Category Archives: Boat AC Topics

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:

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“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!”

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

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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.

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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 if 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

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. 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 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

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.

Key Electrical Concepts For 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.

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.

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 not get as hot. 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 “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 on 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 to those circuits. When the boat is under way, and external power 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”

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 present a discussion of the AC power systems focused 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.

This article will assist readers in having confidence when talking about electrical topics with a professional, marine-certified, electrical technician, either designer or tradesman.


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 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 a 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 shock and electrocution safety. In order to connect a residential electrical system to “earth ground,” one or more interconnected rods of copper are driven into the ground. 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 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 home.

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 safety, but an earth ground is not necessary for electric circuits to operate. The term “common” is useful in electrical design. It is used among power distribution engineers and craftsmen to reference the conductor that returns current flowing in a circuit from the load to the source.  This is the purpose of the “neutral conductor” in AC electric systems. This conductor does not have to be “0” volts with respect 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.”

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.”  The most appropriate term in household electrical systems is “neutral.”  “Neutral” is a specific term that refers to the current-carrying return conductor of residential AC circuits, but it is not a specific reference to “earth ground.”

Direct 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. 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. 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.
  3. current – (ampere; amp) a quantification of the number of electrons flowing through a circuit at any one time.
  4. resistance – (Ohm) a characteristic of an electrically conductive material that tends to retard or impede the flow of electrons through it.
  5. 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.
  6. frequency – (Hertz) the number of times a wave goes through a complete cycle in a standard measurement time interval, usually one second.
  7. 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)
  8. source – the origin from which AC power emerges to energize a circuit.
  9. 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.
  10. common – a portion of a circuit connection or set of connections that creates a direct return path for electrons flowing in an electric circuit.
  11. 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.
  12. ground – a universal  standard earth reference voltage of “0” volts.
  13. fault current – an abnormal path for current flow, usually to ground.  Fault currents represent potentially dangerous conditions.
  14. 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 earth 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.
  15. GFCI (Ground Fault Circuit Interrupter) – an anti-shock safety device that senses leakage currents and disconnects the energized power source.
  16. AFCI (Arc Fault Circuit Interrupter) – a fire protection safety device that senses lose connections and disconnects the energized power source.
  17. GFP/EPD/ELCI (Ground Fault Protection/Equipment Protective Device/Equipment Leakage Circuit Interrupter) – similar to GFCI, but higher disconnect specifications.
  18. 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.
  19. 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.
  20. 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.
  21. Stator – the fixed coils of one design of AC generator, from which sine waves of AC power emerge.
  22. 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.
  23. 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.
  24. 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.
  25. 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.
  26. 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.
  27. NFPA – National Fire Protection Association; owner/creator of the NEC.
  28. 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 110V220V 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 attached equipment.  The occurrence of service voltages outside the specified range (brownouts) was intended to be infrequent.

Following is the chart from 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 day, 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.

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.

What we have not yet discussed is the “safety ground” that is required throughout the residence by the National Electric Code.  This safety ground attaches to every outlet, switch plate, ceiling fan, luminary fixture and appliance in the residence.  In a residential application, there are one or more copper rods driven into the ground outside the building.  The grounding wire is usually of bare #6 or #4 AWG stranded copper wire, and is routed from the buried ground rod(s) to a 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 will be routed to this buss bar.

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: “Isolation Transformers,” incorporated 1/18/2016.
Major addition: “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


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 Ground Fault Protection devices (GFP) have been installed. 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.

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. 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 choices, so even newly purchased, “straight-from-the-factory” boats are not necessarily free from the possibility of denial of power. Boats manufactured “offshore” may not be totally 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.


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 requires that there be no neutral-to-ground bond(s) aboard the boat. For clarity, that firm statement can be modified to read, “there must be no neutral-to-ground bond(s) aboard the boat when operating on shore power.”  if there were a neutral-to-ground bond on the boat, that wrongly-placed bond creates 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. When all of these paths are in parallel, current that should flow only on the neutral will divide and flow in equal amounts on both conductors, and in some amount, through the water itself. By definition, this 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.

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, maybe 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).  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, !0A does not balance 6A, and in shore power Neutral 2, 10A does not balance 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.

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

Splitters come in two use cases:.
1.   For a boat that is fit with two 120V, 30A shore power inlets, if a 240V, 50A pedestal outlet is also available, a “forward-wye” splitter can be used to convert the single 50A pedestal outlet to two 30A receptacles.
2.  For a boat that is fit with a 240V, 50A shore power inlet, a “Reverse Wye” splitter can be used to adapt the shore power cord to two 30A 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 the 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 with which I am familiar.  If a boat fit with a 240V, 50A shore power inlet gets power from a reverse wye splitter attached to a pedestal with ground fault sensing breakers, this cannot be successful.  Since EACH 30A GROUND FAULT BREAKER is looking for balanced current in feed and neutral, it’s doomed to failure.  The description is as in the previous topic.  If one feed (L1) is providing 6A, and the second feed (L2) is providing 14A, the total is 20A.  The single 50A neutral divides into two 30A neutrals inside the reverse wye splitter, downstream of the 30A pedestal breakers.  Each breaker will see 10A, which does not – and never will – balance with the current in the feed conductors.


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, 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 is left behind.  This salt can “bridge” between the blades and conductors of the connector, and form high resistance “shore circuits” WITHIN the connector.  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 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.

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 one case, the bond can’t be on the boat. In the other, 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. To accomplish that, modern 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.”

Some inverter devices may not have automatic transfer relays, and instead accomplish the transfer back and forth from shore power “passthru” operation to “invert” operation with manual switching.   That is OK, as long as the transfer switching is wired correctly.

The operation of the inverter’s internal transfer relay has implications that boat owners should understand. Inverters (or inverter/chargers) that are fully integrated into the boat’s electrical system will cause a 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 the ground fault exceeds 30mA and persists 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 equipment specification or environmental contamination), 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.

TOPIC: Shore Power Transformers

The input side of an isolation transformer (primary winding) connects to the shore power pedestal. The output of the transformer (secondary winding) supplies the AC loads on the boat. The “connection” between the primary and secondary windings 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. There is no continuous electrical connection between the shore power ground and the boat’s ground system. Like an onboard inverter or a onboard generator, an isolation transformer is treated by electrical codes as an onboard AC source, and so the neutral and ground of the transformer output (secondary winding) are 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.

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

One docks, pedestal circuit breakers with ground fault sensors provide two functions. First, they protect against electrical overload currents. The nominal overload set-point is 30A for 125V circuits and 50A for 240V circuits. Second, they 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 shire power 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 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.

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). Galvanic Isolators contain a diode pack (full-wave bridge rectifier) that blocks small DC galvanic currents but allows AC fault currents to flow.

There are three “generations” of Galvanic Isolator technology. The first generation device consisted of a passive diode pack. That passive diode pack was subject to 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 safety risk. 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 created a transient ground fault, and is incompatible with the emerging presence of shore power ground fault sensing equipment on marina docks. 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 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.   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, for some reason I can’t explain, 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.  But in 2020, the majority of marinas do not yet have ground fault sensing pedestal breakers, so I am able to enable the unit and use it as intended.  That works well 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.

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.

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 much 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 Mastervolt® MultiPlus™ line of inverters. 

Shore power 120V residential outlet sources are protected by very sensitive 5mA Ground Fault Circuit Interrupter (GFCI) sensors.  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.  During that synchronization process, the two phases will almost certainly be out-of-phase with on another.  During that time, it’s possible for the inverter to 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.”  It can occur on 120V, 30A shore power sources with 30mA protection.  It can appear as a single, one-time “oops,” or it can be a more persistent annoyance. Owners of this 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.

