced by the “Power Assist” feature. To ensure the inverter does not trip the shore power circuit breaker, assume the inverter’s shore power “Maximum Current” setting is 12A. As long as the “passthru” loads on the boat are less than 12A, the power for those loads comes entirely from the shore power outlet.  It is during these periods of light AC loads aboard the boat that house batteries are charged.

Later there may come a time that the load on the boat jumps up, perhaps because of a toaster, coffee pot, microwave or hair dryer. Assume that at some point the total AC load aboard the boat rises to – pick a number – 22A. Since the inverter-charger is limited to drawing 12A from the shore power outlet, the inverter itself jumps in to “assist” the shore power source with energy drawn from the boat’s batteries. The inverter will sync with the shore power sine wave, and 10A will be provided from the batteries by the inverter. Keep in mind that the inverter is designed to provide this assistance automatically, by monitoring passthru load and automatically jumping in to supplement loads that exceed the pre-set.

Functionally, the above is how the Victron® Power Assist feature works, and it has much user convenience appeal to boaters. However, there may also be an operational downside with the “Power Assist” feature. When this equipment attaches to the Electric Power Grid, it synchronizes it’s 60Hz power waveform with the power on the grid. Emerging experience suggests the synchronization process can cause out-of-phase currents that may trip dockside ground fault sensors.  Owners of these devices should be alert to nuisance trips when connecting to docks with ground fault sensors on pedestals.

Inverters without the “Power Assist” feature have an obvious “one-way” relationship with the Electric Power Grid; that is, they are loads that take power from the grid. Inverters with the “Power Assist” feature are electrically paralleled to the incoming shore power connection and can have a two-way interface with the incoming AC power grid. These two-way inverters are capable of delivering AC power backwards into the electric power grid to which they are attached. The ability to feed power backwards into the grid carries significant safety implications in certain fault scenarios.

“Distributed Energy Resources” (DERs) are AC electricity generating units, typically in the range of 3 kW to 50 mW, that are deployed across the power grid. DERs are installed close to loads, often on customer premises, often on the load side of the customer’s electric meter. DERs are designed to alternately draw power from and return power to the upstream hosting electrical power grid. Worldwide, DERs are a central concept to distributed solar and wind farm (“green energy”) production and to pumped-storage reservoir systems. DER technologies include 25kW to 500kW micro-turbines, 25kW to 250mW combustion turbines, 5kW to 7mW internal combustion engines, 1kW to 25kW Stirling engines, fuel cells, battery-based UPS systems, photovoltaic systems, and wind generation systems.

In the US, the NEC, state Public Utility Commissions, code enforcement Authorities Having Jurisdiction (AHJ), and the ABYC, have all recognized the safety implications related to DERs. While it would be rare – in 2019 – for power generated on a boat to be fed back into the local electric power grid, with a DER-capable inverter, it is possible. The “Power Assist” capability enhances living convenience for boaters as it does for land-based DER users, so its likely that inverter-type DER devices for applications aboard boats will only increase in availability in the future.

ABYC A-32, July 2017, is the most current electrical standard that governs the two-way interface of DER equipment when installed on boats. ABYC, A-32, AC Power Conversion Equipment and Systems, Diagram 1, is shown below. This diagram is the electrical “model” the ABYC has adopted for inverter-type DERs installed on boats. Referring to this diagram, the earlier discussion of neutral-to-ground bonding still applies. The relay that accomplishes that is shown in the green oval.

Inverter Safety – “Anti-Islanding”

In residential neighborhoods (and aboard boats), power arises from the local Electric Power Utility. If power is lost, the implication is that some part of the utility power grid failed. Causes can include electrical device failure, severe weather, floods, terrorism or severe mechanical insult (tree-fall on wires, vehicle into utility pole, hot air balloon into wires, etc). A loss-of-power event leaves some local geography without electricity; home(s), police/fire station, shopping center, hospital, farm, airport, etc., an entire neighborhood, a entire town, etc. Many affected entities have mission-critical needs for uninterrupted power, and use DERs to achieve that goal. The footprint of the area of lost power is referred to as an “Island;” that is, an area that is physically cut-off and isolated from the power grid.

For the safety of residents, rescue personnel and repair personnel working to restore power within the “island” of disruption, DER’s operating at the time of a power failure must immediately detect the loss of grid power and disconnect themselves to prevent back-feeding power into the “island.” Again referring to the ABYC diagram, the relay shown in the red oval is the means by which DER Inverters disconnect themselves from the grid. ABYC requires that the disconnect occur within 100mS of the loss of power. Note: the inverter may continue powering some or all of its attached loads, within the rated capacity of the inverter and the capability of the battery bank.

Boaters are NOT expected to understand or care how all this happens.  The net here is, boaters need to buy and install MARINE-CERTIFIED equipment for installation aboard their boats. Equipment from discounters like Harbor Freight does not meet these complex safety requirements.

Behind the words “MARINE-CERTIFIED” is a very complex series of electrical standards that spans the worldwide membership of the IEC. These standards define the mutually-cooperative manner in which DERs must interact with National Electric Power Grids. At the end of this article is “Addendum 1” that describes the safety and testing standards involved with DER equipment for those interested.

About Motors – Single-Phase

Single phase motors are more complicated than three-phase motors. Even small sized single-phase motors are more complicated – electrically and mechanically – than three-phase motors. The reason is that it is much more difficult to create a rotating magnetic field with just one, single-phase. The “natural” rotation of the phases of a three-phase machine does not exist in a single-phase machine.

There are several different techniques used to create a rotating magnetic field in a single phase motor. All of these motors have high inrush “surge” currents.

A shaded-pole induction motor is a relatively simple and inexpensive motor. There are no brushes. Starting torque is low, so these motors are used for fan and blower motors and other low-starting torque applications. Creation of the torque to start rotation is done by means of one or two turns of heavy copper wire around one corner of the field coil. When the field is energized, inrush current is induced in this heavy coil. This induced current is out-of-phase with the power line current. This results in a second, offset, magnetic field, which is enough to start motor rotation. These motors are generally made in fractional-horsepower sizes.

Where medium and medium-high starting torques are required, the split-phase induction motor is more appropriate. These motors also do not have brushes. Split-phase induction motors are built with two field windings. One of the windings is called the “start” winding and the other is called the “run” winding. One of the windings is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current is commonly created by feeding the winding through a capacitor. A common variation of this design is a switch that disconnects the capacitor when the motor is up to operational speed. In this design, a centrifugal switch is internally mounted to the armature. The switch opens to disconnect the start capacitor when the rotor reaches operating speeds. Often in motors of this type, there is an audible click of the centrifugal switch transfer as it opens and closes. This is normal. In compressor applications, another variation is to have capacitors in both the start coil circuit and the run coil circuit. These alternatives involve complexity and cost.

In addition to a start/run capacitor, another way to achieve a rotating magnetic field is with a second field winding with significantly different values of inductance from the main winding.  This effectively results in an out-of-phase current in the second winding.

Where small physical size and high torques are needed, the Universal Motor is preferred. Universal motors are expensive to build and require periodic maintenance. These motors have carbon brushes and complex internal components that create a strong, consistent magnetic field at all rotational speeds. They can start to rotate against high stall loads. These are commonly used in handheld tools (drills, saws, etc.) and kitchen appliances like mixers and blenders. These motors often are not rated for continuous use, because they generate significant internal heat in operation.

About Motors – Three-Phase

Three-phase motors are very simple electrical machines. Recall that in a generator, there was a rotating magnetic field inside three fixed armature spaced at intervals of 120°. Three-phase motors have field coils that are physically mounted at intervals of 120°. The incoming three-phase power is connected to the windings of the motor’s field coils. As the voltage in the phases rises and falls, each in turn, in the 60Hz sinusoidal rhythm, a magnetic field strengthens and weakens around the field coils. An aggregate rotating magnetic field is produced by the rise and fall of current in the three individual field coils. That aggregate magnetic field rotates around the diameter of the machine’s field coils at a rate of 60 times per second. Reversing the connections of any two of the incoming three phases will reverse the direction of rotation of the magnetic field, and therefore, the direction of rotation of the motor itself.

A characteristic of motors is that they have high start-surge currents. At the moment when power is first applied to the machine, this surge is at its greatest. As the motor spins up to its running speed, the current settles down to its steady-state running level. Motors have separate ratings for start and run currents. Circuit designers need to allow for start-surge currents in selecting the gauge of wiring to the motor. Large horsepower motors have special controllers that limit inrush surge, but small frame motors found in boats generally do not need these sophisticated controllers. Because of the inrush surge, motor circuits are generally set up with slow-blow circuit protection.

The strength of the magnetic field determines the amount of torque the motor can deliver. The work will be to turn pumps, fans, windshield wipers, machine tools, refrigeration compressors, etc. Starting torque is large because of large start-surge currents. Running torque is the steady state torque the motor produces. Engineers select motors to match the torque required by the machinery the motor will drive.

About Motors – Raw Water Pumps

In motor-driven water pumps used in terrestrial applications – a residential hydronic heating systems, for example – an electric motor connects to the pump via a mechanical shaft. A rubber “lip seal” is used in the pump housing to prevent leaks at the shaft. This design has it’s limitations. Over time, the lip seal will harden, crack and fail and/or the shaft will become scored from mechanical wear, leading to leaks. Obviously, this design represents a future maintenance activity for the owner.

Boat raw water pumps are of different design. Instead of a mechanical shaft, the motor is fit to a strong permanent magnet. The pump impeller is also magnetic, and rotates on a shaft mounted inside a Fiberglass Reinforced Plastic (FRP) housing. The pump housing is designed so that when fit together with the motor, the magnet fits inside the metal-containing impeller’s housing. Since there is no shaft penetration through the housing, there is nothing to leak. As the motor spins, the magnetic field acts through the FRP housing and causes the impeller to spin. Good installation practice is for the assembled motor and pump be mounted vertically with the motor above the pump.

This design is leak free. The impeller can jam, but the pump motor will not overheat and will not be damaged if it does. These motors generally need little maintenance, but check the manufacturers instructions to verify the needs of your pump motor.

About Motors – Maintenance

Routine maintenance for electric motors includes, first and foremost, periodic lubrication of sleeve bearings. Use machine oil, not automotive motor oil. Most motors have lubricating ports – small holes – for applying machine oil. Use only a couple of drops of oil. Avoid the temptation to flood the bearing. If you do, the motor will just throw the excess all over the place.

If a “capacitor start” or a “capacitor start/capacitor run” motor will not start, check the capacitor. When a capacitor fails, the motor may overheat and either will not start or will not run correctly. Capacitors are physically located outside the frame of the motor, and are much less expensive to replace than the motor. This is particularly true if the motor is an air conditioning/heat pump compressor sealed into a refrigerant system.

Brushes wear in normal service and are normal maintenance parts. Replacements are available from the tool or appliance manufacturer. Typically, the motor will show symptoms of impending failure. Brushes wear in operation to the point where they no longer make good electrical contact. Often, a small external physical bump will cause the motor to start. That’s a sure sign that the brushes need replacing. Order replacement brushes when symptoms first appear, or the tool will surely fail when you most need it, before replacement brushes are on-hand.

Motors are very reliable devices. Motors will generally give many years of satisfactory performance. The down side of that is that your specific model may not be available when you do need to replace it. If a motor will not start due to internal failure, you do have options. I recently had occasion to help a friend with a blower motor for his onboard air conditioning unit. The manufacturer wanted over $400 for a replacement blower. Instead, we took the motor to a local motor refurbisher, and for $60, the refurbisher replaced the bearings and rebuilt the motor. Centrifugal switches are also replaceable. Electric motor refurbishers are available in most medium sized and larger communities across the country. Don’t overlook this option. Look under “Electric Motor – Repair” in the Yellow Pages! Yes, folks, I grew up using Yellow Pages.

Refrigeration compressors have built-in safety circuits. One is a thermally operated switch that’s mounted to the case of the compressor. It is designed to open and disconnect power to the compressor motor if the compressor case gets too hot. Another is a pressure operated switch that is designed to disconnect the motor if the refrigerant gas pressure gets too high. Some units can also detect low refrigerant pressures. These switches can fail, and their failure rate is higher than the failure rate of the compressor itself. If a compressor fails to run, check the safety switches before changing the compressor or changing the entire fridge or air conditioner/heat pump unit. Many an unsuspecting soul has paid to have a compressor replaced and only gotten a $20 switch for the money!

Qualifications of Personnel

The above discussions illustrate an important safety consideration which I know some will find restrictive and controversial. Simply stated, people who are not thoroughly familiar with marine electrical standards and requirements should not install or modify boat electrical systems! Many excellent residential electricians and many skilled DIY “practitioners” who learned terrestrial NEC compliance techniques in residential applications are simply not qualified to perform work on boats. Switching requirements are different on boats than they are on land, yet it is true that cheaper switches incorrectly selected for use on a boat may appear to work. The details of neutral-to-ground bonding are much more extensive on boats, yet man-made wiring errors may go hidden and without symptom for weeks, months or years. Work performed by one who is simply unaware of boat equipment requirements can lead to unintended but serious safety faults for friends and family to discover at some random future time.

The frustration of encountering a no-power situation because the boat trips a ground fault sensing pedestal breaker on a cruise is unwelcome for the boat owner, but is truly unsettling to the spouse and guests aboard. Diagnosing man-made wiring errors is expensive and frustrating by any definition. It is extremely important to know, understand and comply with the low-level details of the ABYC electrical standards. Boats in marinas are in very close proximity to their dock neighbors. All marina residents – whether longterm or transient – depend on the safety of neighboring boats. When hiring someone to do electrical work on your boat, make sure the person you hire is actually qualified by training and certification to perform marine installation, maintenance, troubleshooting and repair services.

Incidental Topic – Dockside Ground Fault Sensors

While not actually a boat-side AC electrical topic, GFIs on docks is a topic that does apply to any discussion of boat AC electrical systems. The problems that cause dockside ground fault sensors to trip are all caused by conditions that exist on the boat. Many (the great majority) of these issues were caused by unqualified but well-intended DIY practitioners who did the wrong things without realizing it. I have written in detail about dockside GFI problems and solutions. Articles on this website that discuss these issues include:

  1. Electric Shock Drowning
  2. Emerging AC Electrical Concern
  3. AC Safety Tests for Boats
  4. ELCI Primer
  5. Ground Faults and Ground Fault Sensors
  6. Ground Faults: Difficult to Hire Skilled Troubleshooter

Incidental Topic – Galvanic Corrosion

Also not an AC electrical topic, this heading is included because the Galvanic Isolator
is fit into the main safety ground conductor of the boat. The submerged metal parts of boats are comprised of a mix of dissimilar types of metals. Boats commonly have
stainless steel drive shafts and rudders, bronze propellers, struts, rudders and thruhulls, and Aluminum trim-tabs. When immersed in sea water, these different metals and metal alloys follow the same laws of electrochemistry as found in a battery, albeit not optimized in construction and materials purity as they would be in a made-for-purpose battery. The action of this electrochemistry results in “metal wasting” corrosion of some of the underwater metals.

Another very common form of galvanic corrosion is “single-metal” corrosion (ex: “rust”
in iron-containing metals, “poultice corrosion” in aluminum, “pitting corrosion” and
“Crevice corrosion” in Stainless Steels). A serious and often unrecognized form of
single-metal corrosion occurs in the all too common brass plumbing fittings bought in
big box and hardware stores, and even in some marine chandleries. Brass is a metal
alloy containing primarily copper and zinc. We know zinc is a galvanically active metal
(anodic) that will sacrifice itself to protect more noble metals (cathodic). Brass fittings
flooded in sea water suffer from a phenomena called “Dezincification.” The zinc
wastes away, leaving the remaining metal structure of the brass alloy porous, with a
pink appearance, and physically very weak.  WARNING: never use brass fittings
below the waterline or in raw water circuits used by heat pumps aboard the boat.
“Sacrificial anodes” of zinc, aluminum and magnesium are usually attached to valuable underwater metals to protect the more valuable metals from galvanic corrosion wasting damage. Zincs are most effective if electrically located on the metals they protect.  Zincs waste away as they give up positive ions to the electrolyte of the galvanic cell.An “Impressed Current Cathodic Protection” (ICCP) device is an electronic approach to
managing galvanic corrosion on boats built with metal hulls (steel, aluminum). An
ICCP is able to protect the relatively very larger surface areas of metal hulls than can
be done effectively with individual sacrificial anodes.

About – Galvanic Isolation

The ABYC recommends some form of galvanic corrosion control on boats. Aside from
the active electronics of an ICCP, there are three passive ways to achieve this control.
One modifies the electrical makeup of the underwater collection of metals. The other
two act by disrupting the flow of the small but destructive DC galvanic currents. The
latter two approaches impact upon the design of the boat’s shore power safety ground.

The first and most common approach to reduce galvanic wasting is with the use of
sacrificial anodes. These anodes modify the makeup of the underwater metals in a
way that makes them waste, rather than more valuable metals.

The second approach is with the use if a Galvanic Isolator (GI), which eliminates the
electrical path for galvanic currents to use. Electrically, this device is placed in the
main safety ground wire where the ground conductor enters/exits the boat; that is,
electrically at the shore power inlet(s). The newest generation of GI is the “Fail Safe”
device. It consists of a solid state, full wave, bridge rectifier and a large capacitor. This device will allow AC fault currents to flow normally in the safety ground, should
that need ever arise. The physics of the diode junction effectively blocks the small DC
voltage that drives the flow of galvanic currents. Without a galvanic isolator, zincs can
be consumed in weeks. With a galvanic isolator, zincs should last many months.

The third approach to interrupting the flow of galvanic currents is by installing an
onboard AC “shore power transformer.” An Isolation configuration eliminates the path
from the boat’s grounding network to shore. A polarization configuration keeps the
shore path, so should include a Galvanic Isolator. There are subtle pros and cons to
this choice. This author prefers the polarization configuration for maximum safety.

