Category Archives: Electrical Lingo/Jargon

Electrical Lingo: What Is He Talking About

10/18/2021 – Initial post
10/19/2021 – Added “AIC”
11/28/2021 – Added “Charge Acceptance Rate,” “Self-Discharge”
12/12/2021 – Added “Power Quality,” “Switch Mode Power Supply”

Following are my attempts to explain and describe what is meant by some of the “lingo” that appears frequently in Internet and dock conversations about electrical topics. Hopefully, this will make the inevitable future conversations about electrical topics on your boat less intimidating. It is not possible to discuss electrical topics without encountering some “jargon.”  Even in fairly basic discussions, some technical background knowledge is always assumed. The very basic stuff, like Ohm’s Law, is High School science, but that is often not quite “enough.” Online forums are places for people to exchange information and learn. Some minimal “knowledge” is required to read and understand electrical technical exchanges. individuals directly involved in these discussions may understand the context, but those who are generally not familiar with electrical topics may not know what the lingo being used really means. Those who would otherwise benefit from the information exchange may decide to skip the topic, and miss the benefit. This article is an effort to try to help electrical “laymen.”

Some of the descriptive sections below are longer than just simple descriptions.  That’s because I have not written about them before.

I don’t know that I’ve included all of the lingo that those unfamiliar with “things electrical” (“laymen”) may find obscure or confusing. So if ANY READER is interested in terminology that I have not captured here, please contact me to let me know that. My plan is to maintain this document as a “living document,” not a one-time article. Please help make it more complete and understandable.




Standards organization; American Boat and Yacht Council; creates minimum manufacturing performance standards for various aspects of pleasure craft; primary jurisdiction is manufacturers in the United States, and manufacturers who sell boats into the US market, but ABYC standards are used in Canada, and have a very influential role in Europe and SE Asia as well.

Return to TOC


Standards organization; National Electric Code; created and owned by the National Fire Protection Association. Adopted in the United States by individual state, county and municipal governments as the minimum legal requirements for electrical installations performed by installers of electrical systems.

Return to TOC


Standards organization; National Electrical Manufacturers Association; provides mechanical and electrical standards for electrical equipment and components. NEMA publishes more than 700 electrical and medical imaging Standards and technical whitepapers that cover millions of Member products.

Return to TOC


Standards organization; National Marine Electronics Association; NMEA created uniform interface standards for digital data exchange between different marine electronic products; most familiar are:

  1. NMEA0183 and
  2. NMEA2000.

NMEA2000 is an adaptation of CANBUS to suit marine-specific application equipment needs.

Return to TOC


A “Surveyor” is an independent tradesman in the marine industry. “Surveying” is the process of evaluating the current condition of the boat against objective criteria. Surveying necessarily involves a number of objective standards, subject-matter knowledge, work experience and subjective personal judgement on the part of the surveyor.

In evaluating boats, surveyors make extensive reference to industry-recognized and accepted standards. The principle collection of objective standards in North America are those of the United States Coast Guard (USCG) and American Boat and Yacht Council (ABYC). ABYC standards are voluntary, and boats are “grandfathered” to compliance standards that applied at the time the boat was built (not necessarily to the standards in place as of the time of survey). If the boat has undergone “significant retrofit and upgrade,” current ABYC standards apply.

The surveyor’s written work-product is a “Survey Report,” or “Survey.” Survey content, and the way that content is phrased, can result in insurance companies mandating certain repairs, determine if the client can get insurance at all, and can affect the pricing of premiums.

There are several kinds of “surveys,” including:

  • Condition and Value
  • Insurance
  • Electrical
  • Engine
  • Corrosion

Statistically, electrical problems account for a large percentage of maintenance and performance issues that boaters typically encounter during their period of boat ownership. Previously-owned boats may have had OEM equipment installed that cannot meet today’s requirements or may have had prior electrical work done by unqualified or under-qualified tradesmen or DIY owners. Buyers, particularly of larger, previously-owned boats, should consider commissioning a pre-purchase “Electrical” Survey. Buyers of wooden-hulled boats or of metal-hulled boats should also commission a pre-purchase “Corrosion” Survey. Electrical and Corrosion surveys are done by technicians who specialize in those specific marine specialty areas.