TOPIC: Harmonic Distortion on the Electric Utility supply

Like the topic “Inverters and inverter/chargers; an obscure cause of trips,” this particular cause is an obscure likelihood.  But 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 the space or 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 and the neutral conductors, 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 sensor 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.

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.

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.

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.

The way surge suppressors work is by dumping surge energy “spikes” to ground. There are special diodes in the surge suppressor 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 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 the same discussion about damaged diodes that applied to the diode pack of a galvanic isolator. The diodes in the surge suppressor can be damaged in a way that leaves them partially-conductive. That is 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.

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.

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.

TOPIC: Neutral-to-Ground Connection Incorrectly Made At 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.


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 fundamentally 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 be severely overloaded and becomes a fire risk. AN open ground conductor means protection from electric shock is absent or compromised. Corrective action should be taken with any boat found to have wiring issues. It’s just the safe thing to do!

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

AC Safety Tests For Boats

Note;  Major revisions to this post, with additional test added, June 20, 2015.
Note: Editorial and spelling corrections; alternative test added for Test 9B, April 23, 2018
Note: Editorial update to include 240V Breakout Tool added, July 28, 2019.

This article describes a series of test measurements intended to be performed safely by boat owners/operators.  Little or no prior knowledge or skills with electricity, electric circuits or the regulatory codes, components, materials, workmanship and techniques involved in installing and servicing AC electrical systems is needed to perform these tests.  There is never a need to contact a “live” electrical circuit.  The tests assess the safety status of a boat’s AC electrical system.  These tests can a) expose non-compliance and/or b) confirm compliance with a key subset of safety elements of the ABYC E11 electrical standard.   As of 2018, compliance of all boats to ABYC electrical standards is progressively more and more important.  As shore-side facilities upgrade to the requirements of newer versions of the National Electric Code (NEC), boats that do not comply to the ABYC Electrical Standard risk being left without the ability to connect to AC electric power. For background information on that concern, see my article, “Emerging AC Electrical Concern,” on this website.  For owners of boats fit with 120VC/240V, 50A shore power cords, addition detail on the electrical behavior of the 120V AC circuits is described in my article “Electrical Behavior of a 208V/240V Boat,” on this website.


  1.  Shore power cords labeled “2-wire plus ground” contain three conductors.  The conductors are individually coated with green (ground), white (neutral) and black (hot) insulation.
  2.  Shore power cords labeled “3-wire plus ground” contain four conductors.  The conductors are individually coated with green (ground), white (neutral), black (hot1) and red (hot2) insulation.
  3.  The testing described in this document is presently limited to AC shore power pedestals and boat shore power circuits equipped with either 1 or 2, “2-wire plus ground,” 120V shore power circuits.  These descriptions do not apply to boats with “3-wire plus ground,” 240V shore power systems.
  4.  The purpose of these tests is merely to IDENTIFY and EXPOSE the presence of conditions that represent immediate or potential ELECTRICAL SAFETY RISKS or create incompatibility with shore power facilities such that shore power may be interrupted to the affected boat.  Boat owners may wish to engage a ABYC-certified marine electrical technician for help in performing or interpreting these tests.
  5.  The fault conditions that these tests can expose have an extensive array of possible causes.  It is not possible to present a practical DIY list of effective corrective action(s).  If fault conditions are identified in testing, it is necessary for electrical novice boat owners to engage an ABYC-certified electrical service technician to perform further diagnosis and corrective actions.
  6.  Any boat equipped with two 120V shore power inlets should have these tests performed on both of the incoming shore power circuits.
  7.  These tests require the use of commonly available, inexpensive test equipment:
    • Digital Multimeter (DMM)
    • Clamp-on ammeter
    • AC Circuit Tester
    • Home-made test jig cordset
  9.  Ground and leakage fault currents that originate on a boat may cycle “on” and “off” with the automatic operation of equipment installed on a boat.  Boat owners should have as much of the electrical equipment on the boat in actual operation at the time of testing as possible, including water heater, battery charger(s), range/oven(s), heat pumps, washing machine/dryer, microwave, refrigerator, ice maker, space lighting, entertainment systems, etc.  Equipment that may harbor an electrical fault may not show symptoms unless it is actually running.  If all devices aboard can not be run at the same time, testing may be performed in stages to ensure all onboard electrical equipment is tested.
  10. A “hot basin” is a facility where an “electric charge” exists in the water.  “Hot basins” can have many causes, including:
    • a longterm unattended boat fit with power cord(s) with degraded insulation draped in the water,
    • damaged or degraded dock wiring or dock wiring periodically immersed in basin water, and
    • a nearby boat with an on-board ground fault.  (Ground faults cause power to flow into the water, which will then return to their source on the ground wires of all nearby neighboring boats.)
  11.  This testing does not include DC System tests.
  12.  This testing does not cover DC galvanic currents or the testing or operation of Galvanic Isolator diode packs.
  13.  This testing does not include advanced tests, procedures or test equipment that certified service technicians may utilize.

References to the following items appear in the test procedures:

30A Shore Power Plug/Receptacle Blade Layout: 

Bent Blade   –  Safety Ground (Green)
Shorter Blade –  Neutral (White)
Longer Blade  –  Hot (Black)

L5-30P - Plug L5-30R - Receptacle

L5-30P – Plug
L5-30R – Receptacle

Tests 9, 9A and 9B: 

Clamp-on Ammeter;
Theory of Operation.  Upper drawing for 120V Breakout Tool, lower drawing for 240V Breakout Tool.

Clamp-on Ammeter test configuration for Test 9.


Tests 9A and 9B: Suggested “home-made” test “tools;” these “breakout cable” tools allow access to the individual conductors contained within a shore power cord and at the same time protect against actual contact with live electrical conductors. test-jig


Tests 10 and 11:

AC Circuit Tester for 120V, 15A and 20A outlets; a suggested test “tool.”

AC utility outlet tester including GFCI

AC utility outlet tester including GFCI

NOTE: Tests 1 – 3, following, are performed with the dock pedestal receptacle energized (turned “on”).