Electrical Emergencies

True electrical emergencies are rare. Electrical emergency situations will always
become less dangerous if power is quickly disconnected.

Be wary and suspicious of unfamiliar, unpleasant or pungent odors. Transformers,
motors and many other electrical devices that are in the process of failing often
overheat and cause insulating materials to emit strong, pungent odors. TURN OFF
POWER and use your nose to track down the source. Turning power off will also
shut down air circulating blowers that circulate odors and make locating their origin
difficult. Treat strong odors as an pre-emergent true emergency. The goal is to
find the offending device before it ignites! Turning off the power will stop the self destructive process and allow the failing device to cool off. Do not re-start a device
that has overheated in operation to the point of emitting strong odors! This type of
over-heating often causes secondary internal damage that you cannot see.

In an emergency, the most important commodity you can have is time! Time to
think and act. To buy time, install smoke detectors. Install smoke detectors that
have dual mode incipient gas sensors as well as visible smoke sensors. Install a
model that communicates with other units so that when one alarms, they all alarm. I
placed a dual-mode smoke detector on the overhead of my electrical locker. That
locker is a small, closed space behind my AC and DC branch circuit panels, and is
where the shore power inlets and the Generator Transfer Switch are located. That is a
good place to install a smoke detector, placed there in order to buy me some time.

Emergencies – Avoidance

When working around electricity, use insulated tools, especially when working around batteries. Batteries contain enormous amounts of stored energy. A metal tool across the terminals of a battery may actually weld the tool metal to the battery terminals. If this happens, the tool metal will become extremely hot. Whenever you plan to work around a battery, pre-plan to have a two foot piece of 2”x2” wood stock, or a wood handled carpenter’s hammer, readily at hand. If the worst should happen, use the wooden 2”x4” as a mallet to forcefully knock the tool away from the battery terminals. Once this cascade of events starts, the only way to stop it is to break the tool free of the battery terminals. Act quickly. The battery can get hot enough to melt and start a fire.

Many electrical emergencies are avoidable. Always comply with standard electrical
safety rules and practices. This is not a exhaustive list. As you plan your projects,
plan for safety.

    1. Never work on live electrical circuits. Turn power “off” before accessing.
    2. Never work alone; always have someone with you who can disconnect power
      and call for help in an emergency.
    3. Never wear watches or jewelry when performing electrical work.
    4. Never parallel multiple small gauge wires to achieve a larger current carrying
      capacity (“ampacity”). That virtually guarantees trouble in the future.
    5. Install protective insulation and safety covers to prevent accidental contact with
      bare electrical connections and terminals.
    6. Periodically, go on an “inspection tour” of the boat’s electrical system; make this
      a part of your scheduled preventive maintenance checklist. Specifically,
  • Screws work loose over time; with power off, periodically go through the boat
    and tighten electrical connections.
  • Crimp connections corrode and loosen over time; avoid crimp connections
    wherever possible; given the choice to splice an existing wire or run a new
    wire, run the new wire; with power off, check crimps by firmly pulling on the
    wire at the crimp. Replace any connections that show any signs of heating
    or of being or becoming loose!
  • Secure loose or dangling wires.
  • Check wiring bundles where they ride over or round obstructions or through
    bulkheads. Vibration injures insulation and wiring, so support and insulate
    bundles in these areas to prevent wear spots.
  • Leave adequate slack in wire runs so they are not under tension.
  • Repair cuts, cracks or gouges in insulation immediately. Don’t wait.

In Case Of Fire

“Experts” all agree, in any fire on a boat, 1) there is very little time to act, and 2)
the odds of successfully fighting a fire are against you from the beginning.

If there is any doubt about being successful at extinguishing a fire aboard, use the
precious little time available to get your crew and yourself safely away from the fire.

    1. Alert your crew:
      • If you decide to fight a fire, do not use water! Water can conduct electricity,
        and you may wind up with both fire and electrocution emergencies. To fight
        an electrical fire, use a dry-chemical extinguisher rated for “Type ABC” fires.
      • Crew calls “m’aidez” (“May Day”) via VHF-16, or 911 via telephone. Do not
        hang up the phone until the 911 dispatcher tells you to.
    2. Disconnect Power:
      • If on shore power, turn power “off” at the pedestal!
      • If on genset, shut down the machine!
      • Shut down DC power to any inverter or inverter/charger!
      • Disconnect the main battery bank!
    3. Once the fire is extinguished, monitor the involved area to be sure it’s cool
      enough that it will not self re-ignite.
    4. Make repairs before re-applying power.

Appendix 1

The following is more in depth than I usually write, and will be of interest to advanced
DIY practitioners and electrical professionals interested in how electrical safety and
testing codes are applied. This material adds to what has been presented above, but
is not necessary to understanding.

Acronyms and Abbreviations

ANSI – American National Standards Institute
AHJ – Authority Having Jurisdiction
CSA – Canadian Standards Association
DER – Distributed Energy Resources
DOE – United States Department of Energy
EPS – Electric Power System
ETL – Intertek® registered testing mark (Electrical Testing Laboratories)
IEC – International Electrotechnical Commission
IEEE – Institute of Electrical and Electronics Engineers
NEC – National Electric Code (United States)
NREL – National Renewable Energy Laboratory
PUC – Public Utility Commission
REPC – Rural Electric Power Conference
SGIRM – Smart Grid Interoperability Reference Model
UL/ULc – Underwriters Laboratories® testing mark

ABYC A-32, AC Power Conversion Equipment and Systems

All ABYC Standards follow a common layout format (“boilerplate”). Following is an
excerpt from the “References” section of ABYC A-32, July, 2017:
32.3 – References
The following references form a part of this standard. Unless otherwise noted the
latest version of the referenced standard shall apply.
32.3.1 – refers to several other ABYC standards
32.3.2 – IEC 62116 Test procedure of islanding protection measures for utility interconnected photovoltiac inverters (IEC 62116 is a European standard)
32.3.3 – IEEE 1547 Standard for Interconnecting Distributed Resources with the
Electric Power Grid (IEEE 1547 is a US ANSI Standard (North America power grid))

Author’s note: emphasis and comments added for clarification.

Relationship of IEEE 1547 and UL 1741

Safety Standards define minimum feature and function capabilities for the design of a
particular class of equipment; in this case an inverter-charger DER. Testing Standards
define test specifications that a device must meet in order for the manufacturer to claim
compliance to the design standard. This leads to some very complex relationships
between different national regulatory authorities and between and among multiple
independent, private enterprise businesses. Following is a pictorial that shows the
relationship of the safety and testing standards that define the INTERFACE between
devices in the class of DERs to the North American Electric Power Grid as deployed in
the United States:

In the above Figure 6, the IEEE 1547 Standard defines the minimum design
requirements of DER equipment. IEEE 1547.1 and UL 1741 together define the
minimum test conditions that the completed device must meet. In addition, ABYC A32,
32.9.2 calls for disconnect protection in less than 100 mS after loss of incoming AC
power. The NEC, Article 705, defines what the National Electric Power Grid is


Figure 7 shows the Certificate of Conformity for the Victron® MultiPlus™ device family.  At the bottom of the Certificate, readers can see that the device complies to UL 1741-2016 (2nd Edition) and the Canadian National Standard, CAN/CSA 22.2, No. 107.1-16, (4th Edition).


  1. Victron® MultiPlus™ and Quattro™ inverter-chargers are grid-attached DERs,
    even though their purpose when installed on boats is not to supply power
    backwards onto the grid.
  2. ABYC Standard A-32 incorporates the requirements of IEEE1547. All of the
    ABYC standards operate in the same way, by including (incorporating) other
    relevant IEC, EN and IEEE standards into themselves.
  3. IEEE 1547 has been adopted as an American National Standard by the
    American National Standards Institute. IEEE1547 and subs (1547.1 through
    1547.8) state the design and testing requirements that DERs used in the US
    must meet; in this case, we are specifically interested in the Victron® MultiPlus™
    and Quattro™ inverter/charger devices. Victron® complies to UL1741, which is
    compatible with the NEC in the US.
  4. At this writing, I am still investigating, but I believe it is true that when UL 1741 applies to a device, that certification supersedes UL 458. UL458 compliant
    devices disconnect the incoming mains when the device is operating in “invert”
    mode. UL1741 compliant devices do the same thing, but for a broader set of
  5. Per Victron®*, “…we disconnect/get isolated from the AC source within 20mS.”
    That is well within the July 2017, ABYC A-32 requirement of 100mS.

* email to the author dated 3/15/2019, signed:

Mr. Justin Larrabee
Sales Manager
Victron Energy
70 Water Street
Thomaston, ME 04861

AC Electricity Fundamentals – Part 1

2/14/2019:  Initial post
6/6/2020:    Added: “US Utility Voltage Standards vs. Common Language”
10/6/2020:  Added section: “Electric Circuit ‘Wire’ Naming and Identification“;
                      other typo and grammar edits
10/10/2020:  Revised the sections “Ground” and “Common/Circuit Common.”
2/4/2023: Added section: “World-Wide AC Service Voltages/Frequencies”

About this article

This article discusses the concepts and terminology of AC electricity at an introductory level. The scope of the article is limited to the AC power systems found in North and Central America. In Part 1 (this part; already plenty long enough), I will discuss the basics of AC power generation and the delivery of AC power to single-family residential neighborhoods and homes. In the Part 2 article,  I discuss the AC power systems as found on cruising boats.

I chose this two part approach for two reasons. First, almost all homeowners have some familiarity with household AC electricity. At the very least, most homeowners can find the circuit breaker panel and reset tripped breakers. Second, and more important, boat AC electrical systems are just a specific subset of what is found in a single-family residential AC installation. Boat AC systems are equivalent to sub-panels in a residence. Sub-panels are subordinate to the main service disconnect panel in a residential building, and in the same way, boats are subordinate to the AC electrical infrastructure of a marina. A basic understanding of household AC electrical systems puts boaters 75% of the way towards understanding boat AC electrical systems. Where boats differ from land-based residential buildings, the reasons are based on specific safety issues that emerge in, and are unique to, the marine environment. Boat AC electrical systems are significantly more complex than single family residences.

My goal is to explain electrical topics in a way that enables readers to relate new information to what is already somewhat familiar, so that boaters can identify and summarize problems or questions, and communicate with, interpret and understand the service professionals with whom they may need to interact.


There is one absolute, always rule whenever you must deal with electricity. VIRTUALLY ALL ELECTRICIY CAN BE DANGEROUS TO PROPERTY AND LIFE. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.  The large batteries found on boats can produce explosive gasses and store enough energy to easily start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! If you will be working in noisy environments, with running engines or other loud machinery, WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!

The rule is, “if you aren’t sure what to do and how to do it, stop. Don’t do anything until you’re sure of the “what,” “how” and “why!”


Electrocution is a biological insult that starts with an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death.  Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electrocution, there are several things to do immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM!  You can not save someone else if you panic!
  3. SCREAM FOR HELP! ATTRACT ATTENTION!  Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again!  Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate.  CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.

Basic Electrical Working Concepts  – Volts/Amps/Ohms

Like gravity, electricity is invisible. A common analogy used to explain electrical concepts is to liken an electric system to a community water system. Consider the familiar garden hose fit with a nozzle. In the garden hose, when the nozzle is opened, “pressure” in the system makes water flow.

In this analogy, the water in the hose is analogous to electrons in a wire. “Voltage” is the “propulsive energy,” or “pressure,” that makes electrons flow through the wire. The greater the water pressure, the more water flows per unit time. Similarly, the more voltage that is present across a circuit, the more electrons will flow through the circuit per unit time. The amount of water that comes out of the hose is measured in “gallons.” The flow of electrons through wire is measured in “amperes,” or “amps.”

In a water hose, the nozzle restricts the flow of water through the hose. The flow of electrons is restricted in electrical circuits by the electrical property of “resistance.” All materials that conduct electricity have some amount of resistance. Silver and gold have little resistance per unit length. Pure copper has only slightly more, and aluminum has slightly more again. Even the small resistance of a copper wire is extremely important in power distribution applications. Electrical “resistance” is measured in “Ohms.”

Assume we have a 3” diameter water hose and a 1/2” diameter water hose, both attached to the same water source. Only so many molecules of water can fit through the small hose in a minute, but many more molecules of water can fit through the large hose. This concept is called “carrying capacity.” Only so many electrons can “fit” through a wire per unit time.  The larger the wire, the more electrons.  Electrical “carrying capacity” is called “ampacity.” “Ampacity” is a rating assigned to wires.  Wires of the same metal, of different sizes and covered by insulation with different thermal and chemical properties, have different rated “ampacities.” The ampacity rating is the safe maximum current the wire can carry within the temperature rating of the wire’s insulation. Ampacity tables are widely available on the Internet.

Ohm’s Law – Memory Aid

The mathematical relationship between voltage, current, resistance and power is defined by “Ohm’s Law.” Ohm’s Law is probably the most fundamental relationship there is in the entire realm of electricity.  Folks who deal with electricity regularly have this relationship emblazoned in their brains, but for the rest of us, this “memory aid” is  extremely helpful! First, decide what variable you want to calculate. It’s unusual not to know at least two of the necessary variables. For example, today I saw a TV advertisement for a small, portable, plug-in electric space heater. The device plugs into a 120V outlet, so we know E = 120V. I went to the website and found that the unit is rated at 600 Watts, so we know P = 600. For use on the boat, I wondered how much current the device would draw. From the two known variables, we can calculate that the unit will draw about 5 Amps of AC current, which indeed may be OK for some uses on a boat. We also know the unit’s equivalent resistance is 24 Ω (“Ω” is the Greek Letter “Omega,” and is used as shorthand for “Ohms.”)


The word “Ground” has multiple meanings in different contexts.  To a residential electrician, “ground” should mean the “safety ground” required by the National Electric Code (NEC).  But in casual conversation around docks, “ground” may mean lots of things.  In DC discussions, “ground” probably refers to a “return conductor” for DC electric current.  But in AC discussions, the “return path” for electric current is the “Neutral.”   So, what does the word “Ground” really mean?  Unfortunately, it means all of the above, and more.

The Earth – the crust of our beloved home planet – is electrically conductive. It has many minerals and mineral salts which provide “free electrons.” In the presence of an electric voltage, electrons flow from point-to-point around and within the earth’s crust. By far the most dramatic example of this is the natural phenomena called “lightening.”

The electrical potential of the earth is defined to be “zero” volts. It is the standard reference point for taking electrical measurements.  In the rest of the world, connections to the earth are referred to as “Earthing,” but in North America, we talk about connections to the earth as “grounding.”  In order to connect a residential electrical system to “earth ground,” one or more interconnected rods (usually of copper) are driven into the earth. The neutral return point of the residence’s electrical system is physically connected to the network of copper grounding rods.

The concept of “earth ground” is not absolutely essential for the safety of people, pets, farm animals and wildlife.  The entire electric distribution grid of the country is connected at innumerable points to rods driven into the earth (the “electric grid” is a “multi-earthed system”).  Every residential property has an “earthing” connection at the service entrance to the residence.  Earth ground is also the reference for rendering static and lightening safe.  In electrical system design, protecting manmade structures from lightening involves routing the discharge arc around the structure to be protected, not through it.

Under some unique and undesirable circumstances, electric currents can be found flowing through the earth.  When that happens, it can be dangerous – even fatal – to people, pets, farm animals and wildlife.  This is most likely to occur, and most serious, around high voltage power distribution lines and high voltage switchyard facilities.

The essential point here is that “earth ground” is a universally understood reference point for all power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the total absence of any voltage.  We will return to this concept over and over as we proceed in our discussion.

Circuit Common/“Common”

The concept of “earth ground” is essential for electrical system designers to manage weather catastrophes and other accidental insults to electrical systems, static and lightening, but an “earth ground” is not necessary for electric circuits to operate.  Most, if not the vast majority of, portable generators ARE NOT grounded, and yet household appliances and tools run by portable generators run just fine without that ground connection.  People may get hurt doing that, but the appliance and tools themselves are able to run just fine.

The term “common” is useful in electronics applications.  The term “neutral” is used in residential AC electrical systems to reference the conductor that returns current flowing in a circuit from the load to the source.  The “common” conductor does not have to be “0” volts with respect to ground. Unless the neutral is specifically referenced to ground, the “common return” is a “free-floating” conductor. It is extremely important to understand the difference between the concepts of “ground” and “common.”

Unfortunately, the term “common” or “circuit common” is not often used in routine conversation.   The common return of a circuit is frequently – colloquially – called its “ground,” to mean the return path for current.  “Neutral” is a specific term that refers to the current-carrying return conductor of residential AC circuits, and although in AC electrical systems, the “neutral” is connected to “ground,” the term “neutral” is not a specific reference to “earth ground.”

Direct physical contact with energized high voltage is completely safe as long as you are not “across” two or more electrical conductors. For current to flow, there must be a connection between two conductors where there is a voltage difference between them (that is, “across a voltage”). Consider birds sitting on high tension transmission lines, or squirrels running along neighborhood overhead wires. They are safe because they are on, but not across, a voltage. The animal’s entire little body is raised to the voltage of the wire upon which they sit, yet they are perfectly safe because there is no path for current to flow THROUGH the body. The electrical activity of their brains and hearts is not affected by the voltage they are sitting upon. But, a human-being on a wet concrete floor wearing leather shoes best not come into contact with a “hot” wire. That concrete floor is made with salt-containing minerals, and most definitely is electrically conductive, especially when wet. A person standing on that floor and simultaneously touching an energized wire is “across” an electric voltage. That is a shocking experience!  Maybe, a fatal, shocking experience.