🎓Click link for additional information on Boat Surveys

Return to TOC


Interoperability” is the technical capability that allows consumers to buy products from different manufacturers with confidence that the products will operate properly in a system containing equipment from multiple different manufacturers.

The word “interoperability” is a conceptual expression used across organizations and manufacturers involved in communications technologies.  It means that various different pre-programmed electronic devices made by different manufacturers can be networked together and will then be able to “communicate with,” “understand” and “respond appropriately” to one another. This is done by defining a common “language” (called a “data protocol”) to which individual manufacturers agree to design their products to be compatible.  For example, “interoperability” means that products made by Garmin can interact correctly with products made by Sitex, Sitex products can interact correctly with products made by Raymarine, Raymarine products can interact correctly with products made by Furuno, and Furuno products can interact with products made by Garmin, all can interact with various engine manufacturers, and all can be reported by Mareton network monitors

The marine standard communications language “protocol” is either NMEA0183 or NMEA2000, which are different vintages of communications protocol.  They are mutually incompatible.  The standard communications protocol in the computer world is Ethernet (IEEE 802.3) or Wireless Ethernet (IEE802.11 a/b/g/n/an).  The cellular communications standards include GSM and CDMA. Older equipment protocols, like RS232, are still found in use in legacy equipment, like Raymarine Seatalk, but are no longer found in new equipment, and are disappearing in favor of newer, faster, more functionally rich, network technologies.

🎓Click link for additional information on Marine Data Networks

Return to TOC

DC Electricity
AC Electricity

Direct Current (DC) is electricity that flows in one, and only one, direction from the source, through a circuit, and back to its origin. By convention, engineers treat DC current as if it flows from a “positive” terminal to a “negative” terminal.  Common sources of DC electricity are batteries, solar cells and fuel cells.

🎓Click link for additional information on DC Electricity On Boats

Alternating Current (AC) is electricity that flows in two directions, with the “positive” and “negative” terminals of the source periodically reversing their relative roles of outbound and return conductors. The time period of the reversal of direction is called “frequency.”  In North and Central America, the standard frequency for commercial AC power is 60 Hertz, meaning it reverses “60 times per second.” In other parts of the world (Eastern Europe, Asia and Oceana), the standard frequency is 50 Hertz. The most common sources of AC electricity are rotating machines (generators) driven by water or steam turbines or fossil fuel engines, and inverters that produce AC electronically from DC sources.

🎓Click link for additional information in AC Electricity Fundamentals, Part 1
🎓Click link for additional information in AC Electricity Fundamentals, Part 2
🎓Click link for additional information in Electrical Behavior of a 208v/240v Boat

Return to TOC

Single phase
Three phase

“Single Phase” and “Three Phase” are terms that specifically refer to types of alternating current electricity. These terms refer to how the AC electricity originates in generating equipment and is then distributed to end users.

Single Phase” (or 1∅) refers to alternating current systems in which any/all of the voltages in the system rise and fall together, at the same time.

Three Phase” (or 3∅) refers to alternating current systems in which there are three separate voltages in the system which  rise and fall at intervals displaced 120° in time apart from one another.  In English, that means they do not all rise and fall together, but rather in a time-displaced, repeating pattern.

This distinction is extremely important to electrical system and equipment designers, but it is of no significant interest to end users of electric power in buildings or boats except as noted in the following section on 208V vs 240V services found in marinas and other facilities that provide shore power to boats.

🎓Additional description and drawings in AC Electricity Fundamentals, Part 1

Return to TOC

120V/208V AC Power
120V/240V AC Power

Throughout North America, the AC power present in commercial buildings (apartment houses, condos, light commercial businesses), can be either 120V/240V or 120V/208V.  The AC power delivered to single family residential homes is 120V/240V. 120V/240V originates with Single Phase sources, so that the end user gets 120V/240V from the source; 120V/208V originates with Three Phase sources, so the end user gets 120V/208V.  For 120V-only appliance and attachment needs, both are identical and building occupants would be unaware of any difference at all.  There is only a significant difference for devices that might need 240V to operate correctly.

In larger buildings and multi-building complexes, Three Phase distribution is significantly less expensive to install and maintain than Single Phase distribution, so whichever type is installed in a facility is the facility owner’s capital cost choice.  Because of the voltage differences between the systems, the facility manager is generally the designated party responsible for replacing dwelling unit equipment (ranges, dryers, water heaters) that requires both 120V and either 208V or 240V voltages. Making equipment replacement the facility manager’s responsibility means the occupant of the dwelling unit doesn’t need to know or care about their electric service.