Test 1:
Purpose of Test 1. Verify the electrical integrity and wiring of the dock feeder wiring to the dockside pedestal;
2. Verify the pedestal’s 30A receptacle connections.The governing electrical standard is NFPA 70 (NEC, Article 555).
Test Setup 1. Shore Power cord(s) disconnected at pedestal and set aside;
2. Pedestal breaker(s) set “on;”
3. Digital Multimeter (DMM) set to measure AC Volts
Test Measurement Points At the pedestal outlet (NEMA L5-30R, 30A):
measure for AC voltage between the Ground and Neutral receptacle contacts.
Nominal Result 0.0 VAC
Possible Findings Less than 5 VAC;
Greater than 5 VAC
Fault Findings Requiring Correction Anything greater than 5 VAC;
(Note: A finding of 120V is an urgent emergency; immediately report this finding to facility management.)
Considerations Affecting Test Setup or Outcome 1. Long length of dock feeder cables;
2. Amount of power being drawn by boats connected to the feeder.
3. Reversed Polarity of Neutral and Hot conductors.
Test 2:
Purpose of Test 1. Verify the electrical  integrity and wiring of the dock feeder wiring to the dockside pedestal;
2. Verify the pedestal’s 30A receptacle connections. Governing standard is NFPA 70 (NEC, Article 555).
Test Setup 1. Shore Power cord(s) disconnected at pedestal and set aside;
2. pedestal breaker(s) set “on;”
3. Digital Multimeter (DMM) set to measure AC Volts
Test Measurement Points At the pedestal outlet (NEMA L5-30R, 30A):
measure for AC voltage between the Hot and Ground receptacle contacts.
Nominal Result 120 VAC
Possible Findings Normal “System Voltage” can range from around 105V to 125VAC;
0.0 VAC
Fault Findings Requiring Correction Less than 105V (hot, summer days; severe “brownout” conditions);
greater than 125 VAC.
0.0 VAC (Note: potential safety issue if open ground; immediately report this finding to facility management)
Considerations Affecting Test Setup or Outcome 1. Demand conditions within the utility distribution system;
2. Long length of dock feeder;
3. Amount of power being drawn by other boats connected to the same dock feeder
4. Reversed Polarity of Neutral and Hot conductors.
Test 3:
Purpose of Test 1. Verify the electrical  integrity and wiring of the dock feeder wiring to the dockside pedestal;
2. Verify the pedestal’s 30A receptacle connections. Governing standard is NFPA 70 (NEC, Article 555).
Test Setup 1. Shore Power cord(s) disconnected at pedestal and set aside;
2. Pedestal breaker(s) set “on;”
3. Digital Multimeter (DMM) set to measure AC Volts
Test Measurement Points At the pedestal outlet (NEMA L5-30R, 30A):
measure for AC voltage between Hot and Neutral receptacle contacts.
Nominal Result 120 VAC
Possible Findings Normal “System Voltage” can range from around 105V to 125VAC;
0.0 VAC 
Fault Findings Requiring Correction less than 105V (hot, summer days; severe “brownout” conditions);
greater than 125 VAC.0.0 VAC (Note: potential safety issue if open ground; immediately report this finding to facility management)
Considerations Affecting Test Setup or Outcome 1. Demand conditions within the utility distribution system;
2. Long length of dock feeder;
3. Amount of power being drawn by other boats connected to the same dock feeder
4. Reversed Polarity of Neutral and Hot conductors.
Test 4:
Purpose of Test 1. Verify the electrical integrity and continuity of the Shore Power Cord and boat AC Shore Power inlet connections
Test Setup 1. Shore Power cord(s) disconnected at pedestal but connected at the boat;
2. Genset(s) “off,” Inverter AC output set “off.”
3. DMM set to measure Resistance (Ohms, Ω). (For convenience of measuring, drag the male end of shore power cord onto the boat.)
Test Measurement Points Measure for resistance and continuity between the Ground blade of the cord’s male plug end (NEMA L5-30P) and the main AC Safety Ground Buss aboard the vessel.
Nominal Result Less than 1 Ω
Possible Findings A range of resistance from 0 Ω to  Ω
(infinity); (missing/defective AC Safety Ground is an urgent emergency)
Fault Findings Requiring Correction Any varying or fixed  value of greater than 1 Ω.

Note: This fault is one of the two that are necessary for boats to  dump AC power into the water; urgent attention is required

Considerations Affecting Test Setup or Outcome Presence of Galvanic Isolator; consult manufacturer for procedures for testing the device’s diode pack. Presence of Isolation Transformer, which is considered an on-board, not shore power, source.  The onboard ground does not connect to the shore power service ground.
Test 5:
Purpose of Test 1. Confirm that the boat’s AC Safety Ground is bonded to the Ship’s common DC Ground.

Governing standard is ABYC E11,

Test Setup 1. Shore Power cord(s) disconnected at pedestal but connected at the boat;
2. Genset(s) “off,” Inverter AC output set “off.”
3. DMM set to measure Resistance (Ohms, Ω).(For convenience of measuring, drag the male end of shore power cord onto the boat.)
Test Measurement Points Measure for continuity between the Ground blade of the cord’s male plug end (NEMA L5-30P) and the Engine Block of the Propulsion Engine aboard the vessel.
Nominal Result Less than 1 Ω
Possible Findings A range of resistance from 0 Ω to  Ω
(missing/defective AC/DC system bond; high priority attention required)
Fault Findings Requiring Correction Any varying or fixed value of greater than 1 Ω.
Considerations Affecting Test Setup or Outcome Presence of Galvanic Isolator; consult manufacturer for procedures for testing the device’s diode pack.Presence of Isolation Transformer, which is  considered an on-board power source.  The onboard ground does not connect to the shore power service ground.
Test 6:
Purpose of Test 1. Confirm that cross-connections are not present between the ground buss and the neutral buss for the 120V circuit aboard the boat.

Governing standard is ABYC E11, and

Test Setup 1. Shore Power cord(s) disconnected at pedestal but connected at the boat;
2. Genset(s) “off,” Inverter AC output set “off.”
3. DMM set to measure Resistance (Ohms, Ω).
Test Measurement Points At the cord’s male plug (NEMA L5-30P 30A plug), measure the resistance between Ground and Neutral blades
Nominal Result ∞ Ω
Possible Findings less than 1 Ω
Fault Findings Requiring Correction Any varying or fixed  value of less than ∞ Ω

Note: If the boat is fit with one or more inverter’s which comply with UL458, low resistance (<1.0 Ω) is a normal finding.  It will be necessary to isolate the inverter’s automatic neutral-to-ground bond to complete the test.  The process of isolating the inverter will be unique to the particular inverter manufacturer   If unfamiliar with this process, consult a marine-certified electrical technician for assistance.

Considerations Affecting Test Setup or Outcome 1. Incorrectly isolated genset, inverter or isolation transformer
2. Incorrect selection (design/capability) of device transfer switch
3. Defective Reverse Polarity detection circuit(s);
Test 7:
Note 1: this test does not apply to (cannot be performed on) boats with only one AC Shore Power Cord.
Note 2: this test correlates with Test 9C, which should also be performed if a non-infinity resistance value is found.
Purpose of Test 1. Confirm that the AC Neutrals aboard a boat with two 120V AC shore power circuits are isolated from one another.

Governing standard is ABYC E11, and

Test Setup For boats with two 120V shore power circuits aboard:
1. Shore Power cords disconnected at pedestal but connected at the boat;
2. Genset(s) “off,” Inverter(s) set “off;”
3. DMM set to measure Resistance (Ohms, Ω).
Test Measurement Points Measure the resistance between the Neutral blade of Shore Power cord #1 (NEMA L5-30P) and the Neutral blade of shore power cord #2 (NEMA L5-30P)
Nominal Result ∞ Ω
Possible Findings Any range of values
Fault Findings Requiring Correction Any varying or fixed  value of less than ∞ Ω
Considerations Affecting Test Setup or Outcome
Test 8:
this test cannot be performed on boats with only one AC Shore Power Cord
Purpose of Test Confirm that the AC Grounds aboard a boat with two 120V AC shore power circuits are connected together and continuous with one another.

Governing standard is ABYC E11,

Test Setup For boats with two 120V shore power circuits aboard:
1. Shore Power cords disconnected at pedestal but connected at the boat;
2. Genset(s) “off,” Inverter(s) “off.”
3. DMM set to measure Resistance (Ohms, Ω).
Test Measurement Points Measure the resistance between the Ground blade of Shore Power cord #1 (NEMA L5-30P) and the Ground blade of shore power cord #2 (NEMA L5-30P)
Nominal Result Less than 1 Ω
Possible Findings Any range of values
Fault Findings Requiring Correction Any varying or fixed  value of greater than 0 Ω.
Considerations Affecting Test Setup or Outcome
Test 9:
Purpose of Test 1. Determine if abnormal leakage currents are present in the shore power cords supplying power to a boat via two 120V shore power circuits.

1. This test is only an easy-to-perform screening test.
2. Any observed fault means that either a) power is being dumped into the water by this boat, b) power is returning to its on-shore source on an unintended conductor, or c) power which has already found its way into the water of the facility’s basin is finding it’s way back to its on-shore source via this boat’s ground system.