“Conventions” vs. Facts

Within the study of electricity as a science, there are hard electrochemical and materials facts, and then there are shorthand ways people talk to each other about complex concepts.  This happens in all professions, of course.  It’s all fine until the terminology confuses an understanding of the true concepts.  Some examples:

  1. It is a fact of physics that electrons carry a negative electrical charge, which means electrons flow from a more negative voltage in a circuit to a more positive voltage.  However, by universal agreement, or “by convention,” the entire practice of electricity and electronics treats current as flowing from positive to negative.  The direction of electron flow has no practical importance, but to properly interpret electrical diagrams, you need to understand the conventional way current flow gets represented by arrow-containing symbols.
  2. The symbols on electrical drawings are all agreed by “convention,” or “working agreement.”  Industry-specific symbols are agreed by international standards organizations.  Where there are symbol differences, their meaning is often obvious.  Some differences occur across international boundaries.  The power industry uses different symbols than are used in the electronics industry.
  3. The “single phase, center tapped, three wire” service is the residential standard in use, by convention, all across North and Central America.  It is institutionalized in the National Electric Code of the US and The Canadian Electric Code in Canada.  Completely different systems are used in other parts of the world, including Europe, Asia, Oceania and southern South America.
  4. The insulation used to coat electrical conductors is colored.  The colors, by convention, identify the use to which wires are put.  Understanding the color schema for wires is essential to electrical safety.  Mistakes here can be fatal.  The meaning of colors vary from country to country.  There are numerous differences between the United States and the nations of the European Economic Community and Oceania.  For those interested, tables are available on the Internet that document color meanings.

Science and Craftsmanship

The laboratory study of “electrical energy” is a theoretical and conceptual science.
Electrical craftsmanship is practical.  I will discuss only a tiny subset of the technical terms and concepts that are necessary to understanding low voltage AC as found in residential and boat applications.  Craftsmanship involves selecting materials, employing fabrication techniques, installing and maintaining electrical equipment, all with the goal of accomplishing some intended design purpose.  Craftsmanship is performed by electricians or electrical technicians and governed by formal regulatory controls called “electrical building codes.”

I view craftsmanship in two stages, which can be sequential or iterative.  If you have ever done an electrical project, you’ve performed both of these functions.

The first stage is the domain of the “circuit designer;”  i.e., the person who designs a branch circuit for installing a ceiling fan with a single switch to turn the fan “on” and “off.”  Or a slightly more complex branch circuit with three switches to turn a light “on” and “off” from different locations.  Or a much more complex array of multiple branch circuits to power a “man cave” or “she shed.”  Or the system for an entire home.  The designer must have solid knowledge of the National Electrical Code (NEC).  Electrical designers for boating applications must be thoroughly familiar with the American Boat and Yacht Council (ABYC) electrical standards.  The NEC and ABYC standards have as their purpose avoiding or minimizing present and future loss of life or damages to property.  The work product of the designer is a system drawing that defines the purpose of a circuit and the manner in which that purpose will be achieved through the use of electrical equipment, components and materials.  The work product includes a the bill-of-materials of the components required to implement the project.  For most projects, a reliable cost estimate can be produced at this stage.

The second craftsmanship stage is the domain of the skilled technician who is charged with the doing of the thing.  This craftsman must know how to use and interpret the designer’s drawings and how to use an enormous array of electrical meters and mechanical tools in the safe fabrication, construction, installation and maintenance of electrical circuits.  This craftsman must understand current assembly techniques, materials and supplies, and must understand and deeply respect industry safety practices.  Safety practice involves knowing when to and when not to work around, and with, energized electrical circuits.  On boats, because of the special safety implications of an electrical system on a floating structure, this craftsman must understand not only what to do and how to do it, but in fact, why things are done as they are, in making an electrical installation safe.

Key Concepts and Terms

  1. Ohm’s law – describes the mathematical relationship between voltage, current, resistance and power.
  2. Kirchhoff’s Laws
    • Kirchhoffs Current Law: “the total current entering a node is exactly equal to the current leaving the node.
    • Kirchhoffs Voltage Law: in any closed loop network, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop.
  3. voltage – (Volt) the quantification of “Electromotive Force” (EMF) (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive Force is measured across two points in a circuit.
  4. current – (ampere; amp) a quantification of the number of electrons flowing through a circuit at any one time.
  5. resistance – (Ohm) a characteristic of an electrically conductive material that tends to retard or impede the flow of electrons through it.
  6. power – (elect: Watt; Joule) (mechanical: inch-pounds, foot-pounds) elect: the amount of “work” that electricity performs in its application.  In purely resistive applications, light or heat.  In turning a motor, torque.
  7. frequency – (Hertz) the number of times a wave goes through a complete cycle in a standard measurement time interval, usually one second.
  8. ampacity – (Amp) a rating of the ability of a conductor of given material, diameter and insulation properties to conduct an electric current within the temperature limit established by the properties of the wire’s insulation characteristics.  (current: amperes; amps) (temperature: degrees Centigrade)
  9. source – the origin from which AC power emerges to energize a circuit.
  10. load – the components of an electric circuit where energy is consumed to do useful work; “useful work” includes production of heat, light, or torque via a motor.
  11. common – a portion of a circuit connection or set of connections that creates a direct return path for electrons flowing in an electric circuit.
  12. neutral – a special case in an AC circuit of a non-ground return path for electrons flowing in a North American standard residential electrical service.
  13. ground – a universal  standard earth reference voltage of “0” volts.
  14. node – a junction, connection or terminal within a circuit were two or more circuit branches are connected together.
  15. fault current – an abnormal path for current flow, usually to ground.  Fault currents represent potentially dangerous conditions.
  16. short circuit – a specific category of electrical fault resulting from an unintentional direct connection of an energized conductor to either a return circuit or an equipment ground.  This low-resistance, unintentional connection results in the flow of extremely large fault currents, and causes overload protection devices (fuses, circuit breakers) to open in order to disconnect the energized power source.
  17. GFCI (Ground Fault Circuit Interrupter) – an anti-shock safety device that senses leakage currents and disconnects the energized power source.
  18. AFCI (Arc Fault Circuit Interrupter) – a fire protection safety device that senses lose connections and disconnects the energized power source.
  19. GFP/EPD/ELCI (Ground Fault Protection/Equipment Protective Device/Equipment Leakage Circuit Interrupter) – similar to GFCI, but higher disconnect specifications.
  20. chase, raceway, conduit, “emt” – enclosed containment spaces in a building or a boat through which wires are run to achieve access to distant locations or to protect wiring from accidental physical damage.
  21. Field Coil – the rotating part of one design of AC generator; this coil can be a DC permanent magnet (typical in small machines), or a DC electromagnet.
  22. Voltage Regulator – the device that determines the strength of the magnetic field in an AC genset by adjusting DC current flowing in the spinning field coil.
  23. Stator – the fixed coils of one design of AC generator, from which sine waves of AC power emerge.
  24. Armature – the power-producing component of a generator; the rotating part of a DC generator; the fixed coils (Stator) in one design of AC generator.
  25. switchgear – a generic term for all disconnecting devices (fuses, circuit breakers, switches, power panels).  This term is used across the electrical power industry, from generating station to transformer yards to residential locations.
  26. Inductance (Ohm)/capacitance (Farad)/Power Factor (unitless) – technical characteristics common to the behavior of AC electricity in circuits that significantly affect large motor driven appliances and all electronic devices.  These become increasingly important as voltages, frequencies and power consumption rise.
  27. Managing the collapse of a magnetic field – a design consideration of any magnetically operated electrical devices (motor, generator, relay, etc), and many solid state devices.  A significant safety consideration for maintenance craftsmen.  When a magnetic field collapses, it creates a very high energy spike, which sometimes includes an electric arc.
  28. ABYC – American Boat and Yacht Council, Annapolis, MD.  This organization produces a very comprehensive set of electrical standards applicable to boat manufacturers, the marine insurance industry and boat owners.
  29. NFPA – National Fire Protection Association; owner/creator of the NEC.
  30. NEC/CEC – National Electric Code (USA), Canada Electric Code (Canada).  electrical design standards for political subdivisions and the construction industry.  Ranges from codes for residential housing, light commercial and industrial buildings, elevators, hospitals, airports, and heavy industry.

Generation (Source) and Consumption (Load)

There are three primary divisions of all electrical power distribution systems, including the global system we call the “nationwide electrical power grid.”  They are 1) the source of the electrical power, 2) the transmission system, or interconnecting wires and switches that carry power from the source to the point where it is consumed, and 3) the load, or the part of the system where the electrical energy is transformed into useful work.

At the level of the US national electric power grid, the source of AC electric power is one or more generating machines located in one or more generating stations.  Often, the term “alternator” is used interchangeably with the term “generator.”  These generating stations range in size from enormous, industrial-sized installations to small rural hydroelectric dams to units suitable for individual residential applications.

The substations, switchgear and wiring that connects sources of power to load centers are extremely complex, involving may hundreds of miles of high tension power lines, enormous transformers and highly complex switches.  Transmission equipment  can also be as simple as an extension cord run from the garage to the hedge clipper.

Electrical loads fall into the entire range of electrical equipment, from the largest commercial synchronous motors to the smallest and most humble LED electric clock.

About AC Generators

Typical AC electric generators have a rotating magnet (imagine the big bar magnet you played with in grade school science) which has a north pole and a south pole.  That magnet may be driven by a belt, wind or water turbine or direct drive, but ultimately, it’s a spinning magnet mounted on a shaft.  The north and south poles of the spinning magnet travel in a circular path.  A pick-up coil is positioned just outside the edge of the circle.   As the magnet spins on it’s shaft, the poles of the magnet approach the fixed pick-up coil, producing an electric voltage at the pick-up coil.

As the spinning magnetic pole gets physically nearer to the pick-up coil, the voltage at the pick-up coil gets progressively larger.  Once the magnetic pole moves past and away from the pick-up coil, the voltage at the pick-up coil gets progressively smaller again. When the north and south poles of the magnet are both equally distant from the conductors of the pick-up coil, no voltage is produced at the pick-up coil.

The voltage induced in the pick-up coil by the passage of the north magnetic pole is equal in magnitude and opposite in polarity from the voltage induced by the passage of the south magnetic pole.  One pair of north and south magnetic poles that sequentially rotate past the pick-up coil produce one cycle of AC voltage at the pick-up coil on each revolution. The speed, in revolutions per minute (RPM), of the spinning magnet determines the frequency (Hz) of the generated voltage.  The resulting AC wave form is called a “sine wave,” which is centered around “0” volts.  Sine waves rise and fall in smooth, graceful fashion with no sharp transitions in the shape of the wave.

In the preceding diagram, there is a geometrically balanced arrangement of a spinning magnet and a geometrically balanced arrangement of pick-up coils.  The output power of the generator is directly proportional to the strength of the magnetic field, up to the limits of its materials and mechanical design.  The output consists of two wires, and is referred to as “Single Phase” AC.

Many physical arrangements of the magnet poles and pick-up coils are possible, but the basic principle is the same for all AC generators.  To produce 60Hz AC, a single phase, two-pole, gasoline-driven, big box store genset (2500W to 6kW) typically spins at 3600 rpm; a single phase four-pole Marine genset (7.5kW to 25kW) typically spins at 1800 rpm. Because of the enormous weight and mechanical forces involved, multi-megawatt commercial generators may have 24 poles and spin at 200 rpm.

The rotating magnet in an AC generator is called the “field coil.”  The field coil is just a spinning DC electromagnet.  DC is fed to the field coil via slip rings and brushes on the spinning shaft.  The fixed pick-up coil in an AC generator is called the “stator coil.”  It is wrapped around iron support columns that are fixed in position around the perimeter of the frame of the machine.

Occasionally, the term “armature” may be heard; an “armature” is defined as the power-producing component of a generator.  In a DC machine, it is the armature that spins, with field coils stationary in the frame of the machine.  Fixed field coils with a spinning armature is a construction alternative for small frame AC alternators (<25kW).  This is both more costly to build and much more complex mechanically, so not common in the generator sizes found in the consumer retail market.

The amount of power that a generator can produce depends on many aspects of the physical construction of the machine, the amount of energy available from the driving motive source, the size of the internal conductors and underlying metal components, the strength of the internal magnetic field, and many other factors.

“Single-Phase” and “Three-Phase Power”

In reading through questions and discussions on various Internet boating bulletin boards , the differences between “single phase” AC and “three-phase” AC is often a point of confusion.  Three-phase power is extremely rare in residential settings, and few people have any life experience with it.

Consider the above preceding description of generator concepts.  Commercial power plants are fit with enormously large and heavy generators.  For several reasons, it is advantageous for these very large machines to spin slowly.  These generators are built with a large number of physical pick-up coils.  These pick-up coils are arranged as pairs in sets of three.  Logically – not physically – the machine appears as shown in this diagram.  These sets of pick-up coils are placed around the perimeter of the circle of the rotating magnet, at geometric intervals of 120° around the 360° circle.

As the bar magnet spins, the voltage in each of the pick-up coils rises to its positive maximum, falls back to zero, then rises to its negative maximum, and falls back to zero. This happens in each set of coils, in turn.  The result is three sinusoidal waveforms being produced by the same rotating magnet (“field”).  The three wave forms are displaced in time by 1/3 of a cycle (120°) of rotation of the rotor.  Enter here, the short-cut language the electrical industry has for this: “three-phase AC,” often shown on electrical diagrams written as “3-ϕ” or “3-phase.”

For commercial power plant operators and distributors, three-phase power is far more economical to generate than single-phase power.  Worldwide, all commercial electric power is created in generators configured as 3-ϕ machines.  The phases are designated as “Phase-1,” “Phase-2,” and “Phase 3;” this terminology can also be “Phase-A,” “Phase-B,” and “Phase C.”  Three-phase derived power is of special interest for boaters with 120V/240V, 50A shore power connections since it results in 120V/208V voltages.  More details in the section on “Special Situations.”

Single-phase AC is the type of electric service found in virtually all single family residential applications because it is easily derived from three-phase distribution systems, in two ways.  The first is to connect a load between any one of the phases of a three-phase service and a suitable electrical return point, usually the common of a 3-phase wye configuration.  This is how residential neighborhoods are serviced.  The second way to obtain single phase AC is to connect the load between any two phases of the 3-phase distribution system.  This is common in commercial applications and in apartment and condo buildings, but not in single family residential services.

US Utility Voltage Standards vs. Common Language

In the US (throughout North America), just what voltage standards do we have for residential and light commercial use?  Interesting question.  Is it 110V, 115V or 120V?  Is it 220V, 230V or 240V?  When people speak about residential voltages, it’s quite common to hear one or more of these numbers.  In actuality, they all mean the same thing.

Standardized utility voltages evolved over the years from 110V/220V systems to 115/230V to 117V/234V to 120V/240V.  In the 1970’s, the American National Standards Institute (ANSI) adopted the now current 120V/240V voltage standard via ANSI National Standard C84.1-1970.  This standard specifies two voltage ranges which included a specification for “service entrance voltage” and a standard for the voltage that would appear at user attached devices, called “utilization voltage.”   “Service entrance voltage” is measured at the meter, and “utilization voltage” is measured at the terminals of attached equipment.  The occurrence of service voltages outside the specified range (brownouts) was intended to be infrequent.

Following is the chart from ANSI C84.1.  The Range A service voltage range is plus or minus 5% of nominal. The Range B utilization voltage range is plus 6% to minus 13% of nominal.

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The occurrence of service voltages outside the Range A limits should be infrequent. Household equipment is designed and rated to give fully satisfactory performance throughout this range.

Range B includes voltages above and below the Range A limits that necessarily result from practical design and operating conditions in utility or user systems, or both. Although such conditions are a part of practical operations, they should be limited in extent, infrequent, and of short duration (brownouts). If they occur on a repetitive or sustained basis, corrective measures should be undertaken within a reasonable time to improve voltages to meet Range A requirements.  Household equipment is designed to give acceptable performance in the extremes of this range of utilization voltages, although not necessarily as good performance as in Range A.

Table 1, below, is useful for an understanding of the relationship of power supplied by a power utility and the standards to which household appliances are manufactured.  The “Nominal” column is what we always talk about in common, ordinary discussion.  The “Service” and “Utilization” column are as discussed above.  The “Nameplate” column shows the voltage that will appear on an item you buy, such as refrigerator, dishwasher, washing machine, air conditioner, TV, Stereo, drill press, air compressor, etc.  The “NEMA” column (National Electrical Manufacturers Association) shows the tolerances used by manufacturing designers in creating the devices you buy.  These are all slightly different perspectives on the same thing.

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There are two net messages here.:

  1. The voltage delivered to any residence will vary throughout the day, throughout the month, and throughout the year.
  2. The equipment in the residence is made to tolerate variation in utilization voltage.

This is very much more important to appreciate and understand when the scene shifts to boats in a marina, or more completely, when the scene shifts to boats cruising from place to place with widely different shore power services.

World-Wide AC Service Voltages/Frequencies

This article describes AC electric power as found in North America.  The site is primarily oriented towards cruising boaters in an effort to increase practical understanding by boat owner/operators of the electric power, electric systems, electric equipment, electric wiring and electrical safety codes and requirements involving boats. Electro-physics theory is universal, but:

  1. Boats have unique safety considerations not found in land-based residential settings, and
  2.  country-by-country across the earth, there are significant differences in service voltages, line frequencies, equipment applications wiring details and installation code requirements that will be of interest to some long-range cruising boaters.

For international cruisers, the following link may be of interest, as it describes country-by-country service voltages and frequencies: Those using these data are encouraged to perform their own due diligence relative to electric power service in the countries of intended visitation, and treat that as just one more item on the list of items requiring advance preparation.

Residential Neighborhood

For simplicity, I started with power as delivered to single family suburban homes excluding light commercial buildings.  Light commercial buildings (condos, townhouses, apartments, offices, stores, and marinas) can all be served with single phase AC electric service, just as single family residences are, but more commonly, they are served by 3-phase utility service.  I will talk about these buildings later in the section on “Special Situations.”