In boating, many marinas are served with Three Phase systems, which means it is inevitable that boaters will encounter 120V/208V power at some marinas, and 120V/240V power at other marinas.  Boat equipment that requires both 120V and 208V/240V voltages (especially refrigeration and HVAC equipment and water pumps) MUST BE DESIGNED to tolerate both 208V and 240V sources. The boat owner is responsible to ensure that equipment is properly rated, and to know what the supply voltage is to the vessel when hooking up to shore power.  This is an area that can impact boaters who install 120V/240V household appliances aboard boats.

🎓More information in “Why Do I Get 208VAC At Some Marinas?

Return to TOC



Electrically, “Ground” is just a physical connection to the earth.

  • The NEC defines “Ground” as: “The Earth.”
  • ABYC defines “Ground” as: “11.4.16 Ground – the potential of the earth’s surface. The boat’s ground is established by a conducting connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.”

“Grounding” is the act of making an electrical connection to the earth.  Because the earth is electrically conductive, the act of electrically connecting one of the current carrying conductors of an electric circuit to the earth “references” the voltage of that conductor to the voltage of the earth itself.  This also establishes the “polarity” (discussed further down) of the conductors in the distribution system.

The “conductivity” of the earth from place-to-place varies greatly, and the earth is not a particularly good conductor of electricity. It is not “good enough” to provide any kind of guarantee of electrical shock safety.  Yes, the act of referencing a building’s electrical system to earth is a safety mechanism of sorts, but it is not primarily about electric shock mitigation to protect living beings.  It is primarily about mitigating facility and equipment damage caused by spike voltages from static electricity, lightening and abnormal events that can occur in the commercial electric power grid.


So if the system’s ground connection isn’t about shock mitigation, then how is shock mitigation accomplished?

Throughout North America, AC systems in buildings and on boats have a conductor (green insulation on boats, bare copper in buildings) that is commonly called the “Safety Ground.”  It’s called the “Safety Ground,” or “Ground” by almost everyone EXCEPT the national standards-writing authorities, who call it an “Equipment Grounding Conductor” (EGC).  All metal objects (plumbing, furnace, air ducts, heat pumps, appliance cabinets, utility outlets, etc) installed in the building/boat are connected together by a network of EGCs, and that entire network is attached to the earth ground back at the power’s source.  If there is an electrical fault anywhere in the system which causes power to be wrongly applied to metal contact surfaces anywhere in that network of connected metal objects (dangerous shock “touch potential” to living beings), that EGC network provides a low resistance electrical path that will cause a circuit breaker to “trip,” thereby clearing the fault by removing electrical power from the faulting circuit.  “Bonding” or “Bonding conductors” are terms commonly applied to EGC conductors.

So the way we boaters might see “Grounding” discussions is context sensitive.  It could be in reference to either one of two different applications:

  • Connecting the system to the electrical potential of the earth, or
  • Interconnecting metal objects in a network to provide a low resistance safety path that will trip circuit breakers.

By far the most common way the term “ground” will be encountered by homeowners and boaters is the latter case; i.e., a reliable low resistance path that will clear a fault by tripping a circuit breaker.

Note: the terms “ground” and “grounding” as used in conversations of VHF and UHF radio transmitting equipment and antennae is a related but very different technical discussion.

🎓More information in Earthing and Grounding
🎓More information in Bonding System Design and Evaluation

Return to TOC

Circuit Breakers –

  • single pole
  • double pole
  • multi-pole

The purpose of any “Circuit Breaker” (fuse or mechanical switch) is to protect its attached wiring from electrical overload.  Wiring that is overloaded will become hot, and with a large or prolonged overload, can get hot enough to cause ignition of nearby flammable materials or its own insulation.  It is urgently important to ensure this DOES NOT HAPPEN with wiring that is hidden inside the walls of a home, or on or behind liner panels of a boat, where the wiring is likely to be surrounded by, or in contact with, flammable materials.  In such situations, overheating can cause a fire danger long before a human occupant becomes aware of impending danger.