Test Setup 1. Shore Power cord(s) connected at pedestal and connected at the boat;
2. Pedestal breakers set “on.”
3. All possible boat AC loads turned “on” and actually running/operating.
4. Genset(s) “off,” Inverter(s) set “off.”
5. Clamp-on Ammeter set to measure AC Amps.
Test Measurement Points Sequentially clamp the ammeter around the outer body of each 120V shore power cord.
Nominal Result In salt water, less that 50mA;
in fresh water, less than 25mA. 
If fault current is within above limits, skip to test 10. If fault current readings exceed the above limits, perform tests 9A and 9B.  Also see notes titled, “IF TEST 9 IS POSITIVE,” following below.
Possible Findings A value ranging from 0.0 Amps to several Amps
Fault Findings Requiring Correction Any varying or fixed value greater than 50mA (0.050 Amps) in salt water.
Any varying or fixed value greater than 25mA (0.025 Amps) in fresh water.Note: There is imprecision and debate among experts around the absolute value of the above numbers.  I feel these recommendations are a reasonable compromise between human/pet life-safety and the practical sensitivity of inexpensive, utility-grade clamp-on ammeters available OTC from community sources (marine chandleries, big box and hardware stores) at prices that lay buyers are willing to pay.  Furthermore, periodic screening checks for ground fault and leakage fault currents is vastly better than never checking, which results in being unaware of the actual presence of a potentially serious problem.Any doubt about life safety should be immediately referred to a marine-certified electrical technician.
Considerations Affecting Test Setup or Outcome As much of the electrical equipment installed on the boat as possible should be “on,” running and in operation at the time of the test.  This includes water heater(s), battery charger(s), inverter/charger(s), heat pump(s), refrigerator, ice maker, watermaker, range/oven, washer/dryer, lights, entertainment devices, etc.  If it is not possible to run all equipment at the same time, perform this test in multiple steps, each with different equipment running and in operation.  The more equipment that is running at the time of the test, the more comprehensive the test results will be, and the greater the confidence about the results.

Comments on Test 9A:

Test 9A can be performed on boats with either one or two 120V, 30A shore power cords.

On boats equipped with only ONE shore power cord, or when only ONE shore power cord is physically connected between the test boat and the shorepower pedestal at the time the test is performed, a non-zero test finding indicates that power is escaping from the boat into the surrounding water.

On boats equipped with two 120V, 30A shore power cords when both shore power cords are physically connected between the boat and the shore power pedestal, a non-zero test finding is only an indication that abnormal currents are present, but the test does not distinguish between a Ground Fault and a Leakage Fault.

A Ground Fault is a condition that results in power leaking into the water surrounding the boat.

A Leakage Fault is a condition in which power returns to its on-shore source, but it returns on an unintended conductor of the AC system.

Test 9A:
Purpose of Test Determine if fault currents are present. The two types of fault current are:

1. Ground Faults causing power to escape from the boat into the surrounding water (“dumping power into the water”); and
2. Leakage Faults, which are not a Ground Fault, but result in current following in an unintended path back to its on-shore source.

Test Setup 1. Install the recommended test tool jig fixture* between the pedestal outlet and the shore power service cord in order to gain access to the three individual shore power conductors (Black, White, Green).
2. Shore Power cord(s) connected at pedestal and connected at the boat;
3. Pedestal breakers set “on.”
4. All possible boat AC loads turned “on” and actually running/operating.
5. Genset(s) “off,” Inverter(s) set “off.”
6. Clamp-on Ammeter set to measure AC Amps.
* The test tool I suggest for this testing is a “home-made,” short, 30A “extension cord” fit with an L5-30P male plug on one end and an L5-30R female receptacle on the other end.  Carefully remove about 8” of outer jacket insulation to expose the individual electrical conductors (green, white and black) inside the cord.  Only remove the outer exterior insulation jacket.  Be careful not to cut or damage the green, white and black insulation covering the individual conductors.
Test Measurement Points Install the clamp-on ammeter around BOTH the White and Black wires of the shore power breakout cord at the same time, but excluding the green wire.
Nominal Result 0.0 A
Possible Findings A value ranging from 0.0 A to several amps Any non-zero amps (positive) finding means a fault current is present. If this test is positive with only ONE shore Power Cord physically connected between the boat and the shore power pedestal, it should be treated as an URGENT EMERGENCY.

If this test is positive on a boat with two shore power cords connected between the boat and the shore power pedestal at the time of testing, continue to Test 9B.

Fault Findings Requiring Correction Any varying or fixed value greater than 50mA.
Considerations Affecting Test Setup or Outcome On-board equipment running at the time of the test.  The more equipment that is actually in operation at the time of the test, the more  comprehensive the test results will be.

Comments on Test 9B:

Test 9B is indicated any time a non-zero AC current reading is found in either one of the two 120V shore power cords in Test 9A. The purpose of this test is to differentiate between the presence of a Ground Fault or the presence of a Leakage Fault.

Rationale – Ground Fault:

In order for power to flow from a boat into the surrounding water, two faults must by present simultaneously. The first necessary fault is an open circuit or high resistance in the boat’s green safety ground connection to the shore power infrastructure (identified in Test 4). The second necessary fault is the presence of a condition that has unintentionally energized the safety ground circuit. This can be caused by a wiring error or equipment malfunction aboard the boat.

If the safety ground connection from the boat to the shore power infrastructure is sound, and if incorrect neutral/ground wiring connections are present aboard, fault currents can exist in a hidden, non-symptomatic state. Fault currents which evolve to become symptomatic can be lethal to people and animals in nearby waters.

Defective ground connections are easy to avoid and easy to correct. Ground faults can be very difficult to isolate and identify. Several causes of ground faults can come on over a long period of time. Multiple ground faults can exist at the same time in any electrical system, making isolation and correction extremely costly and time consuming. Identification of ground fault cause is beyond the scope of this article. Diagnosis and correction of ground faults requires advanced electrical skills and electrical diagnostic experience. If Test 9A confirms this condition aboard a boat, the professional services of a certified marine electrical technician should be engaged.

Rationale – Leakage Fault:

If the neutral conductors of a boat with two 120V AC shore power circuits are cross-connected, a potential parallel path for AC current returning to the on-shore source is created between the two circuits. That potential parallel path becomes an actual parallel path when BOTH shore power cords are physically connected between the boat and the shore power pedestal. The actual parallel path created by cross-connected neutrals is present even if only one of the pedestal disconnect breakers is set “on,” and/or even if the main disconnect breaker aboard the boat is set “off.”

Cross-connected neutrals create Leakage Faults. Leakage Faults result in AC current flowing in conductors where it is not, at that time, intended to flow (the other neutral). This condition it is not a Ground Fault and does not result in power flowing in the water surrounding the boat, but it will result in tripping a Ground Fault Protection (GFP) device.

Cross-connected neutrals are also a fire risk. If one of the neutrals were to become compromised, the other would take over the load intended to be carried by the failed neutral conductor. With both 120V, 30A circuits highly loaded, the remaining neutral conductor would be significantly overloaded. For example, assume the house circuit is loaded at 18 amps and the heat pump circuit is loaded at 26 amps. In the normal case, with conductors rated at 30A, both circuits are operating within their safe ampacity. But with cross-connected neutrals, if one of the neutrals were compromised or open, the worst case is that the other neutral would carry all of the current of both circuits, or 18 + 26 = 44 Amps. Now, that neutral conductor is operating well above it’s safe ampacity, will overheat, and would cause a fire.