Utility company power transformers have input sides, called the “primary,” and output sides, called the “secondary.”  Physically, both the primary and the secondary coils of the transformer are independent windings of wire wound around an internal metal core.  The windings are electrically isolated from each other; i.e., “insulated” from each other, but are “coupled” to each other by a shared magnetic field.  As the incoming primary voltage rises and falls, the magnetic field in the metal core strengthens and weakens.  As that magnetic field strengthens and weakens, voltage appears at the secondary.

Residential Single Phase “Street” Transformer (Typical)

The “load” for the transformer outside your house usually consists of four or so residential homes.  Throughout North and Central America the transformer is matched to the primary voltage to produce 120V/240V at the secondary.  The utility company transformer reduces the primary voltage to the residential requirement.  The range of transmission system primary voltages in a three-phase grounded wye configuration include; 34,500/19,900 volts; 22,900/13,200 volts; 13,200/7,620 volts; 12,470/7,200 volts; and, 4,160/2,400.  The first number in these number pairs represents the phase-to-phase voltage; the second number represents the phase to neutral voltage.  A single phase primary in a residential neighborhood is most commonly 7,200 volts, measured phase-to-neutral.  In rural residential primaries, 13,200 volts is common.

Transformer coils can be built with one or more “taps” on both the primary and secondary windings (coils).   The secondary winding of a residential power transformer is built with a single tap at the electrical midpoint of the coil.  This configuration is called a “center-tap.”  The three wires that come to a single-family residential  home from the utility pole are the two end-points of the secondary coil and the center-tap.  That center-tap conductor becomes the “neutral” within the building’s distribution wiring.

In the world of the electrical craftsman (electrician), it is desirable practice in a residential building or boat to have about ½ of the total household load attached to each side of the service transformer.  This practice balances the load on the secondary windings of the transformer on the street, and balances the concentration of heat that builds up within the windings and metal core of the transformer.  Transformers are oil cooled, and under heavy loads, they can get very hot.  Thus, balancing heat dissipation is crucially important in periods of very high electrical demand.  Days that are 104°F on the Chesapeake Bay or -30°F at International Falls are not times you’d want the transformer that serves your home to fail!

The definition of a North American residential standard power distribution system is a “single-phase, center-tapped, three-wire” service (alternatively, “single-phase, center-tapped, three-pole” service); in this case, the term “pole” represents a current carrying conductor.  Other common terms for this systems include “grounded neutral” and “split phase.”  The three parts of this definition are:

  1. single phase
  2. center-tap (gives rise to the system “neutral;” “grounded neutral”)
  3. three wires (two “hot” and one “neutral”)

From time-to-time, professional electricians and DIY lay technicians incorrectly refer to the residential “single phase, center tapped, three-wire” configuration as consisting of two phases.   The “evidence” is that one leg, “L1,” is 180° out-of-phase with the other leg, “L2.”  While “true,” this misleading factoid is a measurement curiosity caused by performing the electrical measurement from an inappropriate reference point.  Voltages from the two halves of our residential service will appear to be out-of-phase if measured with an oscilloscope from Neutral to “L1” and then from Neutral to “L2.”  The false appearance is the result of looking at the secondary of the transformer with reference to its center tap rather than across the entire winding.  This measurement curiosity is not present if the secondary is measured from “L1” to “L2” (or vice versa).  Think of it this way.  There is only one magnetic field alternately rising and falling in the transformer, driven by the rise and fall of the single-phase input at the primary.  That is the defining characteristic of “single-phase” equipment.  In a three phase device, there are three independent magnetic fields rising and falling within the equipment.  That is the defining characteristic of 3-phase equipment.  This distinction becomes extremely important when describing rotational torque in a 3-phase motor.

The above discussion is somewhat of a “technicality” issue, which has no practical importance in real life, and can safely be ignored!  When I was a pup, and first worked for an electrician in the early 1960s, I learned to refer to the two residential “hot” lines as “legs” instead of “phases.”  Doing so distinguishes the in-residence wiring from the conductors of the utility distribution system.  Frankly, except for the concepts involved, it’s really not important how you refer to this as long as you don’t let it confuse you!

Service Entrance – Single Family Residence

So now we understand that the electrical service entering a single family residence is a “single-phase, center tap, three-wire” service.  In our single family residence, if there are overhead wires and utility poles in the street, the three wires coming from the transformer are routed to a weather head or anchorage on the home, where they are spliced to wires leading to the electric meter.  In most jurisdictions in the US, the wires coming from the street are owned by the utility company.  The weather head, meter box and the wires from the “street splice” to the meter box are customer-owned.  The meter itself is owned by the utility company.

The customer-owned wire to the electric meter and from the meter to the main disconnect panel inside the building is comprised of two insulated wires (usually black) surrounded by a wrapping of bare wire strands.  This entire cable assembly is insulated as a single triplex unit.  This cable has a flat rectangular cross-section and is known as “Type SE,” or “Service Entrance” cable.  The two hot lines are routed to the input side of the “main” circuit breaker in the main disconnect panel.  The uninsulated neutral wire of the Service Entrance cable is routed to the neutral buss bar in the main panel.  The neutral buss bar is insulated from everything else in the service disconnect box, including the metal of the box enclosure, itself.  If the residence has an underground service, wires from a transformer located on a ground-level concrete pad will all be individually insulated wires rather than a triplex assembly.  They will be routed through underground conduit into the electric meter and then to the service disconnect panel.

The output side of the main circuit breaker in the disconnect panel is attached directly to metal “buss bars,” to which individual branch circuit breakers are fitted.  These buss bars are referred to as “L1” and “L2,” because they are on the overload-protected load side of the panel’s main circuit breaker.  The input side of the main disconnect breaker is referred to as the “Line” side and the output side is referred to as the “Load” side.

Electric Circuit “Wire” Naming and Identification

All electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”

In single phase 120V residential circuits in North America, the two conductors are named for on their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained shortly), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

In electrical engineering, “earth” is the single reference point in an electrical circuit from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the code for AC systems in buildings has required safety ground. This safety ground is a network of conductors that attach to every outlet, switch plate, ceiling fan, luminary fixture and wired appliance in the residence.  In a residential application, there are one or more copper rods driven into the ground outside the building.  The “System Grounding Conductor” (wire) is usually of bare #6 or #4 AWG stranded copper wire, and is routed from the buried ground rod(s) to a ground buss bar located in the service disconnect panel.  That buss bar is physically mounted on, and electrically connected to, the service disconnect panel’s metal box enclosure.  All of the ground wires that come from outlets and appliances everywhere in the building are routed to this buss bar.

In the NEC, each individual conductor that is a component of the “ground system” network has its own specific name, but for purposes of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”  The purpose of the safety ground conductors is to provide a low resistance fault-clearing path, so under normal conditions in a properly wired electrical system, the safety ground conductors DO NOT carry current. The ground conductor ONLY carries current when there is a “fault” in the system. In buildings on land, the ground conductor is often bare copper wire. On boats and in appliances, the ground wire is solid green in color, or green with a yellow tracer.

NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new work, always buy the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape.

However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.

Main Service Disconnect Panel

We know from earlier discussion that the “neutral” in the building is a free-floating return line for power that arrives from the transformer hot lines.  But a free-floating return point is unlikely to be at “zero” volts, which is required to avoid electric shock in the home.  The NEC requires that the neutral line in a residence be bonded to earth ground “at the derived source of the electricity.”  For a home, the “derived source” is defined to be the main service disconnect panel.

In one design of main service disconnect panel, there is a buss bar dedicated to collecting branch circuit neutral conductors and a physically separate buss bar dedicated to collecting safety ground conductors.  In this style panel, there is a screw – usually dyed green in color – in the neutral buss bar. That screw is the “system bonding jumper,” or “bonding screw.”  This design allows the panel to be used either as a main disconnect panel or as a sub-panel.

If the disconnect panel is to be used as the Main Disconnect Panel, the bonding screw must be seated into the panel’s metal enclosure housing to electrically “bond” the “neutral” buss bar to the “safety ground” buss bar.  That screw is not for any mechanical purpose; it is the electrical bridge that make the “neutral” to “earth ground” connection.  THIS IS A CRITICALLY IMPORTANT SAFETY FEATURE.  NEVER OMIT OR REMOVE THE BONDING SCREW!


Sub-panels, a special case of residential switchgear, are used for several reasons:

  1. reduce the number of wire runs from the main service disconnect panel,
  2. manage the round trip length for long branch circuit wiring runs,
  3. manage the number of wires run in hidden chases/raceways/conduits, and
  4. reduce the cost of the installation.

The NEC does not limit the number of sub-panels that may be installed in a residential electrical system. Larger residential systems may have sub-panels located in several places around the home; ex: attached or detached garage, detached “guest quarters,” workshop, greenhouse or yard shed, pool house, Man Cave, She Shed, attic-space mechanical service (air conditioning compressor or attic vent fans), etc.  To install a sub-panel in residential applications, a single, appropriately sized 4-conductor cable, “Type SER,” is run from the main service entrance panel to the sub-panel (red arrow, below).  This 4-wire configuration carries “L1,” “L2,” “N” and “G” to the sub-panel switch box.  Because the sub-panel is subordinate to the main disconnect panel, the neutral-to-ground bonding screw is NEVER used in any sub-panel switch box.  By definition, the sub-panel is not the “source” for these branch circuits.  The main disconnect panel remains the “defined source” of the circuit.

The configuration of sub-panels in a residence is exactly analogous to the configuration of a boat attached to a marina shore power pedestal.  Notice the 240V, 3-pole, 4-wire feeder (red arrow) that connects the residential Main Disconnect Panel to the remote sub-panel.  This feeder is exactly analogous to the 240V/50A shore power cord of a boat.  The sub-panel “feeder cable” is “Type SER.”  It contains three current-carrying conductors and a safety ground.  Rather than the flat, rectangular cross-section of “Type SE,” “Type SER” cable features a round cross-section.  A boat’s “feeder cable” (shore power cord) is “Type SO” or “Type SOW,” which are very flexible cords.  Net: a boat looks like a sub-panel to the marina’s shore power system, and that is why the ABYC electrical standard seems so closely aligned with the requirements of the NEC.  Notice also in the drawing that the sub-panel safety ground leads back to the neutral buss in the main panel.  The neutral-to-ground bond is made only at the “derived source,” which is the Main Disconnect Panel.   Likewise, on a boat connected to shore power, there should never be a neutral-to-ground connection anywhere on the boat.  In both cases, the neutral-to-ground connection is made at the “derived source,” which is the main distribution panel in a residence, analogous to the marina shore power system for a boat.

Note: This main disconnect panel drawing shows a single buss bar which is shared by the neutrals and the grounds of branch circuits.  This arrangement is an NEC-compliant variation in a main disconnect panel.  Many main disconnect panels and all sub-panels will have physically separate busses for the neutrals and the grounds.

Branch Circuits

Branch circuits are where useful work gets done in the home.  There are three use cases:

  1. Between legs “L1” and “L2” alone, without “N,” we can power 240VAC, two-wire (two-pole) appliances; for example, the 240V motor of a deep-well pump, 240V baseboard electric heat radiator(s), or a 240V hot water heater.
  2. With “L1,” “L2” and “N,” we can power 240V, three-pole appliances; these appliances require 240V for some internal functions and 120V for other internal functions; for example, an electric dryer, range cooktop or oven; all of these appliances require 240V for the heating elements, but 120V for the motor and control circuits.  Or, central air conditioning system, which requires 240V for the compressor, but only 120V for the control circuits.
  3. Finally, with either “L1” or “L2” alone, and “N,” we can power the entire panoply of 120V, two-pole household loads; oil or gas furnace, dishwasher, incandescent and florescent lighting, computers, printers, routers, wireless telephones, TVs, VCRs, stereo, refrigerator, freezer, microwave oven, coffee maker, toaster, crock pot, waffle iron, blender, mixer, hair dryer, steam iron, battery chargers, shop tools, CPAP, oxygen concentrator, etc; you get the idea!

Branch circuits originate at a circuit breaker located in either the main service panel or a subordinate sub-panel.  Branch circuits feed either convenience outlets or feed into the attachment enclosure of a permanently installed appliance.  For convenience of installation and maintenance, the individual black, red, white and bare wires of a branch circuit are packaged together within a sheath of plastic outer insulation.  Most residential wire sold in big box, hardware stores and home centers is “Type NM,” meaning “non-metallic.” This is often called “Romex.”  “Type NM” intended to power 120V circuits is called “two-wire with ground,” or “two-pole, three-wire.”  “Type NM” intended to power 240V circuits is called “three-wire with ground,” or “three-pole, four wire.”    Another common residential wire is “Type AC.”  “Type AC” has an armored metallic sheath around the individual colored conductors instead of a plastic outer sheath.  “Type AC” is used for furnace controls for LPG and oil burners, hot water heaters and other appliance in an equipment room or basement, as well as when installed in areas exposed to being physically disturbed or damaged, such as workshops or garages.  Carefully match the wire you buy to the application you have, based on NEC and local electrical codes.

In the U. S., the color of the insulation on individual wires is important; “L1” is black, “L2” is red, “N” is white and “G” is uninsulated copper in convenience and appliance circuits, but can be green or green with a yellow tracer when insulated.

Occasionally, you may encounter a wire in a service disconnect panel or a junction box that has a piece of electrical tape of another color  conspicuously wound around it near its connecting end.  In a residential building, you may see red or black electrical tape wound on a white insulated wire, or you may see a piece or white electrical tape wound on red or black insulated wires.

Do not remove these pieces of tape; they are not an accidental left-over!  It means the installing electrician has “changed” the meaning of the base color of the insulation of the wire.  In residences, the most common place to find it is in wall boxes containing switches that control lighting or fans from multiple doorway locations, or wall boxes at the top and bottom of staircases.   If you ever see this, always triple-verify how the wire is actually being used before proceeding or disturbing the connection.

I have spent a lot of time talking about the current that arrives at the load in one of the energized conductors, “L1” and/or “L2,” and returns to the source in the neutral, “N.”  I have not discussed the use of the green ground wire, “G.”  In a correctly wired, normally operating home or boat AC electrical system, the ground wire should never have any current flowing in it.  The purpose of the safety ground wire is to provide an emergency path for current in order to trip the supplying circuit breaker to remove power from a faulting circuit.  By definition, current flowing in a safety ground is symptomatic of an electrical fault condition.  Fault currents originate from the hot line(s), but return to the source in the safety ground instead of the neutral.  This condition is also known as a “ground fault.”  Never use wire covered with green insulation as a current-carrying conductor.

Circuit Breakers

Contrary to popular belief, circuit breakers/fuses do not protect attached loads!  Circuit breakers do not protect TVs, entertainment systems, computers, microwaves, coffee pots, pumps or compressors.  CIRCUIT BREAKERS/FUSES PROTECT THE POWER-CARRYING WIRING THAT IS HIDDEN IN WALLS AND/OR ENCLOSED IN CHASES, RACEWAYS AND CONDUIT THROUGHOUT YOUR HOME OR BOAT!  They protect the WIRING of your home/boat.  This is a critically key concept.

When wires overheat, their colored insulation can melt, exposing the live conductor.  At that point, energized conductors can touch other now uninsulated conductors, and sparks can fly.  Wires in closed spaces, unusually warm spaces, or chases/raceways/conduits warm up more than wires in un-congested, cool, spaces where there is plenty of air circulation. Overheating softens the insulation.  Wires can get so hot that they will literally melt and can weld themselves together.  This process can cause adjacent nearby wood and composite building materials to burst into flame.  So, circuit breakers protect wires from overload, and therefore, protect the insulation from overheating, melting, failing and causing fires.

There are several common types of circuit breakers, and several manufacturers of circuit breakers and compatible service disconnect panels.  Circuit Breakers for 120V circuits are singe-wide; for 240VAC, they are “stacked” or “doublewide.”  Doublewide breakers have mechanically linked operating levers, and must be doublewide so that they can be physically installed in a service panel in a way that allows them to mate to both the “L1” and the “L2” buss bars at the same time.   If one leg of a 240V circuit – say, “L1” – develops a fault that causes the circuit breaker to trip, the mechanical link causes the other leg – in this example, “L2” – to also be disconnected from it’s source.  Never remove the mechanical linkage between doublewide breaker operating levers.

Switchgear on Boats – Residential vs. Marine-certified

Circuit Breakers should be selected based on the size of the wire they protect.  A 15A circuit breaker protects #14 AWG, Type NM cable; a 20A breaker protects #12 AWG Type NM, and a 30A breaker protects#10 AWG Type NM.  These numbers are based on the 60℃ temperature rating of “Type NM” wire.  Wire ampacities are higher with the 105℃ temperature rating of “Type BC5W2” boat cable.

Circuit breakers used for “over-current protection” (OCP) have rating of 15A, 20A, 30A or 50A.  That said, modern, sophisticated circuit breakers actually carry several ratings.  In a true short circuit, an over-current fault can instantaneously be as high as several hundreds of amps.  By arcing, that extreme amount of current can weld the contacts closed and permanently damage the circuit breaker’s contact points, rendering the breaker inoperable.  Circuit breakers and all switching devices carry an “Ampere Interrupt Capacity” (AIC) rating.  AIC is the amount of current the device can interrupt without being damaged by arcing.

Modern circuit breakers can also have multiple purposes.  Besides OCP, one added purpose is “Ground Fault Protection” (GFP) and another purpose is “Arc Fault Protection” (AFP).  GFP breakers contain a circuit that compares the amount of current being delivered in the hot wire(s) to the amount of current returning in the neutral.  Any difference in outgoing and returning current is a “ground fault.”  Household “Ground Fault Circuit Interrupter” (GFCI) breakers are designed to trip “off” if the difference between supplied and returned current is as little as 4mA – 6mA.  “Equipment Leakage Circuit Interrupters” (ELCI) onboard boats – and Equipment Protective Devices (EPD) on dockside pedestals – protect the whole boat, as a sub-panel.  ELCI/EPD are designed to trip “off” in less than 100 mS if the difference between supplied and returning current exceeds 30mA.