Test 9B:
Note 1: this test does not apply to (cannot be performed on) boats with only one AC Shore Power Cord.
Purpose of Test Differentiate between positive fault findings to identify either:
1. a Ground Fault leaking current into the water, or
2. a Leakage Fault returning power to its source on an unintended conductor.
Test Setup 1. Install the recommended test tool breakout cord* between the pedestal outlet and the shore power service cord in order to gain access to the three individual shore power conductors (Black, White, Green).
2. Shore Power cord(s) connected at pedestal and connected at the boat;
3. Pedestal breakers set “on.”
4. All possible boat AC loads turned “on” and actually running/operating.
5. Genset(s) “off,” Inverter(s) set “off.”
6. Clamp-on Ammeter set to measure AC Amps.
* The test tool I suggest for this testing is a “home-made,” short, 30A “extension cord” fit with an L5-30P male plug on one end and an L5-30R female receptacle on the other end.  Carefully remove about 8” of outer jacket insulation to expose the individual electrical conductors (green, white and black) inside the cord.  Only remove the outer exterior insulation jacket.  Be careful not to cut or damage the green, white and black insulation covering the individual conductors.
Test Measurement Points Install the clamp-on ammeter around the White and Black wires of BOTH shore power cords at the same time, but excluding the green wires. In this setup, you will have clamped around BOTH black wires and both white wires; four wires in all.

Test Setur for Test 9B

Measurement configuration for Test 9B

Nominal Result 0.0 A
Possible Findings A value ranging from 0.0 A to several amps A non-zero finding in excess of 50mA means a ground fault defect on the boat is dumping AC power into the water. Significant leakage can be lethal to swimmers, divers and pets. This finding is an URGENT EMERGENCY.

A zero amp finding means all power is returning to shore, and no power is being lost to the water. However, because the measurements in the INDIVIDUAL CORDS were not 0.0A, this finding means the neutrals for the two shore power circuits are cross-connected on the boat.

Fault Findings Requiring Correction Any varying or fixed value greater than 50mA.
Considerations Affecting Test Setup or Outcome On-board equipment running at the time of the test.  The more equipment that is actually in operation at the time of the test, the more  comprehensive the test results will be.

Note: There is also an  alternative way  to get the result provided by test 9B. It is easier, but requires more interpretation on the part of the person doing the testing.   It is very useful if the shore power cords have clean contact blades making good electrical contact.   If blade surfaces or contacts are degraded, it can be less clear. In the hope that it may be useful for some, following is how to approach it.

When the neutrals of two 30A shore power circuits are cross-connected on the boat, they create a parallel path back to the shore power source.   To perform this alternative test, connect both of the shore power cords to the pedestal outlets, but turn power “on” for only one cord.   I like to power the heat pumps, because the compressors and raw water circulator create a large and obvious load.   For this example, let’s assume that the heat pump circuit draws 26 amps. Normally, that 26 amps should return to shore ONLY on the neutral conductor of the heat pump shore power cable, but if the two neutrals are paralleled on the boat, 1/2 the current will return on shore power cord A and 1/2 the current will return on shore power cord B.

With power applied to, and load on, only one shore power cord, clamp each individual shore power cord, one after the other.   If they make good electrical connections and show nearly identical ampere readings, that is a telltale that they are paralleled aboard the boat.   That is, they are cross-connected together, in error, on the boat.   In the above example, the total of 26 amps for the heat pumps would divide and return to shore on both neutral conductors.   The meter would read 13 amps on EACH  individual cord.  If both cords are clamped at the same time, the reading would be 0.0 amps.

Yes, this condition will absolutely trip a GFP sensor on a dock pedestal.

Test 9C:
Purpose of Test 1. Definitively determine if power is entering the boat’s grounding system from the water in which the boat is floating (“hot” basin).
Test Setup 1. Install a test tool breakout cord* between the pedestal outlet and the shore power service cord in order to gain access to the three individual shore power conductors (Black, White, Green).
2. Clamp-on ammeter set to measure AC Amps
* The test tool I suggest for this testing is a “home-made,” short 30A “extension cord” fit with an L5-30P male plug on one end and an L5-30R female receptacle on the other end.  Carefully remove about 8” of outer jacket insulation to expose the individual electrical conductors (green, white and black) inside the cord.  Only remove the outer exterior insulation jacket.  Be careful not to cut or damage the green, white and black insulation covering the individual conductors.
Test Measurement Points Clamp around ONLY the Green wire of the shore power cord, excluding both the Black and White wires.
Nominal Result 0.0 A
Possible Findings 0.0 A up to several amps Any positive finding means that AC power is flowing from the water of the facility’s basin back to shore through the boat’s ground system.  This is a “hot basin,” which can be caused by a fault in the shoreside infrastructure or by AC power leaking into the water from a neighboring boat.
Fault Findings Requiring Correction Any non-0 A value
Considerations Affecting Test Setup or Outcome Report this finding to facility management.
Test 10:
Purpose of Test Verify the integrity and wiring of various on-board AC utility outlets.
Test Setup 1. Shore Power cord connected to pedestal; pedestal breaker set “on;”
2. All utility branch circuits set “on” at the boat’s AC breaker.
3. Test device is an AC Circuit Tester.
Test Measurement Points Insert the circuit tester into any energized outlet receptacle.
Nominal Result Indicator lights show “Normal” condition.
Possible Findings Indicator lights reveal abnormal condition.
Fault Findings Requiring Correction Indicator lights reveal abnormal condition.
Considerations Affecting Test Setup or Outcome n/a
Test 11:
Purpose of Test Verify correct operation of all GFCI-protected outlets.Governing ABYC Standard for minimum GFCI protection is E11,
Test Setup 1. Shore Power cord connected to pedestal; pedestal breaker set “on;”
2. All utility branch circuits set “on” at the boat’s AC breaker.
3. Test device is an AC Circuit Tester.
Test Measurement Points With the AC circuit tester inserted into an energized outlet and displaying normal circuit condition, depress the “GFCI test” button.
Nominal Result Circuit should instantly de-energize
Possible Findings  Circuit fails to de-energize.
Fault Findings Requiring Correction Circuit fails to de-energize.
Considerations Affecting Test Setup or Outcome Test does not apply to outlets not protected by GFCI device.


Test 9 by itself is AT BEST a “screening test.”  Any non-zero reading of AC amperage is only an indication that more testing is required. The cause(s) of a non-zero reading could be aboard the boat in the boat’s electrical system or it could be in the shore power infrastructure of the facility basin waters surrounding the boat. If Test 9 is “positive,” what a boat owner/operator does next may be informed by the following considerations:

  1. If these tests have never been previously performed, a test cordset that allows access to the individual black, white and green conductors of the shore power cord is required for Tests 9A, 9B and 9C. If a test cordset is not available or cannot be borrowed, an electrical layman may want to call in a qualified service technician to continue diagnosis. Any non-zero result for Test 9A is an indication of an electrical fault on the boat. Electrical faults are dangerous, and can develop on any boat at any time.  Electrical faults on a boat will require technical electrical skills to resolve.
  2. If the boat owner has been performing periodic screening tests of the shore power cords, and there is a history of normal Test 9 clamp tests (0 Amps), then there is a high confidence (not 100%, but high) that the boat is correctly wired. Additional testing is required to rule out the recent onset and development of a net-new fault condition.

When out cruising, cruisers are constantly encountering new facilities.  Owner/operators of otherwise fault-free boats would expect to experience “normal” screening tests (less than 50mA) in all facilities.  The logic is, the boat is known from previous testing to be fault-free, and the new basin is assumed to be fault-free. If one encounters a positive screening test at a new facility:

  1. If staying at a facility as an overnight or short-term transient guest, one may opt to “just ignore” this single finding. One can’t be certain, but because there is a known history of normal tests on the boat, this case is probably caused by a hot basin at the facility. When the boat is subsequently relocated to another facility, re-test to confirm. The probability of encountering two hot basins in a row is very low, but not absolutely zero. If a positive test result follows the boat and recurs at a new location, the owner should then perform both tests 9A through 9C.
  2. If one encounters a positive test when clamping their own boat’s shore power cords, then clamping around the shore power cord(s) of the immediate dock neighbor (whether 30A or 50A; the normal reading in either case is less than 50mA) is advised.  If non-zero readings are also present there, although one can’t be absolutely certain, in the case of a boat with a known fault-free history, it’s likely the facility basin is hot.
  3. If one observes fault readings on both the test boat and a neighboring boat(s), the owner should disconnect their shore power cords from the pedestal and re-check the dock neighbor’s cord(s). If the fault reading at the neighbor’s cord is gone, that suggests the test boat could be the source of the fault current. Unchanged non-zero readings at the neighbor’s cords suggests a hot basin. Reconnect the test boat and re-check. Unchanged non-zero readings at the neighbor’s cord(s) means the test boat is not the source of the fault current.
  4. If one has confidence that an on-site facility employee will understand the subject, report the observed condition to have management check for a hot basin. However, it’s important to talk to someone knowledgeable, generally not “kids” who work afternoons/weekends.  If staying at a facility as an overnight or short-term transient guest and only “kids” are available, call back to the office later and talk to someone who is knowledgeable, or at least responsible enough to pass the concern on to someone who is knowledgeable.
  5. For long-term facility stays, boat owners should periodically check their own cords and those of immediate dock neighbors. If all of these cords show non-zero fault currents flowing, perform Tests 9A thru 9C. Report positive 9C findings to a responsible facility employee to have the basin checked.
  6. For long-term facility stays, boat owners should periodically check their own cords as other transient boats come and go.  I have been able to identify boats with ground-fault problems just by screening my own cords.  If a “hot boat” slips near me, I’ll see that in the readings of my own known-good cords.

An AC fault current flowing in the water of a boat basin generally won’t hurt the underwater metals of boats, but because current is flowing through the water, it is obviously dangerous to swimmers and pets.


Emerging AC Electrical Concern

Initial Post: 4/29/2015
Added “Isolation Transformers:” 6/16/2015

A significant AC electrical concern is emerging for some owners of older boats. For those who may be, or are affected, I urge you to investigate sooner rather than later.


Land-based building and construction codes are usually implemented through government rule-making and have the force of law.  In the United States, the National Fire Protection Association (NFPA) is a private fire safety association that creates and maintains the US National Electric Code (NEC).  In Canada, the Canadian Standards Association (CSA) maintains C22.1, and various other departments maintain industry specific Canadian Electric Codes.

In the US, the NEC is updated and published on a three-year cycle.  The most current version was published in 2014.  Across the United States, the 50 individual states, and many counties and municipalities, “adopt” the NEC as their construction and building standard.  Adoption occurs at random intervals and varies from locale-to-locale.  Code enforcement is through the permitting process, and designated local code enforcement officers called the “authority having jurisdiction;” the “AHJ.”  Today, some jurisdictions are operating on the 2008 NEC, some on 2011 and some on 2014.  People who own properties in multiple states may indeed be working with different versions of the NEC.

For boats and boat owners, there are no “permitting” processes for performing upgrades, and there are no official “authorities having jurisdiction.”  The only “codes” that apply to pleasure craft are very limited federal requirements (enforced by the United States Coast Guard) and a very broad set of voluntary standards developed by the American Boat and Yacht Council (ABYC).  Principally for pleasure craft, the standards of the ABYC take precedence.  The ABYC electrical standards align and integrate tightly with land-based codes, except that compliance is “voluntary.”  As boaters, the closest we come to any kind of formal “code enforcement” is through the business requirements of a lender or, more commonly, an insurance carrier.  To obtain hull insurance, boaters usually obtain a “survey.”  If a surveyor identifies a discrepancy or noncompliance to one of the provisions of the ABYC standards, the lender or marine insurance carrier may require the boat owner to implement corrective measures.  However, in practice, this process is flawed in significant ways.  Most surveyors are not expert in assessing AC electrical systems, and are unlikely to check for, or detect, AC ground fault or related current leakage issues.  Since compliance is voluntary for the boat owner, nothing is actually required for those owners who choose to self-insure.  Periodic surveys are not required by law.  ABYC compliance is not a legal requirement but rather, is loosely and inconsistently based on the business transaction between individual boat owners and the various marine insurers.

The Emerging Issue:

The scope statement of the National Electric Code, Article 555, is “… the installation of wiring and equipment for fixed or floating piers, wharfs, docks, and other areas in marinas, boatyards, boat basins, boathouses, and similar occupancies.  This article does not apply to docking facilities or boathouses used for the owners of single-family dwellings.”  The equivalent language in the scope statement of the National Fire Protection Association standard (303, Chapter 5, 5.1.1) is:  “This standard applies to the construction and operation of marinas, boatyards, yacht clubs, boat condominiums, multiple-docking facilities and multiple-family residences, and all associated piers, docks and floats.” In the NEC, the term “similar occupancies” include yacht clubs, condominium docks, restaurant docks, etc.  Because land-based codes are mandatory but boat codes are voluntary, there is an emerging issue (in 2015 and later) for boat owners, and definitely for owners of older boats, at all non-single family, residential docks.  These defined facilities are the point where the voluntary compliance requirements of cruising and transient boats interface through the shore power code with the mandatory requirements of the land-based NEC.  We as boat owners are squarely in the middle.

The average age of one fleet with which I am familiar is 27 years.  Across that span of years, electrical standards, materials and best-practices have all evolved significantly.  Across those years, many “previous owner modifications” may have been made to a boat’s AC electrical system, and perhaps not always implemented “in the right way.”  Furthermore, some boats built new, offshore, contain equipment choices selected for cost and do not comply, new, out-of-the-box, with ABYC requirements.  And finally, across the years, systems and components can deteriorate and fail, sometimes in obscure and non-symptomatic ways, invisible to and hidden from the boat owner.

The issue to which older boats in particular are exposed, then, is that they may have one or more true ground faults and/or leakage faults aboard.  Simply put, boats that are not compliant with the 2012 and 2015 versions of ABYC E11 will be INCOMPATIBLE – by definition and in fact – with the shore-side requirements of the 2011/2014 National Electric Code for ground fault protection on docks.

Requirements placed on land facilities:

The National Electric Code, Article 555.3 (entitled: Ground Fault Protection) says:  The main over-current protective device (OPD) that feeds the marina shall have ground fault protection not exceeding 100mA.   Ground-fault protection of each individual branch or feeder circuit shall be permitted as a suitable alternative.

In the case of a complying facility, then, the statement, “The main over-current protective device (OPD) that feeds the marina…” would refer to the AC feeder to EACH individual dock.   So EACH dock would have to be GFP-protected to no more than 100mA.  Also, the statement, “Ground-fault protection of each individual branch or feeder circuit shall be permitted as a  suitable alternative” means there are three practical choices for a facility operator to achieve compliance with 2011/2014 NEC 555.3:

1. The ENTIRE DOCK would share a single feeder with 100mA GFP, or
2. A CLUSTER OF SLIPS (two, four, six, etc) would share a feeder protected at no more than 100mA, or
3. EACH PEDESTAL would have a “home run” to the mains distribution panel, and EACH INDIVIDUAL PEDESTAL would be protected at no more than 100mA.