Finally, for use on gasoline powered boats and environments of potentially explosive gas, circuit breakers (and other electrical switching devices) must be rated as “ignition protected.”  This means that any internal arcing (sparking) caused by the contacts opening under load must not be able to come into contact with any airspace outside the breaker’s enclosure.  If explosive gasses were able to infiltrate the breaker’s enclosure, the vapors would be able to cause an explosion.  Of course, common residential circuit breakers are not made to the standard of “ignition protected” devices.

In general, in my opinion, it is bad practice to use “big box” and hardware store electrical switchgear equipment, circuit breakers or wire made for residential applications on a boat.  Residential switchgear is not made to withstand humid, salt-containing air, is not suited to the materials properties required by ABYC, and is not equivalent in temperature ratings for the ampacities of given conductor sizes.  NEVER, NEVER use solid core household wire on boats.

Aggregate Electrical Load – Residential Building

“How  much electrical “stuff” can we run “all at once” in our single family residential home?”  This is a key question for both residential applications and boats.  For boaters, it relates directly to discussions about 30A and 50A shore power cords and inlet wiring sizes.

Today, if you have a home of 2000 ft2 or more with an oil or gas-fired furnace, you’ll have a service entrance with at least a 200 amp service capacity.  If your home has electric baseboard heating and/or central air conditioning, it’ll probably have a 400 amp capacity. In the 1960s, we simply didn’t have as much “electrical stuff.”

What does it mean to “have a 400 amp capacity electrical service?”  In a moderate-sized residential building, if the individual capacities of all of the branch circuit breakers in your residential service disconnect panel were added up, there would probably be between 500 and 800 amps of distribution capacity.  For example:

8 – 30 amp double pole breakers for baseboard heating
1 – 50 amp double pole breaker for the range/oven
3 – 20 amp single pole breakers for the dishwasher, washer, and microwave
1 – 30 amp double pole breaker for the clothes dryer
1 – 40 amp double pole breaker for the hot water heater
1 – 40 amp single pole breaker for that great air compressor in the garage
20 or more – 15 or 20 amp single pole breakers for convenience outlets
1 – 50 amp double pole breaker for the air conditioning compressor

Hmmm…   Adds up to 810 amps (+/-) of branch circuit distribution capacity.  Take out the baseboard heating and you still have 570 amps.  However, that service panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at maximum breaker capacities.  Remember, breakers protect wires, so the individual breaker capacity is to protect the wire, not the attachment.  What happens if you exceed the capacity of the 200A/400A main breaker?  Well, in that case you’d blow the main breaker, but without blowing any of the individual branch circuit breakers.  Hmmm…

So to the question, “what does it mean to have a 200A or 400A electrical service?”  A “200 amp service” means that the installed utility-owned drop from the street, the conductors of the ”Type SE,” 3-wire service entrance cable to the electric meter housing, the conductors from the electric meter to the service disconnect panel, the service disconnect panel itself, and the earth ground connection are all sized and designed to operate in a safe manner when handling up to 200 amps for a 200A service, or up to 400A for a 400A service. If you exceed that capacity, that set of essentially unprotected electrical components may fail.  In effect, 200A/400A is the “ampacity” of the unfused and unprotected service entrance feed components.  So even though you have 500 to 800 amps of branch circuit load attachments, if you never exceed a combined aggregate load of 200 total amps, the distribution box will serve you just fine.  If every you do blow the main 200A/400A breaker in the home, have the cause determined by a qualified electrical professional!

Aggregate Electrical Load – Boat

The previous analysis of loading a residence main disconnect panel applies in exactly the same way to boats.  Most cruising-sized boats with 30A shore power will have well in excess of 30A of branch circuit capacity; likewise, boats with 50A shore power will have proportionally more branch circuit capacity.  That power is delivered onto the boat through a (30A)(50A) onboard main disconnect breaker, or compatible ELCI.  As with the residence case, the service distribution panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at their maximum breaker capacities.  If you exceed the maximum main breaker capacity, you blow either the main disconnect breaker, or the Shore Power pedestal breaker, generally without blowing any of the individual branch circuit breakers.

The ABYC requires an AC Main Disconnect Circuit Breaker within 10 feet of the shore power inlet.  Nothing is allowed to be connected ahead of that main disconnect breaker except the actual shore power inlet connector.  Recall, the purpose of circuit breakers is to protect wiring, and in particular, wiring hidden from view, and away from reasonably easy access, and running through spaces containing combustable materials.  The AC Main Disconnect Breaker protects the boat’s main inlet wiring (the boat’s “service entrance cable,” if you will) up to the main distribution panel that serves the boat’s individual branch circuits. Remember, the ampere rating of the disconnect breaker must be matched to the ampacity of the wiring between the power inlet plug and the main disconnect panel on the boat.  In the case of boats, the wiring installed by the boat manufacturer should reflect what the naval architect sped’ed for the boat.  Remember, the wires we’re talking about provide power to the AC circuit breaker panel of the boat, and carry the total aggregate current load for the whole boat.  Sizing shore power cords smaller than necessary could be dangerous.

AFCI and GFCI-protected Protection

Since 2008, the NEC has constantly extended the AFCI requirement to now include all habitable areas of a home, including kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar places.  Some states have modified these requirements when adopting the NEC as statewide regulatory code (building codes of all kinds are done on a County-by-County basis in Maryland).  Check local building codes before proceeding.

Since 1971, the NEC has continually expanded the coverage requirements for GFCI protection. Today, GFCI protection is required in all “wet” locations in residential buildings, which includes bathrooms, outdoors locations, rooftops, crawl spaces, unfinished basements, kitchen countertop areas, sinks, laundry areas, bathtub/shower stall areas, boathouses, locker rooms, pool areas: you get the idea.  On boats, the ABYC requires GFCI-protected outlets in heads, galley, machinery spaces and everywhere on the weatherdeck.

Should you wish to retrofit AFCI and GFCI-compliance into an older home (a good idea), a reasonable approach is to replace the conventional circuit breakers in the main  disconnect panel or sub-panel that serves affected branch circuits with combination AFCI/GFCI-protective circuit breakers.  That way, all outlets served by that circuit breaker are AFCI-protected and GFCI-protected.  Combination breakers are available from many manufacturers for about $35 – $45 apiece (as of January, 2018).  Discounts are available for volume purchases.   On the boat, physically compatible GFCI-breakers are not generally available, so GFCI-protected outlets are recommended.

GFCI-protected devices do present some unintended consequences.  A common scenario is for boaters to use adapters to enable a 30A or 50A shore power cord to use a standard 15A or 20A, 120V GFCI-protected utility outlet on a dock.  This provides power for a fridge, a battery charger, and maybe a reading lamp, for a night or two.

In the case of deteriorated, cracked insulation on a shore power cord lying in the water, a ground fault current could easily be large enough to trip a GFCI breaker, and that fault would not go away over time.  That condition is a true ground fault.  Not all trips are caused by true faults.  Sometimes, electronic components (capacitors and inductors) within the familiar portable computer “power bricks” can cause “momentary” surge currents that can trip sensitive GFCI protection devices.  Insulation breakdown on blower motors, pumps  and air conditioning compressors, as well as aging hot water heater elements, can cause transient power leaks.  Power spikes on power lines can trip GFCI devices.  All GFCI implementations are exposed to false faults resulting in “nuisance” trips.  When attaching to GFCI-protected outlets, it’s a good idea to set all AC breakers “off” first, then plug in, then turn branch circuits “on” one at a time.

For marinas and boatyards, starting in 2011, the NEC has required ground fault protection on new construction docks (except residential, single family docks until 2017).  These devices are called Equipment Protective Devices (EPD), and are also subject to “nuisance trips.”  To reduce the incidence of nuisance trips, the NEC has adopted two accommodations to lessen the occurrence of false trips on docks.  First, the size of the leakage current – 30mA – that would cause a marine pedestal EPD to trip “off” is greater than (less sensitive than) a 15A/20A GFCI convenience outlet.   Second, the length of time (duration of) the leakage current needs to be present – up to 100mS – has been made longer.  Since 2011, the rollout of these EPD sensors at marinas has been slow, but they are beginning to appear in greater numbers, and all boaters should expect to see EPD protection of marine outlets on docks with increasing frequency over the next few years.  The electrical knowledge and skills found among dock staff are unlikely to resolve problems for those who do experience nuisance trips at a marina.  Particularly on holidays, weekends and off-hours, high-school and college summer help are not likely to be able to assist transient boaters.

“Nuisance trips” may or may not mean you have wiring errors or equipment faults on your boat, but the fact is, many boats do have wiring errors and equipment faults that until recently have been silent and non-symptomatic.  Obviously, “troubleshooting” this scenario could be very complicated and time consuming.  If you have the skills to do it yourself, it’ll cost lots of time.  If you hire a marine electrician to do it for you, it’ll cost lots of money.  Either way, it won’t be easy or inexpensive.  It may well be that you just have older switchgear equipment, like a reverse polarity light with a filament that provides a “leakage path” from “neutral” to “safety ground.”  This is not an unsafe condition, but it will trip some EPD devices.  What is nasty about this is that “your boat is at fault,” and that’s precisely what you’ll get from the marina operator.

Special Situations – Life’s Little Complications

There are two types of three-phase wiring configurations: “wye” (or “star”) and “delta.” Three-phase distribution systems are used in commercial facilities and larger industrial facilities.  Within this category, I include condos, townhouses, strip mall offices, shopping centers, marinas and boatyards.  So, consider for example the case of three-phase distribution systems feeding end-user attachments in a condo or apartment.

In our “single family suburban residence” model, we learned the US standard voltages of a “single phase, center tapped, three wire” service entrance would be 240VAC/120VAC.  For many technical and economic reasons, light commercial and multi-family residential buildings are supplied from a three-phase, wye-connected service.  In a wye configuration, a 4-pole, 4-wire distribution system comprised of  “ϕ-1,” “ϕ-2,” “ϕ-3” and “N” is delivered into the building.  What is finally delivered, in turn, to the individual occupancy units is a 3-pole, 3-wire feeder analogous to the single phase street feed.  It is not, however, derived from the secondary of a single phase transformer.  Rather, it consists of any two of the three phases that came into the building, together with wye’s “N.”  As an example, suite 100 may receive “ϕ-1,” “ϕ-3” and “N,” and suite 102 may receive “ϕ-2,” “ϕ-3” and “N,” and so forth.

In the wye configuration, the voltages delivered to individual occupancy suites are not the standard 240VAC/120VAC.  Between “N” and any of the phases, the suite would see 120VAC. But between the two phases, the suite would see only 208VAC.  This service is written on paper as “208V/120V Y,” to indicate the phase-to-phase voltage, “208V,” the phase-to-neutral voltage, “120V,” and the fact that the configuration is a wye connection, “Y.”  This practice is common enough in the US that household appliances built for 208VAC/120VAC are commonly available in retail outlets for condo and townhouse dwellers.

Fortunately, 240VAC/120VAC appliances connected to 208V/120V Y services will usually work. Many are made to tolerate the lower line-to-line voltage.  The downside is, appliance efficiency may be reduced.  The power available across the “L1” and “L2” lines of phase-to-phase connection service will electrically be only 85% of the power available from the full design voltage.  As boaters, we need to be aware that many marinas are configured in this way.  If a boat has 240VAC appliances aboard – air conditioning, hot water heater, range/oven, washer/dryer, etc. – those appliances will receive “low voltage” if the marina is configured to provide “208V/120V Y.”

The most significant impact might be to 240VAC pump and compressor motors.  With a low voltage on the appliance, efficiency will be compromised, and motor overheating might occur.  Three phase “Y” distribution configurations are common in marina’s.  Boats with one or two, 2-pole, 30A shore power connections would not be affected.  Those connections are 120VAC.  Those with two 30A shore power cords connected to a “Y” adapter into a 50A outlet on a pedestal are also unaffected.  That’s because even though you are bringing the two different phase lines aboard, your boat does not have any 208V/240V appliances, so nothing aboard is affected.  Boats that connect to shore power with 3-pole, 50A shore power cord are potentially affected, as that 3-pole, 4-wire connector provides 240VAC with the expectation that it will be used on the boat.  Without 240V appliances, there is no affect.

The only thing you, as a boat owner/operator, can do to protect your appliances is to measure, with your onboard volt meter(s), the line voltages (2xxVAC/120VAC) provided by the shore power pedestal, each and every time you hook up.  In this way, you will know what the marina is delivering.  I recommend you become meticulous about this.  If you are not receiving 240VAC – if you are receiving only 208VAC – you will have to make decisions about what to do next.  Do not expect the dock hands that help you tie up to know what they have. Some may, but I would assume many would not.  Frankly, even the marina manager may not know.

50A Power from 30A Sources

Facility managers for marinas, yacht clubs, boatyards, condos and municipal walls must make investment choices about the electrical infrastructure that they will install to support their customer’s needs. Systems that provide maximum flexibility in electrical connectivity for boaters are expensive in capital cost and maintenance. In many facilities boaters will encounter more modest wiring alternatives. Wiring configurations will also vary between docks in larger facilities. Different docks at facilities that support a wide size range of both resident boats and transient visitors may be wired differently. Very large boats would normally slip on a dock with other large boats. These docks will likely be powered with only 208V/240V, 50A service outlets. Docks intended for mid-sized boats may have a mix of 208V/240V, 50A outlets and 120V, 30A outlets, or may have only 120V, 30A outlets. Facilities that cater to only transient visitors may have a mix of 30A and 50A outlets, or may have only 20A and 30A twistlock outlets. Electrically, there are many possible code-compliant wiring variations.

Cruisers must assess their personal desire for, and dependence on, shore power. Before departure, cruiser’s should obtain a set of adapters to provide the desired personal flexibility. The specific adapter(s) needed aboard the boat depends on the shore power inlet configuration of the boat. Along the Great Loop route, several variations of shore power may be encountered. The goal would be to have the flexibility to be able to connect the boat’€™s shore power inlet connection to each of the following commonly found power sources:

  1. residential 120VAC, 15A and 20A duplex outlets,
  2. marine 125V, 20A twist outlet (sometimes alone and sometimes in pairs),
  3. marine 125V, 30A twist outlet (sometimes alone and sometimes in pairs),
  4. 208V/240V, 50A marine twist outlet.

We never encountered a 120V, 2-Pole, 3-Wire NEMA type SS1 50A shore power source; they exist, but are very uncommon. We have encountered NEMA 14-50 (residential 240V, 50A outlets used on electric range/ovens and kilns) in various places along the Erie Canal system for use by canal system work boats. Because we had an adapter to access those outlets, we enjoyed shore power when others did not.

Boats fit with 50A Shore Power inlets will regularly encounter situations where 50A outlets are not available. For these situations, 30A-to-50A adapters can provide access to a AC power sufficient to meet short-term needs. Simply put, adapters create options, flexibility and alternatives to boaters. Among the options, adapters can provide enough power to avoid the need to run gensets at a dock.

Note: this article applies to boats which are NOT fit with polarization/isolation transformers.

In North America, the national standard for power delivered to residential and light commercial customers is a “single phase, three-pole, four-wire, center-neutral” wiring configuration. This system is sometimes referred to as a “240V grounded-neutral” system. In these systems, the service’s Neutral (white) conductor is bonded (electrically connected) to the system’s Ground conductor. The bonding point is located at the “derived source,” ashore. Boats connected to shore power systems should never have the neutral and ground bonded aboard the boat. Connections to outlets fed from single phase sources in the utility distribution system will receive service voltages of 120V/240V. Connections to outlets fed from the phase legs of three phase sources in the utility distribution system will receive service voltages of 120V/208V.

Figure 1 shows a typical single-phase residence or dock source fed from a street transformer.  The “secondary” of the transformer is the feed’s source:


Figure 1: Single Phase Transformer with 3-Pole, Center-Neutral Secondary to Customer

Figure 2 shows the common dock power distribution components found on docks at marinas, yacht clubs, boatyards, condos and municipal walls throughout North America. The “derived source” is defined by code to be the point where the Neutral-to-Ground Bond and the dock’s main disconnect circuit breakers are located.

Dock electrical system feeders must be designed to support a number of boats at the same time, which means the current-carrying conductors of the dock feeder need to be quite large. In the US, the National Electric Code, Article 555.12, specifies the ampacity calculations of dock feeders.


Figure 2: North American Dock Shore Power Layout – Typical

Figure 3 shows a simplified example of the most common configuration of 50A shore power outlets found on docks.  This example shows the 3-pole, 4-wire dock feeder with drops to six 208V/240V, 50A shore power receptacles.  The source for the dock feeder can be either 120V/208V or 120V/240V.  This wiring configuration is mandatory in order to support boats fit with 50A shore power services.  Note that the dock feeder is a direct electrical extension of the power transformer shown in Figure 1, and consists of the two energized conductors (L1 and L2), the Neutral conductor (N), and a safety ground conductor, (G).


Figure 3: Typical 240V, 50A, 3-Pole, 4-Wire Dock Feeder

Figure 4 shows a portion of a 120V, 30A shore power configuration.  In this example, each outlet provides 120V at up to 30A to the boat.  Note that adjacent outlets in this wiring configuration are alternately connected to the two energized legs, L1 and L2.  Since both energized legs are necessary for 208V/240V service, this would be the most common way to connect 30A outlets in the case of a dock with a large number of 50A outlets.


Figure 4: Desirable, Best-Case 125V, 30A Dock Wiring Usually Found on Docks with 50A Pedestal Outlets

Figures 5 and 6 show two alternative configurations for providing 30A shore power at a slip.  As in Figure 4, each outlet in Figures 5 and 6 provides 120V at up to 30A to the boat.  The difference in these examples is that all of the 30A outlets are connected to the same energized leg, rather than to alternate legs, of the dock feeder.  For 30A boats, this configuration is functionally equivalent to the example in Figure 4.  Boats requiring two 30A shore power services will never notice or be affected by the difference between the configurations shown in Figure 4, Figure 5 or Figure 6.


Figure 5: Alternate “One” for a 125V, 30A, 2-Pole, 3-Wire Configuration, with Only One Energized Dock Feeder Leg Used for Power


Figure 6: Alternate “Two” for a 125V, 30A, 2-Pole, 3-Wire Configuration, with Only One Energized Dock Feeder Leg Used for Power

For boats that require 208V/240V shore power services via a 50A, 4-Wire shore power cord, a “Smart Wye” splitter can provide shore power from the example shown in Figure 4, albeit at reduced total amperage capacity of 30A, total.  However, the “Smart Wye” will not deliver any power at all if connected to the configurations shown in Figures 5 and 6.

Figure 7 shows the electrical diagram of a “Smart” Reverse Wye Splitter.  To the left are two 30A male plugs which are fit to  30A pedestal outlets.  On the right is a 50A female, which  receives the boat’s regular shore power cord.  In the box at the center, a relay is used to forward power from the pedestal outlets (dock feeder) to the shore power cord.  A 208V/240V relay, K1, is connected between the two energized conductors of the two incoming 120V, 30A lines.


Figure 7: “Smart Wye” Reverse Splitter

Figure 8 shows a “Smart”€ Reverse Wye connected to the dual-leg dock feeder wiring configuration (as previously shown in Figure 4).


Figure 8: Smart Wye Reverse Splitter Connected to Both Energized Legs of a 208V/240V Shore Power Service

This wiring alternative places 208V/240V on the relay coil, K1. The relay “picks,”€ meaning the contacts of the relay close, and this allows 208V/240V shore power to feed through the splitter to the boat. With this adapter, 208V/240V appliances will work. Because the input source is limited to 30A, boat loads may need to be manually limited and controlled to avoid drawing more than 30A. However, with attention, most boat equipment can be used successfully, even if not at the same time.

Figures 9 and 10 show the Smart Wye Reverse Splitter connected to the two alternative single-leg dock feeder configurations. In these cases, since both 30A inputs are connected to the same energized dock feeder leg, there is no voltage between them; that is, zero volts. Since the relay requires 208V/240V to operate, the relay in this case does not “pick,” and no power at all is allowed to pass to the boat.


Figure 9: Both Smart Wye 30A Connections Made to L1


Figure 10: Both Smart Wye 30A Connections Made to L2

In cases where only one energized dock feeder leg is available, the only way to get any shore power at all to this 50A boat – however limited – is with another type of power adapter. Options are available. To understand the options, it is necessary to first understand how the branch circuits aboard the boat are wired.

Figure 11 shows an incomplete but representative view of a 208V/240V boat electrical system. Although I have modified the diagram, credit for the base is to the American Boat and Yacht Council (ABYC), Annapolis, MD. This diagram shows a “typical”€ AC shore power configuration for a boat built with a 208V/240V, 50A AC power system, and found without a polarization/isolation transformer.


Figure 11: Partial View, Representative of a “Typical” 208V/240V Boat Electrical System

The left side of Figure 11 shows the dock feeder discussed above. At the center-right of the drawing, the AC power buss shown in colors is the AC power buss of the wiring of the boat. All boat equipment gets power from the boat’s AC buss via branch circuit breakers. The 120V utility outlets on a 208V/240V boat can be attached to either one of the energized conductors; to L1 alone, or to L2 alone, or some to L1 and some to L2. The drawing shows two utility outlets. The top outlet is fed by L1 and the bottom outlet is fed by L2. Any adapter that’s used to supply some limited power onto a 208V/240V boat must provide that power to both L1 and L2.

Figure 12 shows a 30A-to-50A adapter that will accomplish the goal. Power from one of the energized dock feeder legs is brought through the pigtail to feed both the L1 and L2 blades of the 50A receptacle. Each 50A receptacle blade will have 120V, but because they are fed from the same point, there will be no 208V/240V power.


Figure 12: 30A-to-50A Straight Adapter

Commercial straight pigtail adapters like this are available.  Power is limited to 30A, total.  With this adapter, 208V/240V appliances will not work, but important 120V refrigeration, lighting, entertainment systems and computers connected to 120V utility outlets will be OK as long as the total load is managed to be less than 30A.

Ground Faults: Difficult To Hire Troubleshooter

Several boat owners have ask how they can find an electrical technician who is qualified to troubleshoot ground fault issues on their boats. The answer is, it can be quite tough.  This post will describe the reasons service technicians see this work as bad business.

To review, this overall problem is the result of “backwards incompatibility” between the “real world” as it exists today vs. the noble goals of the National Electric Code (NEC) code and standards writers.  Starting in 2011, the NEC, Article 555.3, requires Equipment Protective Devices (EPDs) on marina docks and at boatyards.  I use the term “ground fault sensor” for EPDs and several other similar devices.  The 2017 edition extends the requirement for ground fault sensors to single-family residential docks.  By requiring ground fault sensors on docks and at boatyards, the NEC standards writers have caught many dangerous problems on boats.

The widespread rollout of ground fault sensor technology on docks is making boating more safe for all of us, and that will only continue as time proceeds into the future. Many boats in the pleasure craft category have had ground fault and leakage fault problems aboard for many, many years. Up to now, these faults have been silent, hidden and non-symptomatic and boat owners have typically been unaware of the presence of these problems unless they led to a fire or injury.

Ashore, these NEC changes have also been disruptive, expensive and frustrating to marina and boatyard operators. Facility upgrades are very expensive, and these changes add significantly to that cost. Once upgrades are completed and the facility is re-opened for business, marina and boatyard operators find themselves faced with complaints about their dock electrical service from unhappy boaters.  To an unskilled boat owner, the argument is: “My boat has been fine for years!  YEARS!  I don’t lose power ANYWHERE ELSE!  This is the marina’s problem!  Fix it!”

Marina operators generally do not have marine-skilled electricians on staff, so boater complaints result in referrals out to the electrical contractor who performed the facility upgrade. The correct response to Mr. Boater: “Sir!  You have one or more problems aboard your boat!”  I’m sure we can all appreciate how well that message is received by some owners!  Ultimately, lots of professional time is wasted, and no one is happy.

Ground faults on boats are often directly caused by work that was previously done – incorrectly –  by the boat’s owner!  Always, ground faults on boats result from failing to know and comply with the ABYC electrical standards for boats.  Marine electrical technicians with the skills to sort through ground fault and leakage fault symptoms and who can troubleshoot these problems are absolutely overwhelmed by their current workload, and this will only get worse in the future as more and more facilities upgrade their shore power systems.  This spike in ground fault troubleshooting workload is entirely IN ADDITION TO the normal types of workload these technicians would otherwise be staffed up to handle.  Demand for these skills far exceeds supply. Furthermore, troubleshooting ground fault/leakage fault problems is not work for beginners.  Diagnosis of these problems involves advanced troubleshooting skills that take time and experience to develop.  (Analogy: Oncologist vs. Family Practice physician).

Complicating the problem is the fact that the vast majority of boat owners don’t know anything about electricity or electric circuits.  Many boat owners are afraid of electricity in all forms.  In fact, when a professional does encounter a knowledgeable layman, the technician may doubt that the layman actually knows what he’s talking about; most laymen don’t, and that is the technician’s life-experience.  So, all this results in largely uninformed and unskilled customers asking for the most advanced and complex kinds of professional services.

Troubleshooting ground faults on a boat is not good business for the marine electrical technician.  When the technician is all finished with the very complex and tedious work he’s done, to the boat’s owner, everything is exactly the same as it always was.  The boat owner is presented with a bill – maybe a big bill – for the complex, tedious and highly skilled work, and there is nothing except the intangible of a “safety improvement” that this owner receives in return.  There are no shiny new LED fixtures, no neat new trash compactor, no nice new HDTV, no new decor lighting, no improvements in heating and cooling efficiency.  Nothing new and glitzy!  Just the same old, same old.

To repeat myself, Troubleshooting ground faults is not good business for the marine electrical technician.  There are very few common themes in diagnosing ground/leakage faults on boats, so virtually every boat requires a customized approach to troubleshooting.  Very few boaters have electrical diagrams of their boats, and what few diagrams that are available are often incomplete and do not contain the low-level detail of wiring for things like reverse polarity detectors or the active control module on a galvanic isolator.  And certainly not for detachable items like user-supplied surge suppressors.  Every technician knows that each of these service calls will probably take a lot of diagnostic and repair time.  At the technician’s billable rate, that translates to a big bill for the boat owner.  Big bills translate into unhappy boat owners.  Unhappy boat owners translate, for the electrical technician or the business manager, into billing disputes, “no-pay” or “slow-pay” customers, and the legal falderal that goes with all that stuff.  In short, everything about this work, from the technician’s point-of-view, amounts to “bad Karma. It should be no surprise, then, that many technicians are refusing this work.

So, to the question: “how can an amateur with minimal knowledge look for [ground fault] problems?”  In some locales, it’s going to be very difficult.  I know of good electrical shops that discourage or refuse this work, either through outright refusal or premium pricing that discourages the boat owner. Boaters will have to keep looking until a technician is found who is BOTH skilled AND wiling to take the work.  What I would recommend is to “ask around” both online and in the local market for references to technicians that other boaters respect and recommend. If the name of a person who’s particularly well respected comes up, and they accept the work, it might make sense to move the boat to their home area just to get that level of excellent service.  (Analogy: findings a doctor or dentist or auto mechanic when you move to a new area.) Some boaters may be fortunate enough to have an established relationship with a qualified technician. If so, get on the work schedule as soon as possible and bite the bullet.  To allow for getting on the technician’s work schedule and for the necessary onsite diagnostic time, boaters should assume and plan that this work will take a few weeks at dockside. Anticipate delays in day-to-day activity. The reality is, emergencies will happen and will have a higher priority for your technician.

The ABYC website at has lists of certified technicians by geographic area. Check to see if recommended technicians hold ABYC certifications.   Some non-ABYC technicians may be able to do this work, too, of course, so in the same way you might ask a surgeon how many surgeries he’s done of the type you need, ask the technician about his/her experience troubleshooting ground faults.  Finally, in general, for better or worse, avoid residential electricians; as a group, they won’t understand the marine environment.  In fact, they do things in residential wiring that will CAUSE ground faults; things that SHOULD NEVER BE DONE ON BOATS.

Ground Faults and Dockside Ground Fault Sensors

Major addition: “Test Tools,” incorporated 12/13/2015.
Major addition: “TOPIC: Isolation Transformers,” incorporated 1/18/2016.
Major addition: “TOPIC: Shore Power Cords,” incorporated 5/24/2019.
Major addition: “TOPIC: Inverters and inverter/chargers; an obscure cause of trips,” incorporated 9/5/2019.
Major addition: “TOPIC: Harmonic Distortion on the Electric Utility supply,” incorporated 4/29/2020.
Major addition:TOPIC: Neutrals MUST NOT be connected to Ground aboard the boat,” incorporated 6/6/2020
Major Addition:TOPIC: Neutrals connected together on boats with two 120V, 30A Shore Power Inlets,” incorporated 6/6/2020.
Major Addition: “TOPIC: Shore Power Adapters (a/k/a ‘Splitters’)”, incorporated 6/6/2020
Editorial Addition: Added “Table of Contents;” minor text edits; 10/22/2021.
Major Addition: “TOPIC: Neutrals connected together on boats with two 120V/240V, 50A Shore Power Inlets,” incorporated 5/20/2022
Editorial Corrections: “TOPIC: Shore Power Adapters,” incorporated 6/3/2022

Topic Contents:


The NEC is updated in a three-year cycle, so the cycles that affect US boaters are 2011, 2014, 2017, 2020, 2023, etc. In the United States, the 2011 revision of the National Electric Code required Ground Fault Sensing equipment at marinas, boatyards, condo docks, municipal docks and other marine facilities shared by multiple-users. The 2017 NEC added the Ground Fault Sensor requirement to private, single-family, residential docks. The rollout of 30mA Equipment Protective Device ground fault sensors (EPDs) at marinas and boatyards is having a big impact on many boats and boat owners. In some places, even more sensitive 5 mA Ground Fault Protection devices (GFP) have been installed. These are the ubiquitous Ground Fault Circuit Interrupters (GFCIs) used in residential and commercial 120V settings. The more sensitive the ground fault sensor, the more likely it is that the conditions discussed below can cause both real and nuisance power interruption problems for cruising boat.

ALL PROPERLY-WIRED BOATS leak very small amounts of power.  This becomes a huge concern for marinas, because several boats, each with leaks that are too small to trip 30mA pedestal breakers, can combine together to trip the 100 mA dock feeder.  If that happens, the entire dock goes down, and all boats lose power.  The “last guy in” gets the blame, but it’s not their fault.

Man-made wiring errors (circuits mis-wired by unqualified personnel) are very common and, because they are constantly present and detectable until they are corrected, are fairly easy to isolate and identify. Some ground faults are “transient” and can appear to “come and go.” Transient ground faults can be difficult, time-consuming and expensive to isolate.

Based on recent experience on several boats, I am more confident than ever that a substantial percentage (20%-50%) of the [pleasure craft] fleet does have man-made wiring errors aboard. In the past (before ground fault sensors), the majority of man-made wiring errors have been hidden, silent and non-symptomatic aboard boats. Not “safe,” but invisible.  These same errors will cause ground fault sensing circuit protection devices on docks and at boatyards to trip AC power “off.” Some of these problems originate with non-complying OEM component designs and selections, so even newly purchased, “straight-from-the-factory” boats are not necessarily free from the possibility of denial of power. Boats manufactured “offshore” may also not be compatible with North American electrical standards.

This article will highlight some ground fault causes that might require service attention from a boat owner. Not all of these conditions will affect all boats. Transient conditions will NOT necessarily affect boats that also have man-made wiring errors. But for those that are affected, awareness of these possibilities will help with problem isolation and correction. Certainly, transient conditions – if present – will complicate troubleshooting of man-made wiring issues on boats.

This article is intended for two audiences:

  1. The “DIY guy” (or gal) who self-describes their personal electrical skills as “high” or “advanced;” or
  2. All others; that is, all boat owners who experience power loss issues and need to hire professional help to resolve the problem.

In the first category, resolving issues will occasionally require contact with “hot” AC circuits, which IS NOT for those without the skills and training to do so. In the second category, the idea is for the boat owner to understand how complex these issues are and to understand what the professional technician is going to need to do to diagnose and correct them. In that second category, there’s gonna be a potentially big billing, and boat owners paying those bills should understand at the top level just what is being done for that money.  Hiring professional help to clean up these issues can be VERY DIFFICULT.  For the technician who does electrical work for a living, this work IS NOT “good business.”


The Fort Pierce, Florida, City Marina completed a multi-million dollar major expansion project in 2016.   The new floating docks at FPCM are equipped with Square D 125V/250V, 30mA Equipment Protective Device (EPD) ground fault sensing circuit breakers located at the slip-side pedestal.

Sanctuary is fit with two 125V, 30A shore power circuits.  When we arrived at FPCM, the dock attendant who landed us suggested we have all AC branch circuits on both panels (“house” panel and “heat pump” panel) set to “off.” That advice is the right advice for all boats in all cases. It is particularly useful for connecting the first few times to docks with ground fault sensors on their pedestals. After attaching to the pedestal, set individual branch circuits “on” one-at-a-time. If the pedestal breaker should trip while powering up, take note of which branch circuit breaker caused the trip. It will become obvious in reading this article why that information is very important to know and valuable to have.


TOPIC: Neutrals MUST NOT be connected to Ground aboard the boat

The ABYC E11 Electrical Standard (AC and DC Electrical Systems on Boats) requires that there be no neutral-to-ground bond(s) aboard the boat. For clarity, that absolute statement can be modified to read, “there must be no neutral-to-ground bond(s) aboard the boat when operating on shore power.”  The underlying principle is, the neutral-to-ground bond should ALWAYS be placed as close to the source of AC power as possible.  In shore power systems, that location is ashore, in the facility’s electrical infrastructure.  So for a boat operating on shore power, If there were a neutral-to-ground bond on the boat, that wrongly-placed bond would create a connection between the neutral conductor and the ground conductor that electrically parallels the two conductors all the way back to the dock-side infrastructure’s correct ground bond. Since the ground conductor on the boat is in direct contact with the sea water in which the boat is floating, this also parallels-in a ground path through the sea water.

The reason this is a problem is that electricity with follow ALL AVAILABLE PATHS to get back to its source.  When several paths are connected in parallel, current that should flow only on the neutral will divide and flow in some amount (proportional to the impedance of the path, based on Ohm’s Law) on all available paths;  so current will flow, in some amount, through the water itself. By definition, this condition is a “ground fault,” and it will trip power “off” if there are ground fault sensors on the dock-side pedestal, but it can also kill people, pets and wildlife in the water. Incorrect neutral-to-ground bonds on boats are a primary cause for AC power leaking into the water, and can lead to incidents of ELECTRIC SHOCK DROWNING. For further information, readers are referred to my article on “Electric Shock Drowning.”

The correction to this problem is to separate the neutrals from the grounds.  Depending on the configurations of the boat electrical system, this can be a complex task.  This problem is frequently created by residential electricians doing work incorrectly in the context of a boat.