As to the capital costs faced by a facility operator in order to comply with Article 555.3:

1. Item 1 is the least capital intensive for the facility to install, but it is also the most troublesome.
2. Item 2 is middle-of-the-road from the perspective of capital cost to install, and middle-of-the-road for being troublesome.
3. Item 3 is the most capital intensive and the least troublesome.

When I use the word “troublesome” above, I am referring to the fact any boat with a ground fault will trip the feeder to which it connects.  So in case 1, a single boat with a ground fault will take down the entire dock.  In case 2, a single  boat with a ground fault will take down a cluster of slips.  In case 3, a single  boat with a ground fault will only affect one pedestal, but NOT its neighbors.

Obviously, for a large marina intending to support a largely transient customer base, case three would be the conservative, preferred option to ensure uninterrupted electrical service to its customers.  In a private club or condo dock, where there is virtually no transient traffic, a less conservative approach may be acceptable.  Obviously, the least risk of service disruption is associated with the greatest capital maintenance costs.

What It All Means to Boats and Boat Owners:

In light of the above, consider a cruising boat with a hidden ground fault that arrives at a newly upgraded, NEC-compliant marina or boatyard.  If that boat has a ground fault aboard, it is possible for that transient boat to bring down an entire dock, or a cluster of neighboring slips. In that case, some number of “innocent” neighboring boats will be negatively affected; that is, they will all lose power.

A boat wired in compliance with the 2012 ABYC E11 standard will not have undetected ground faults. New production boats manufactured after 2012 must have ELCI devices installed.  New boats and those retrofit with ELCI devices aboard will not have hidden ground fault problems.  However, many older boats do not – maybe never did – meet the ABYC E11 standard, and there is no legal requirement that they be retrofit to comply.  Therefore, ALL FACILITIES that comply with the 2011/2014 NEC for Article 555.3 WILL ENCOUNTER ground fault issues with some number of transient boats.   What that also means is the owner of a boat with a ground fault condition will be more and more often exposed to not having power in newer, more modern facilities.  And obviously, the owner of the boat that is the cause of a loss of power on a dock will not be a very popular or welcome guest.

I would guesstimate at least 10% to 20% of older boats could have true ground faults, either by wiring performed by the previous owner(s) or by now-obsolete OEM equipment installed years ago at time of manufacture.   Some argue for a much higher number, but whatever the real number, it is significant.  Just yesterday (end June, 2015), I walked a dock at my home yacht club with a clamp-on ammeter.  I found at least one boat with two 120V shore power cords and co-mingled neutrals.  I found at least 6 boats leaking more than 100mA into the water.  All of those boats would cause a GFP device with a 100mA set point to trip and disconnect power to their boat.  Finally, for the individual owner of an older boat who happens NOT to be an electrical geek, there may be significant time and expense in hiring a marine-certified electrical technician capable of correcting ground fault problems on boats!

Looking ahead to the future:

The development of the National Electric Code is a methodical and disciplined process.  Change proposals require many months or years of study and consideration before they are adopted.  The next release of the NEC (NFPA 70) is scheduled for 2017.  On November 5, 2014, ABYC submitted a proposal entitled “Assessment of Hazardous Voltage/Current in Marinas, Boatyard and Floating Buildings” to NFPA which recommends requiring GFCI outlets on all marine pedestals.  Pedestal outlets under this proposal would be spec’ed at 30mA for 100mS.  Assuming this is adopted by NFPA (Section 303, Chapter 5) and incorporated into the NEC (Articles 553 and 555), it would probably speed the overall rollout of GFP on docks, as it is much easier and less expensive than the whole dock solutions discussed above as provided in today’s standards.

Isolation Transformers:

With an isolation transformer, the shore power ground stops at the transformer, where it connects to an internal shield that is electrically isolated from the transformer’s case.  The case of the physical transformer is connected to the boat’s onboard AC safety ground.  ABYC E11 treats the secondary (output side) of an isolation transformer as an on-board power source, like a genset or an inverter.  The primary (Input side) of the isolation transformer is fed from the dock pedestal. The secondary (output side of the transformer) has its neutral and ground conductors bonded (connected together) at the transformer.  With an isolation transformer, there is no through electrical connection between the shore power ground and the boat’s on-board grounding system.

From a GFP-on-the-dock perspective, by design, there is no ground fault path (unless the transformer itself has a physical flaw).  Boats with isolation transformers will not have GFP issues on 2011/2014 NEC GFP-compliant docks.  (That said, it is possible to have wiring errors aboard that would constitute ground faults if configured for direct attachment to shore power instead of an isolation transformer.)

There is one significant “EXCEPT” to the isolation transformer story.  If a wired telephone, wired Ethernet or shielded TV cable is ever brought onto the boat, the ground conductors in those cables create an electrical path between the shore power ground and the boat’s on-board electrical grounding system; that is, they bridge the gap that was deliberately created by the design of the isolation transformer.  So, first, boat owners MUST ALSO have galvanic isolator devices installed in ALL OF THOSE CABLES in order to create the physical separation gap in the ground path. These isolators can be of the magnetic or optical coupling design.  The Cable TV galvanic isolator is easy; $10 off the Internet.  Isolators are made for the other cables, but may be pricey.  Second, if a boat with an isolation transformer does have a ground fault in its on-board wiring, the boat will trip a GFP at the time one of those (telephone/Ethernet/Cable TV) “bypassing shore ground paths” is connected.  The risk of this is low, but not zero.  Third, because the wires in telephone and Ethernet circuits are physically small, if there are hidden ground fault wiring errors on the boat, those physically small wires may act like fuses, and open.  There is also the potential of damage to attached electronics.  Again, the risk is low, but not absolute zero.

Word to the wise:

Investigate the status of your older boat now!  If you have this problem, it is only a matter of time before it bites you!

Genset Installation

A “built-in” genset can be a valuable amenity on any boat, but especially on a cruising boat. The need for a generator, like the need for air conditioning, space heating or a watermaker, depends on the personal preferences and the anticipated needs of the boat owner. In the case of a generator, considerations include the need to operate a battery charger away from shore power, use of 120VAC aboard, anchoring vs. marina preference, availability and costs of mooring fields vs. transient docks in planned cruising destinations, geographical range of intended cruising waters, etc.

If a boat’s battery bank is sufficiently sized, and the boat is moving by propulsion engine each day, battery charging from the propulsion engine alternator may be sufficient to deliver and meet AC electric energy needs, via an inverter. Especially so if AC electrical needs aboard are modest. Gensets can power larger loads like reverse cycle heat pumps for air conditioning and space heating. They can power emergency de-watering pumps, battery chargers, AC refrigeration, range/ovens, watermakers, washer/dryers and entertainment devices. They can provide emergency dockside power when commercial utility power goes away. They enable long term anchoring and the use of mooring balls in New England and Florida Keys destination towns, and along the shores of the Canadian Canal Systems. We aboard Sanctuary used our genset for refrigeration, heat and TV for several days in the aftermath of Hurricane Sandy, prior to the restoration of commercial utility power.

There are two choices of generator type that can be used very effectively on boats and in RVs. The most common choice is the combination of an alternating current (AC) alternator driven by a gasoline or diesel motor. Speed-of-rotation must be held constant – preferably 1800 rpm – to produce stable 60 Hz AC output. Speed control is generally accomplished with a mechanical governor that responds to changes in electrical load by adjusting the throttle of the drive motor. Generators are readily available in sizes of 7.5/8.0 kilowatts (kW), suited to boats with 2, 120V, 30A shore power circuits, or 12 kW, suited to boats with a single 240V, 50A shore power circuit.