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TOPIC: Neutrals connected together on boats with two 120V, 30A Shore Power Inlets

If a boat is served by two 120V, 30A circuits, it is essential that the Neutrals from Shore Power Circuit 1 and the Neutrals from Shore Power Circuit 2 be SEPARATED aboard the boat. The reason is, both of the neutrals run back into the marina pedestal, and may run all the way back to the marina main service panel. If they are connected together on the boat, they become electrically paralleled all the way back to wherever they are ultimately joined together (pedestal junction, panel neutral buss, etc). AS described above, even though the two circuits are not drawing the exact identical amount of current at the same time, all of the current returning from both circuits on the boat will combine at the neutral connection and divide to flow back ashore in equal amounts on both neutrals. For example, assume one circuit is drawing 6A and the other circuit is drawing 14A. Since the neutrals are connected together, the total of 20A coming onto the boat will divide, and each neutral will carry 10A back to the source ashore. In shore power Neutral 1, 10A returning does not balance the incoming 6A, and in shore power Neutral 2, 10A returning does not balance the incoming 14A. By definition, that is a “ground fault” condition as sensed by the pedestal ground fault breakers, which will trip both breakers and interrupt power to the boat. This will happen EVEN IF no branch circuits on one of the shore power panels are powered “on.” All that is needed for this failure to happen is that both cords are plugged in.

The correction is to separate the neutrals from shore power circuit 1 and the neutrals for shore power circuit 2 aboard the boat.

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TOPIC: Neutrals connected together on boats with a 120V/240V, 50A Shore Power Inlet

Many “50A Boats” have lockout switching for transferring power between shore power and generator power sources for onboard use.  Typically, the shore power feed pair and the genset feed pair consist of two mechanically interlinked 120V circuit breakers (a double-pole breaker).  This mechanical interlocking arrangement ensures that only one of the sources can be connected to the boat’s power panel at any one time.  “Fifty Amp Boats” will also sometimes have shore power inlets at bow and stern locations.  In that case, there may be more sets of sliding interlocks, but the ground fault issue remains the same.  The issue is, 120V/240V circuits are three-pole circuits (L1, L2 and N), and paired 120V breakers only switch the hot conductors, L1 and L2.  THEY DO NOT SWITCH THE NEUTRAL CONDUCTOR, and so the neutral originating from shore power and the neutral originating from the genset remain connected together all the time.

This situation is a true ground fault on the boat, and it will trip a 120V/240V, 50A pedestal ground fault sensor.  As I have discussed before, a neutral-to-ground bond must be made as near to the originating source of the power feed circuit as possible.  For shore power, that’s in the shore power infrastructure, at the main distribution panel.  But for the genset, that neutral-to-ground connection (bond) is made at the generator.

So assume a properly wired generator on a boat, and a proper neutral-to-ground bond made at that generator.  Since ONLY the L1 and L2 conductors are switched between shore power and generator sources, and the shore power and generator neutrals are permanently connected together, then when on shore power, the generator’s neutral-to-ground bond creates a true ground fault AS SENSED BY THE SHORE POWER DELIVERY CIRCUIT.  If the shore power pedestal is fit with a ground fault sensing breaker, that breaker will trip.

The fix is to replace the two-pole interlocking mechanisms with a rotary selection switch that switches all three poles of the 240V circuit (L1, L2 AND N) between as many sources as there are feeding the boat.

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TOPIC: Shore Power Adapters (a/k/a “Splitters”)

Splitters come in two use cases:
1.   For a boat that is fit to get shore power from two 120V, 30A shore power inlets, if only a 240V, 50A pedestal outlet is available, a “forward-wye” splitter can be used to convert the single 50A pedestal outlet to two 30A receptacles that will accept the 30A shore power cords.
2.  For a boat that is fit with a 240V, 50A shore power inlet, a “Reverse Wye” splitter can be used to adapt the boat’s shore power cord to two 30A pedestal receptacle outlets.

These two use cases present special problems and special opportunities in marinas where the pedestal breakers are fit with ground fault sensors.


If a boat with two 120V, 30A shore power circuits trips a pedestal ground fault sensors, one possible cause is that the neutrals are connected together aboard the boat (discussed above).  The electrical behavior of 240V circuits is different than the electrical behavior of 120V circuits.  I explain that in this article.  Even though this is a wiring error and needs to be corrected, if the option of connecting to a 240V, 50A shore power pedestal outlet via a splitter is available, the splitter will serve as a temporary work-around to mask that issue.  In this case, the paralleled neutrals are joined back together in the splitter body, downstream of the 240V, 50A pedestal breaker, so the 50A breaker sees the current flowing back to the shoreside source as balanced.


The reverse situation can not be circumvented by any method I know of.  Because of the installation of ground fault sensors on pedestals, this is a PERMANENT RESTRICTION for the rest of time.

If a boat fit with a three-pole, four wire, 240V, 50A shore power inlet (black, red, white and green wires) gets power from a reverse wye splitter attached to pedestal outlets with 120V, 30A ground fault sensing breakers, this cannot be successful.  In this configuration, the reverse wye splitter gets it’s power from TWO 30A GROUND FAULT BREAKERS.  Since EACH breaker is looking for balanced current between its hot feeding conductor and its neutral returning conductor, and that situation can never exist in this configuration, this arrangement CANNOT work.

If one feed (L1, black) is providing 6A, and the second feed (L2, red) is providing 14A, the numerical value of the “shared current” between L1 and L2 will return in the opposite L1 or L2 conductor.   In this example, both L1 and L2 “share” 6A.  The “difference current” between L1 and L2 will return in the neutral conductor, so in this example, the difference is 14A – 6A = 8A.  The single 50A neutral of the boat divides into two 30A neutrals inside the reverse wye splitter, providing a parallel path for neutral current back to shore.  Current follows all available paths, so each of the two paths will carry 1/2 of the total current.  This parallel connection is located downstream of the 30A pedestal breakers in the circuit.  The result is, the 3-pole neutral returning current (8A) will divide into two equal parts, and each 30A ground fault sensing breaker will see 8A / 2 = 4A of neutral return current.  This does not – and never will – balance with the current in the corresponding L1 or L2 hot feed conductors.  Specifically in this example, 6A delivered in L1 does not match 4A returning, and 14A delivered in L2 does not match 4A returning.  But overall in the 240V, 3-pole circuit of the boat, 14A of power supplied on one leg does balance 6A returning on the other leg plus the 8A returning on the neutral.

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TOPIC: Shore Power Cords

Any time any boat “trips” a pedestal ground fault sensor, the boat owner should perform simple testing to rule out issues with the actual shore power cord/cords. Shore power cords live in a very challenging physical environment. They are subject to strong UV, solar heating, humidity up to and including rain, airborne dust and dirt, insects; simply, all kinds of environmental insult. The cord ends of newly manufactured cords are injection molded, so new cords are relatively protected from water and dirt intrusion.

Thirty amp (30A), NEMA L5-30P/R “twistlock” connectors are the marine industry standard for 30A cords, and they are not particularly robust for the environment in which they are expected to serve. In particular, pedestal receptacles get rough treatment over time. It would be the rare boater, indeed, who has never seen blackened, discolored 30A plug blades resulting from high currents drawn through loose, corroded, weak twistlock connector connections.

Burned and damaged cord connectors are commonly repaired in the field with replacement plugs and receptacles which are made and sold by reputable electrical equipment manufacturers. By their very nature, these replacement connectors can’t be injection molded, so there is “empty” air space within the replacement connector housings. Even when weather boots are installed over them (as they always should be), replacement cord ends are vulnerable to water intrusion and environmental contaminants. Air and waterborne salts and other contaminants can and do collect inside connector housings.

Over time, it is quite possible for salt dissolved in the air and seawater to find its way into cord connectors. When infiltrate water later evaporates off, salt (NaCl; ordinary “sea salt”) is left behind. This salt can “bridge” between the blades and conductors of the connector, and form high resistance “short circuits” WITHIN the connector body. Again, as time goes on, it’s quite possible for these salt “bridges” to carry enough current to trip a 30mA pedestal ground fault sensor.

If a boat trips a pedestal ground fault sensor, disconnect the shore power cords AT THE BOAT END. With just the cord(s) plugged in to the pedestal, reset the tripped breaker and turn it on. If it trips again, the cord itself is the cause of the ground fault, and will need to be cleaned, repaired or replaced.

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TOPIC: Inverters and inverter/chargers; most common cause of trips

One major governing concept for all AC distribution systems in North America is that the neutral conductor of the system must be bonded to the safety ground AT THE SOURCE POINT of the AC power. For shore power (AC power source ashore), the neutral and ground are bonded together in the shore facility’s shore power infrastructure. An ABYC corollary is, for boats operating on shore power, the neutral and the safety ground MUST NOT be connected together aboard the boat. However, for inverters operating in “invert” mode (AC power source onboard), the neutral and safety ground MUST be connected together at the inverter, aboard the boat. In the shore power case, the bond can’t be on the boat. In the inverter case, the bond must be on the boat. Contradiction? No, it’s entirely consistent. In all cases, the neutral-to-ground bond is at the AC power source.

An onboard inverter which is integrated into the boat’s electrical system must comply with the neutral-to-ground bonding requirements in the manner described by ABYC, E-11 (that is accomplished by compliance with UL458). Modern UL458 compliant inverter devices have a power transfer relay inside the device. When operating in “invert” mode, the relay joins the boat’s onboard neutral (white) and ground (green) electrically together to create the required bonding connection. The relay disconnects the onboard bonding connection when the device has shore power and is operating in “passthru” mode. The operation of the relay maintains compliance with North American electrical standards. The details of how this works are described on this website in my article entitled : “AC Electricity Fundamentals – Part 2: The Boat Electrical System.”

Inverters (or inverter/chargers) that are fully integrated into the boat’s electrical system will create a very short duration transient ground fault when shore power is first applied to the boat. At the exact moment – the very instant – the boat is connected to shore power, an inverter operating in “invert” mode will have the boat’s onboard neutral and safety ground connected together through the bonding relay. As viewed from the pedestal ground fault sensor, that condition is a true ground fault. In normal operation, when the inverter “sees” shore power, it transfers out of “invert” mode into “passthru” mode. The internal ground transfer relay removes the neutral-to-ground bond, and thus clears the ground fault. The ground fault sensor at the pedestal will not trip unless 1) the ground fault exceeds 30mA and 2) persists for longer than the trip-time of the pedestal ground fault sensor. So for boaters and service technicians, the specification and operation of an inverter’s transfer interval is important. That relay transfer-time should be in a range less than 25mS. If the transfer relay is slow (due to poor equipment specification, environmental contamination or age), or if manual switching for the inverter is required, the transient ground fault may/will persist long enough to trip the pedestal ground fault sensor.

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TOPIC: Shore Power Transformers

The input side of an isolation transformer (primary winding) connects to the shore power pedestal. The output side of the transformer (secondary winding) supplies the AC power panel and attached loads on the boat. The “connection” between the primary winding and secondary winding of the transformer is via an alternating magnetic field that rises and falls with the rise and fall of the shore power primary voltage waveform. With an isolation transformer, there is no continuous electrical connection between the shore power ground and the boat’s ground system. Like an onboard inverter or an onboard generator, an isolation transformer is treated by the ABYC electrical standard as an “onboard AC source,” and so the neutral and ground of the transformer output (secondary winding) must be bonded together on the boat.

When a boat with an isolation transformer arrives at a dock after an outing, there is no alternating magnetic field present in the transformer. At the instant that shore power is applied to the transformer, there is a very high instantaneous “inrush,” or power-on surge, of current. The inrush current can be 10 to 15 times higher than the rated working current of the transformer. For large transformers with low winding resistance, inrush currents can last for several tenths of seconds until nominal operating equilibrium is reached.

In addition, all magnetically coupled devices (transformers, motors, generators) experience small, naturally occurring internal leakage currents. These leakage currents are proportional to current flow, and so are also proportional to inrush-related current spikes.

One docks, pedestal circuit breakers with ground fault sensors provide two functions.

1. Protect against electrical overload currents. The nominal overload set-point is 30A for 125V circuits and 50A for 240V circuits.
2. Protect against leakage currents. The nominal leakage current set point is 30mA.

The moment that power is applied to a transformer, there is a huge inrush current that creates the magnetic field within the transformer. The inrush current looks like a spike to the electrical system. That spiking electric current must stabilize within the time-interval design limit of the shore power ground fault circuit breaker before the breaker decides to trip “off.” Think of this as a “race” between the inrush phenomena reaching equilibrium and the design trip setting of the circuit breaker. The question becomes, does the inrush transient of the transformer fall to a level that is within both the overload and leakage current trip criteria of the breaker in a sufficiently short time to avoid having the breaker trip power “off?” Emerging evidence seems to suggest that there are some instances where the circuit breaker “wins” and trips power “off.”

A friend has a boat with a 50A, 240V shore power input to a Charles Industries IsoBoost Isolation Transformer. The IsoBoost never trips the 50A over-current set-point of conventional marina pedestal breakers. Not anywhere; not ever. So, the overload spike transient does fall within breaker tolerances sufficiently quickly. However, even with all onboard load circuit breakers set to “off,” that transformer routinely trips pedestal breakers containing 30mA, 100mS ground fault sensors. On that boat, the Charles IsoBoost inrush spike does not resolve itself within the trip interval of the ground fault sensor, so successful connection to ground fault protected shore power is not possible. The overload tolerance of the breaker is longer than the ground fault trip tolerance. Charles Industries has developed a “SoftStart” module that clamps and limits the magnitude of the inrush current. That “SoftStart” module is the solution that Charles recommends for tripping ground fault sensing breakers.

It is highly likely that isolation transformers from other manufacturers may also be affected by this inrush spike phenomena. The “ground fault” in this scenario may result from capacitive coupling between the transformer windings and/or ground, or of inductive coupling through the electrostatic shield to ground, or a mix of factors. Whatever, it really doesn’t matter to an affected boat owner. It is something that can be mitigated by design improvements in the future, but those with affected transformers today will have to find work-arounds such as the Charles “SoftStart” module.

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TOPIC: Galvanic Isolators

Galvanic Isolators are devices that are installed aboard the boat. They are electrically located in series with the boat’s safety ground (green wire) at the shore power inlet(s). Galvanic Isolators contain a diode pack (full-wave bridge rectifier) that blocks small DC galvanic currents but allows larger AC fault currents to flow.

There are three “generations” of Galvanic Isolator technology.

  1. The first generation device consisted of a passive diode pack. That passive diode pack was subject to electrical damage by an overload or surge, and that damage could leave the boat’s safety ground conductor electrically non-conductive to AC fault currents. Many of these 1st generation GI devices are still in service. Unknown to their owners, some percentage of those 1st generation devices are inoperative. That is a potentially serious AC safety risk.
  2. In response to an ABYC standards revision, GI equipment manufacturers developed a “second generation” device. The 2nd generation device utilized an electronic control module to periodically “test and verify” the electrical integrity of the diode pack, and verify the integrity of the ground connection. This 2nd generation equipment creates a transient ground fault, and is incompatible with the emerging presence of shore power ground fault sensing equipment on marina docks.
  3. The newest third generation GIs are of the “failsafe” design. They have both a diode pack and a large capacitor, and no longer have electronically active test modules. Third generation devices are designed so that they will not fail in an electrically non-conductive state. If they fail, they fail in an electrically conductive state. In that state, the boat may lose DC galvanic protection, but WILL NOT lose AC safety ground continuity.

Many boats are still fit with second generation Galvanic Isolators manufactured between approximately 2002 through approximately 2008. One such unit is the Professional Mariner (ProMariner) Prosafe 1, which is the device I installed on Sanctuary when we bought our boat. At the time these devices were developed, impressing a small ground fault current on the ground conductor was not a concern in marine shore power systems, because marine shore power services did not have ground fault sensors. Thus, using an intentional ground fault was a viable approach.

The Prosafe 1 tests the ship’s ground connection when the device is first connected to shore power, and periodically thereafter at regular (3-hour) intervals in regular operation. The Prosafe 1 monitor detects the presence of the impressed ground fault, thereby confirming the integrity of the boat’s ground connection through the Galvanic Isolator diode pack and into the shore power infrastructure. The Prosafe 1 ground fault current is specified at 30mA, and it can last a variable period up to several seconds. The net is, that ground fault can cause a shore power pedestal’s ground fault sensor to trip. In my personal experience aboard Sanctuary, the symptoms have been variable depending on the electrical integrity of the ground conductor itself. At Chesapeake City, MD, I was able to connect to the two 30A receptacles but not to the 50A receptacle through my splitter. At Ft. Pierce, FL, I was not able to connect to a 30A receptacle, but was able to connect to a 50A receptacle through a splitter. HOWEVER, at both Chesapeake City and Ft. Pierce, Sanctuary would trip the pedestal breaker at random time intervals. Sometimes the monitor’s “test pulse” would not cause a trip, and sometimes it would. During the overnight period, Sanctuary tripped the shore power breaker, on average, twice. But I was able to reset the breakers.

ProMariner Technical Support has confirmed the above description. ProMariner acknowledges the problem. The Prosafe 1 GI device is now discontinued and “obsolete,” so the company’s advice is, “upgrade the Galvanic Isolator to a ‘failsafe’ design.” I wanted to preserve my investment in the Prosafe 1, so I installed a 4-pole switch that removes the incoming 120V power feed to the test module. I labeled the switch “Enable” when ‘on,’ and “disable” when “off.” At marinas with Ground Fault sensors, I disable the device. Disabling the test monitor DOES NOT affect the operation of the diode pack, so I still have both blocked DC galvanic currents and a safe AC ground path. But in 2020, the majority of marinas do not yet have ground fault sensing pedestal breakers, so I can enable the unit and use it as intended in most places. That works for me. Eventually, the Galvanic Isolator will need to be changed out, but not yet. For other users less determined than I, ProMariner’s technical advice is the right technical advice. The important thing is to be aware that this problem exists. This issue can cause unexpected results and RANDOM loss of power on an otherwise properly wired boat at docks equipped with ground fault sensing circuit breakers.