The second type of AC generator consists of an automotive style alternator (DC output) driven by a gasoline or diesel motor. “DC generators” have internal electronic inverters that convert the DC produced by the alternator to 120V and/or 120V/240V AC at 60 Hz, usually in pure sine wave (PSW) form. These machines use high output DC alternators such as those used in commercial trucks and emergency vehicles, and produce +12V or +24V DC output. The design of DC gensets is mechanically less complex than AC alternators. The motor’s speed-of-rotation is not important to AC output frequency. Batteries can be charged directly from the 12V (or 24V) alternator output.

Regardless of the choice of technology, when installing a generator on a boat, be sure to purchase equipment that complies with the ABYC A27, Alternating Current (AC) Generator Sets electrical standard. Use marine rated materials and installation techniques that comply with the ABYC E11, AC & DC Electrical Systems on Boats electrical standard. Use marine-certified electrical technicians to perform any contracted installation work. The life you save by doing so may be your own, your children or grandchildren, contractors working on your boat, and/or your family pet(s).

As in all things, choosing the electrical rating, in kW, of the unit depends greatly on what one plans to do with it. That analysis and decision is the initial task to complete. One must balance several factors:

  1. continuous (sustained-output) vs maximum (peak-output) load delivery capacity,
    • the average AC electrical load, in Amps, on the boat
    • the maximum AC electrical load, in Amps, on the boat
  2. the percentage load factor (LF) of the generator on the diesel drive engine,
  3. capacity options as available from various manufacturers,
  4. device technology; i.e., DC generator with inverter vs AC generator,
  5. warranty terms and conditions,
  6. budget: i.e., used vs. new unit capital cost, and finally
  7. installation fitup.

Start by looking at the electrical load that must be supported onboard. The most simplistic analysis is based on the boat having either:

  • one or two existing 30A shore power service cords, or
  • a single 50A shore power service cord.

Doing the math for 30A services, the capacity of a single 30A shore power service is up to (30A x 125VAC) = 3750 Watts. A boat with two 30A services could use up to 7500 watts (7.5kW) of capacity at one time. Thus, a genset capable of a continuous output of 7.5kW would be needed to fully power everything aboard to the same level as available with shore power. Since 7500 watts at any one time is rarely needed, a 5kW machine might be an acceptable compromise, but would limit flexibility in maximum case situations. It does not allow for any further future expansion of simultaneous electrical load or for changes in how the Admiral may want to alter electricity use patterns in future months/years.

The capacity of a single 50A shore power service is up to (50A x 240VAC) = 12000 Watts (12kW) to fully power onboard AC electrical equipment. Thus, a genset capable of a continuous output of 12kW is required to fully power everything aboard to the same level as available with shore power.

Aboard Sanctuary, we do occasionally want to have nearly the maximum amount of AC electric power. That happens when we’re running the heat pumps for either A/C or heat, and when we’re also charging our batteries and simultaneously heating hot water and running the microwave. It’s obvious that this does not happen often, but it can and does happen, usually when we’re in a hurry to depart the boat for other (sightseeing) pursuits. That use could be balanced over time, of course; that is, if the time is available.

Unusual AC electrical equipment, like a washer/dryer, high-pressure pump of a watermaker or an air compressor, would also influence the foregoing load-planning scenario. Motor driven appliances introduce two technical considerations. First, they require dramatically higher currents to start the motor turning than they require to keep the motor running. The generating equipment must be able to handle that transient, short-period (peak) load. Second is a technical issue called “Power Factor.” In general, the implication of power factor is that larger ampacity wiring and higher capacity generating equipment is necessary to support the nominal load, often in the range of 20% greater than nominal. If this type of equipment is to be run by an onboard generator, special consideration must be given to wiring and steady-state power requirements.

If one decides to install a traditional rotating-alternator AC genset, I recommend gensets that spin at 1800 rpm vs 3600 rpm to product 60 Hz output. These small engine generators spin at 1800 (four-pole) or 3600 (two-pole) revolutions per minute (rpm) depending on the number of field poles of the stator design. Four-pole alternators are preferred to two-pole machines. The 1800 rpm speed-of-rotation of a four-pole machine offers longer service life and is quieter in operation.

If installing a new genset on a boat that was not previously fit with a genset, I suggest at least evaluating DC gensets that use high-output DC alternators to create regulated DC output driving an electronic inverter to provide the AC. Hybrid DC-with-Inverter gensets may have advantages over native AC alternators, including:

  1. more efficient in battery charging applications,
  2. no speed control issues for maintaining 60 Hz AC output,
  3. better fuel efficiency,
  4. inverter can be run from battery bank when genset is not running, and
  5. easier parts replacement – especially major components – if/when maintenance is required (dependent on manufacturer; go with one that uses non-proprietary components).

With either native AC alternators or DC-inverter hybrids, I recommend diesel-powered drive engine units. *NEVER* gasoline! And, *NEVER* Honda EU2000 or big box household portable units on boats!

The major project cost categories involved in installing a generator are:

  1. The machine, including delivery charges,
  2. the physical installation of the unit on the boat, including the cost of a suitable mechanical platform and a crane or other means of lifting the machine into place,
  3. components, materials and parts (mechanical, plumbing, fuel and exhaust systems and electrical), and
  4. labor.

If installing a net new genset, plan for and fund the following:

  1. the machine itself, ABYC-compliant,
  2. mechanical preparation of the install location on the boat,
  3. raw water engine cooling intake thruhulls, sea strainer, and intake raw water plumbing,
  4. waterlock muffler, exhaust plumbing and exhaust discharge thruhull,
  5. diesel fuel supply, filter(s), and excess fuel return line,
  6. house AC electrical installation, including:
    1. an ABYC-compliant Generator Transfer Switch,
    2. power feed wiring from the genset to the Generator Transfer Switch,
    3. power wiring from the Generator Transfer Switch to the existing AC service panel,
    4. relocation of incoming shore power lines from the existing service panel to the Generator Transfer Switch, and
    5. remote controls for starting, stopping, and monitoring genset operation,
  7. DC power wiring for the genset starter motor
  8. a genset start battery
  9. labor on above items, and of course
  10. sales taxes

Notes on the above:

  1. genset waterlock (muffler) must be sized to handle the exhaust and discharge cooling water,
  2. plumbing fittings must be bronze or non-metallic; never brass,
  3. all parts and custom-fabricated components must be ABYC-compliant,
  4. the selection of Generator Transfer Switch will depend on how you choose to wire the genset (as a 120V machine or a 240V machine; ours aboard Sanctuary is wired as a 240V machine. There are pros and cons about that choice).
  5. electrical and mechanical skills are required to do this task. It can be done as a DIY project of advanced complexity. With time availability and skills, significant labor dollars can be saved on the installation. Labor hours will be significant: 40~50 hours, minimum. I did all the installation work for our genset myself. It took a couple of weeks elapsed time; not full days, just tasks interspersed with other projects and tasks.
  6. the AC wiring, DC starter motor wiring, raw water plumbing and diesel fuel supply and return lines will have to be custom-fabricated.
  7. a fiberglass waterlock will cost around $150.
  8. the Generator Transfer Switch (I used a Blue Sea Systems #9093) was the single most expensive component part, at $350 in 2004.
  9. for AC power wiring, I used a length of 50A shore power cord, #6 AWG, BC5W2, triplex because of the convenience of handling that multiplex wiring package.

Finally, to minimize the number of current and future hull penetrations, consider a single large raw water inlet with a single, large sea strainer, feeding a raw water distribution manifold (“sea chest”).