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TOPIC: Equipment Aging (esp: Hot Water Heater)

The heating element of a hot water heater, by design, lives in a pool of stored water.   That water provides a path for an electric current to flow from the heater element, through the water, to ground (via the plumbing connections) to the frame of the device.   As water heater elements age, and through many years of heating/cooling cycles, micro-cracks develop in the ceramic insulation of the heating element. Electrical contact between the live conductor of the heating element and the water in the heater’s tank will cause transient ground faults as the water heater cycles “on” and “off.”   The physical size of the contact area, the voltage present at the point of contact and the conductivity of the tank’s water (mineral content) will affect the magnitude of ground fault currents. This can be an elusive problem to isolate. If the water is also heated by a propulsion engine hot water heat exchanger, the water in the hot water tank will be hot enough at the end of a day’s outing that the water heater will not cycle when the boat is first connected to shore power. In that case, the ground fault will appear at some miscellaneous and random later time; maybe the middle of the night, maybe the next day, maybe at shower time.

Random equipment aging problems are common in battery charging equipment, household appliances like refrigerators, freezers and ice makers, and other motor-driven appliances like washer/dryers.   If a shore power ground fault sensor trips at random intervals, try cycling one piece of equipment “on” and “off” at a time to isolate the cause.

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SOMEWHAT OBSCURE GROUND FAULTS: (Transient or continuous)

TOPIC: Inverters and inverter/chargers

There is a new emerging inverter design that supplements power from shore sources when only limited sources are available.  For example, there are docks in many places which provide residential 120V, 15A/20A outlets.  These are not enough power to support very much load aboard most cruising boats, but is enough to charge batteries and run refrigeration.  The inverter feature is called “Power Assist,” and one line of devices that provide the capability is the Victron® MultiPlus™ line of inverters. 

Conventional 120V, 15A and 20A residential outlet sources are protected by very sensitive 5mA Ground Fault Circuit Interrupter (GFCI) sensors, not 30mA Equipment Protective Devices that are now becoming more common on docks.  When inverters with “Power Assist” supplement shore sources, the process begins when electrical load aboard the boat exceeds what the shore power source can provide.  The inverter “wakes up,” and  synchronizes its’ 60 Hz AC waveform with the incoming shore power 60 Hz waveform.  Different inverter manufacturer’s may have different designs for handling synchronization, but during that synchronization process, the two sources will almost certainly be out-of-phase with one another.  During that time, there is anecdotal data to suggest some inverters may trip the shore power GFCI.  The most significant variable is where the two waveforms are compared to one another in time when the sync process begins.  

This trip is a true “nuisance trip,” and does not represent an issue that “needs to be fixed.”  It can occur on 120V, 15A/20A circuits with GFCI and on 120V, 30A shore power sources with 30mA ground fault protection.  It can appear as a single, one-time “oops,” or it could present as a more persistent annoyance. Owners of this type of equipment should be alert to this tripping scenario.  Be sure you have access to the source’s GFCI before depending upon the “Power Assist” feature.  GFCI circuit breakers that are locked in a building will mean, once tripped, shore power will no longer be available.

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TOPIC: Harmonic Distortion on the Electric Utility supply

Like the topic “Inverters and inverter/chargers; an obscure cause of trips,” this particular cause is also “obscure.” I bring it to this article because it is a possibility. Harmonic Distortion of the Electric Power Grid is a huge problem for electric utilities, but the technical details are extremely complex and not worth discussion here. Suffice it to say, Harmonic Distortion on the AC service can happen at a level that could be troublesome in some marina environments. The symptom is that a boat without a prior history of tripping ground fault sensing pedestal breakers might suddenly encounter nuisance trips at one particular marina, or on one particular dock at a marina.

A ground fault sensing circuit breaker is an electromechanical device with an imbedded microprocessor. The microprocessor sums the current in the line conductor (black or red) and the neutral conductor (white), and if the result is not within the rated limit (30mA), it disconnects power from the boat. Due to Harmonic Distortion on the power line, a ground fault sensing microprocessor may not sum up high frequency harmonic components on the AC current waveform correctly, and may trip in error.

Note that this is NOT generally a problem on the boat (although electronic equipment on boats on docks can contribute to Harmonic Distortion on power lines). Proving this is the cause of miscellaneous trips is extremely difficult, and requires specialty equipment like the Fluke 1736 Power Logger (at somewhere around $5K) and long-term monitoring of the facility’s utility electric supply. The facility operator may be aware of other power quality issues, like burned up neutrals in 3-phase infrastructure wiring, or miscellaneous complaints of nuisance trips from other boaters. There is nothing a boat owner can do about the quality of incoming facility power. All a boat owner can do is relocate from the affected marina.  However, if all other causes have been meticulously explored, and the boat does not suffer nuisance trips at other places, this phenomena may be inferred to be the cause.

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TOPIC: Reverse Polarity indicators

Boats fit with 125V, 30A marine shore power services require a Reverse Polarity indicator.   A Reverse Polarity device detects, and warns the boat owner of, reversal of the incoming hot (black) and neutral (white) shore power conductors.   This is a very rare but very dangerous condition. AC power distribution panels and several aftermarket devices are built with reverse polarity detectors, so some boats may actually have several such RP detectors aboard. Electrically, they are all connected in parallel.   Both of Sanctuary’s factory-installed power distribution panels have them, our aftermarket Galvanic Isolator (Prosafe 1) has them, and our aftermarket Bluesea Systems Generator Transfer Switch has them.  The ABYC E-11 Standard calls for these devices to present at least 25KΩ of electrical impedance, but of course, several of these devices in parallel can result in much lower net impedance.   Since by definition, these devices are a “ground fault,” their net effective resistance, if too low, can cause random trips of ground fault sensors on docks. And, especially so in combination with other conditions.

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TOPIC: Cable TV and Wired Ethernet

Dockside cable wiring for TV and Internet services, and wiring for Internet service via DSL telephone lines, can cause transient ground faults on connected boats. Normally, cable services and telephone services at marinas are grounded at their point-of-entry to the marina property. HOWEVER, that point IS VIRTUALLY NEVER the same physical point where the shore power ground bond is established. That difference in connection point leads to a phenomena called “ground loops” between and among the dockside services. Ground loop currents can cause transient or continuous ground faults. These ground faults are prone to appear when AC electric demands are highest (hot summer days, cold winter days) on docks. Also, TV and telephone cables are less resistant to corrosion and environmental conditions than the heavy conductors of the AC shore power system. Boaters who use cable TV and/or wired Internet services and experience random trips of ground fault sensors should try disconnecting these services for problem diagnosis and isolation. The long term fix will most likely require the marina to get the various system grounds tied together at the bonding point of the AC power system ashore, but disconnecting those small signal wires from the boat will interrupt the ground fault path and may temporarily alleviate random nuisance tripping of pedestal shore power ground fault sensors.

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TOPIC: Surge Suppression Devices

With the advent of the computer age, surge suppressors have become ubiquitous in homes and on boats. There are whole-house surge suppressors that attach to the building’s incoming power line, and there are supplementary surge suppressors in many forms that attach to residential 125VAC power outlets. Computers, TVs, home routers and any number of electronic toys can be protected from transient spikes on power lines by these devices.  Some devices, such as Uninterruptible Power Supplies (UPS) used in home offices are built with their own internal surge suppressors.

The way surge suppressors work is by dumping surge energy “spikes” to ground. There are special diodes in the surge suppressor (Zener avalanche diodes or Metal Oxide Varistors [MOV] ) that bridge the hot current carrying conductor to ground. Recall here that the neutral current carrying conductor is already grounded in the shore power infrastructure. When a high-energy “spike” occurs, the diode is intended to conduct that transient “spike” energy to ground. Imagine that a boat is in the vicinity of a “near-miss” lightening strike. That lightening strike causes a large but short-lived energy spike on the electric power line. The onboard surge suppressor acts to ground that spike, thus “saving” attached computer and entertainment equipment from damage. This now becomes a discussion similar to that about damaged diodes in the diode pack of a galvanic isolator.

The diodes in the surge suppressor can be damaged in a way that leaves them partially-conductive.  MOVs, for example, can “melt” and remain in a state of variable conductive.  If shorted, an MOV will hopefully trip an overcurrent circuit breaker (OPD).   But partially conductive, it would not trip the over-current protector, and that would be an example of a true ground fault. In this scenario, the ground fault sensor at the dock pedestal will trip when the circuit breaker with the defective surge suppressor is powered “on.” If a utility outlet circuit causes a dockside pedestal ground fault sensor to trip, consider the possibility of a defective surge suppressor.  And, the more numerous these devices are on a boat, the more possible ground fault paths there are, so the more likely “normal leakage” might be to create nuisance trips.

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GROUND FAULTS CAUSED BY INAPPROPRIATE OEM EQUIPMENT CHOICES: (permanent fault; requires wiring fix/equipment replacement)

TOPIC: Generator Transfer Switch

Every boat with an onboard generator integrated into its electrical system in a manner prescribed by ABYC E-11 will have a selector switch (Generator Transfer Switch) to transfer the boat’s distribution panel(s) between shore power or generator power.   That switch MUST transfer BOTH the hot wires (red, black) AND the neutral wire (white).   The reason for the need to switch the neutral conductor lies with the grounding requirements for AC circuits, described above.   The neutral is bonded to the safety ground AT ITS SOURCE.   For shore power (source ashore), the neutral and ground are bonded in the shore power infrastructure ashore, and MUST NOT be connected together on the boat.   For generators, the neutral and safety ground MUST be connected together at the frame of the generator (source onboard).   To comply with the AC bonding requirement, the neutral, as well as the hot wires, must be switched by the Generator Transfer Switch.   Some boat manufacturers have used switches that do not transfer the neutral.   Some aftermarket installers, to save cost, have used switches that do not transfer the neutral. In the past, that was a hidden, silent, non-symptomatic wiring error. Now, the permanent leakage fault that it creates will trip pedestal ground fault sensors.

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TOPIC: Heat Pump Raw Water Circulator

Many boats with two 125V, 30A shore power inlets are wired so that “house” circuits are fed from one of the inlets and heat pump circuits are fed from the other inlet.   However, some boat manufacturers have heat pumps wired to both of the incoming shore power circuits.   In many cases, regardless of how their compressors are wired, heat pumps share a single 125VAC raw water circulator pump across multiple incoming shore power services. In normal operation, any time any of the multiple heat pumps aboard comes “on,” the shared raw water circulator also comes “on.”

The shared circulator pump is activated when any of the individual heat pumps call for heat or cooling.  The pump itself is energized via a controller [black box] that contains either mechanical relays or electronic switching.  The design of the controller must be handled in a way that does not interconnect (bridge, commingle) the two shore power neutral circuits on the boat. If the neutrals are bridged together aboard, that will cause a leakage fault that will trip shore power ground fault sensors.

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TOPIC: Neutral-to-Ground Connection Incorrectly Made Within A Household Appliance

Some household appliance manufacturers,  and some residential electricians, connect the neutral wire of the appliance to the green safety ground wire at the appliance.   That practice has it’s roots in older (1940s and 1950s) residential systems where there was no safety ground in the residential wiring.   In system without a safety ground, attaching the neutral to the appliance frame at the appliance provided some protection from some kinds of faults.   Today, that condition is called a “phantom ground.”   In modern residential  systems, it is an NEC code violation, and on a boat, it is a clear violation of ABYC E-11, in both cases because it results in a man-made ground fault.   If the affected  appliance is permanently wired into the boat’s electrical system, this condition will always and continually trip a pedestal ground fault sensor.   It does not matter if the appliance is powered on, nor does it matter if the circuit breaker feeding the device is set to “on.”   If the appliance is pluggable, physically removing the plug from the receptacle will clear the fault.

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After I put this article up as a post on my website, several readers asked if I could recommend tools that can test for ground faults and leakage faults. I would only suggest a DIY approach to ground and leakage fault diagnosis to people who self-describe their personal electrical skills as “high” or “advanced.” This work requires the technician to have contact with energized AC electrical circuits containing dangerous, life-threatening voltages. ONLY THOSE WHO – BY TRAINING AND EXPERIENCE – CAN SAFELY PERFORM WORK ON AND AROUND ENERGIZED AC CIRCUITS SHOULD ATTEMPT TO DO SO.   PERIOD.

Indeed, there are some test tools that allow technicians (and appropriately skilled boat owners) to detect the presence of faults on their boats. However, identifying the presence of faults aboard a  boat is only the first step, and actually one of the easiest steps, in overall remediation. The follow-on activities of 1) diagnosing cause conditions, 2) sorting out multiple simultaneous causation conditions, and 3) applying corrective actions require a thorough knowledge of boat AC electrical systems.   Many corrective actions for ground and leakage faults can lead to  re-wiring AC circuits; in some cases, re-designing AND re-wiring of systems may be required.   And as I’ve said before, diagnosing ground faults and leakage faults is a  “high” to “advanced” skill.

For those who are confident in their skills and ability to work safely, my first suggestion is to look at the “AC Safety Test” article on this site.   These tests will expose the presence of wiring conditions that result in faults due to wiring errors made by unqualified  personnel. If performed  as I have described them, they require minimal to no exposure to energized electrical circuits.

Three “test-tool” options that come to mind for follow-on self-diagnosis:

1.  There is a local group of businesses in Georgia (Marine Surveyors of North Georgia) that is making and selling a device they call a “Stray Current Sensor” (SCS).  It is a versatile test tool that can be used to track down ground faults.  The tool can accommodate EITHER 50A boats or boats with two 30A inlets.  It sounds an alarm, but DOES NOT trip off the electric service to the boat, when a ground fault is detected.  I like that as a test tool approach, because as a technician, I can keep working without having to reset the whole boat each and every time a fault is detected.  The tool is built upon one of the ABYC-compliant Equipment Leakage Circuit Interrupter (ELCI) current transformers (North Shore Safety Systems, PGFM Control Module, which by the way is also a solution I really like).  Here’s a link to the device:  As I read the MSNG website, it looks like boat owners could rent one of these tools to use for diagnosis and troubleshooting.  That would be a great solution for boaters possessing appropriate electrical skills.  The tool is really only needed on a one-time-use basis.  Once the boat is “cleaned up,” the tool isn’t needed any longer.  So, if the rental charge is reasonable, I’d seriously consider this option.

2.  Home Depot offers an electric panel that’s intended to provide electric service to home Spa pools:  It would be ideal for a test tool for a 50A boat.  It would be possible to wire this box as a tester for two 30A boat circuits, but that would require changing the double pole 50A GFCI breaker to two 125V GFCI breakers.  In either configuration, it would be necessary to add the necessary wiring and connectors, as this is just the raw box.  Hubbell and Marinco marine-rated 50A plugs and receptacles cost about $80 – $100 each. NEMA SS-2 50A male plugs and female receptacle fittings are also available at Home Depot that would be suitable for an OCCASIONAL USE, FAIR-WEATHER-ONLY USE, test tool, for a lot less than that.   Likewise, NEMA L5-30 male plugs and female receptacles are also available at Home Depot.  The breaker that comes installed in this box is a GFCI Personal Protective breaker with trip sensitivity of 5mA.  The ground fault sensors on docks are Equipment Protective Devices (EPD) with a 30mA trip setting.  The implication is, if you have BOTH 1) one or more significant ground faults AND ALSO 2) one or more of the transient types of ground faults, the sensitivity of this breaker could complicate using it as a diagnostic tool.  “Clean” boats will probably operate OK on 5mA breakers; there is some (as yet unpublished) experience that leads to that conclusion.  However, a completely safe boat may also have nuisance trips with 5mA GFCI breakers.

3.  As above in item 2, buy the Home Depot Spa box for the enclosure itself, and then replace the 5mA GFCI breaker with a 30mA EPD breaker.  However, the 30mA breakers are very pricey, and probably not available at big box stores.  MSRP prices for two pole, 50A, 30mA EPDs are in the range of $500.  Go to an electrical supply house to get one.  Electrical supply house counter prices would certainly be better than MSRP.  But, certainly not inexpensive. Most supply houses will sell to the public, but some may not; still worth investigating.  Also contact Ward’s Marine in Ft. Lauderdale, FL, for availability and price for a 50A, double pole, 30mA EPD.  Ward’s has – literally – “all things electrical,” including Euro and Asia form-factor stuff…


Man-made wiring errors can be hidden, silent and non-symptomatic. While they may remain non-symptomatic for many years, they should not be regarded as “safe.”

Electrical codes are intended to protect us when ABNORMAL things happen in electric circuits; when connections get corroded, when insulation fails or is abraded to expose the metal conductor, when wires get disconnected, when short circuits occur. Bridged (commingled) neutrals “work” when everything is in perfect order with good connections, but if one of those conductors fails, the other can become severely overloaded and becomes a fire risk. An open ground conductor means absent or compromised protection from electric shock.

In recent months, I have repeatedly heard that professional marine-certified electrical technicians are recommending isolation transformers to anyone experiencing ground fault problems on the boat. But, there’s a problem with that! Some of the conditions I’ve described above are benign from an electrical safety point-of-view, but some are dangerous. In the “dangerous” category, some create both fire hazards AND electrical hazards. These are a risk to owners, owners families and guests sleeping aboard boats, and to other boats located at the same marina.

It’s “bad business” for electrical technicians to take on the work of locating and correcting ground fault issues on boats. Labor cots are high. Billing disputes and customer satidfaction issues are part of the landscape for the technician doing this work. And yes, an isolation transformer will mask the problem and make the symptoms go away. But, an isolation transformer DOES NOT “correct” anything, so if there are legit safety issues aboard, they are still there and still a risk waiting to bite the boat owner. However “inconvenient,” FIX ISSUES. DO NOT MASK THEM with an isolation transformer!

Reference: An excellent reference article on GFCI, ELCI and GFPE technologies can be found in the publication “Electrical Contracting Magazine.