Circuit breakers (fuses) used for “overcurrent protection” (OCP) in residential dwellings come in a wide range of standard trip ratings, 15A, 20A, 30A, 50A, 100A, 200A, etc. When the amount of current in the circuit exceeds the overcurrent rating of the circuit breaker, the circuit breaker “opens” to protect the circuit’s wiring.  Wire sizes and insulation ratings result in a specification for the amount of current a given wire conductor can carry safely.  That current carrying rating (“Ampacity”) and circuit breaker OCP ratings MUST BE MATCHED to each other.  (Fuses also serve to protect wiring.)

Circuit breakers are made in “single pole,” “double pole” and “multi-pole” configurations. “Single pole” breakers in a 120V single phase system open only one conductor; normally the “hot,” or “energized” “current-carrying conductor” that feeds power to a circuit.  “Double pole” breakers in 120V single phase system opens BOTH the “hot” conductor AND the conductor which returns power from the circuit, but “double pole” breakers in a 120V/240V single phase system open the two “hot” conductors, but not the return conductor.

Multi-pole breakers are available for use in 3∅ systems and for certain special uses in single phase systems.  One such “special case” in a 120V/240V single phase system on a boat is the “Generator Transfer Switch,” which in a 120V/240V case, must transfer BOTH “hot” conductor(s) AND the return conductor.

Return to TOC

  Ground Fault;
“Ground Fault Sensor”

A “fault” is an undesirable, unwanted condition where one of two thing occur.  Either:

  • electric current DOES NOT flow when it should, or
  • electric current flows someplace where it SHOULD NOT go.

Current that flows where is should not flow creates shock hazards that can be dangerous to all living creatures.

Depending on the “conductivity” of the medium through which the fault current flows, “faults” can occur in a range of very small electrical currents to very large electrical currents.  Fuses and circuit breakers protect against large electrical faults.  A large fault will cause a fuse or circuit breaker to “open” (“trip,” “blow”). However, small faults that are too small to cause an OCP device to “open” can exist in any electrical system.

Ground Faults” are a condition where electric current “escapes” from the normal electrical wiring of an electric circuit and finds its way back to its source via some unintended path.  That path can be through soil, or it can be through water, or it can be through any other medium that conducts electricity, such as a wet, wooden or concrete flooring or decks.  The NEC defines a “Ground Fault” as: “An unintended, electrically conductive connection between the ungrounded conductor of an electrical circuit and the normally non-current carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.”  (Note the word “earth,” meaning “soil” and/or “water.”)  The scope of the NEC definition includes all of the things that can and do happen on a boat, including current running through the “earth”.

In the case of a “Ground Fault,” which can be quite small but quite lethal, a different kind of circuit breaker from a standard over-current protection (OCP) breaker is needed to protect people, pets, wildlife and faulting equipment. In residential settings, “Ground Fault Circuit Interrupters” (GFCI) are installed throughout dwelling units.  GFCI devices are intended to protect personnel (living beings) from shock hazards.  In commercial and industrial settings, “Equipment Protective Devices” (EPD) with a higher trip setting are used to protect equipment.  EPDs protect equipment rather than personnel, but in actual fact, they do ALSO protect personnel.  EPDs and GFCIs are a class of device called “Ground Fault Interrupters.”

EPDs have many other names, like “Residual Current Device” (RCD), “Ground Fault Protector” (GFP), “Ground Fault Interrupter” (GFI), “Equipment Leakage Circuit Interrupter” (ELCI), and others. Regardless of what they’re called, these devices all do the same thing; they monitor the amount of electric current going out into the circuits they protect and compare that amount to the amount of electric current coming back. The outbound and returning quantities must match within a very small tolerance (5 mA for personnel protection, 30 mA for equipment protection). If they do not match within circuit breaker’s tolerance, by definition, power is “leaking out” of the circuit somewhere it should not be going, and the “Ground Fault Sensor,” whatever its name, will “open” the electrical feed to protect the equipment from the risks of that leak condition.

🎓More information in AC Safety Tests For Boats
🎓More information in Ground Faults and Ground Fault Sensors
🎓More information in Difficult to Hire Troubleshooter

Return to TOC

Reverse Polarity

There are two ways in the overall world of electricity to think about “polarity.”

  1. As the relative positive or negative relationship of voltage at any given place in a circuit, and
  2. The manner in which the wiring of a system is physically laid out, denoted by the color of the wire’s insulation.

In DC systems, there is a permanent positive and negative relationship of the voltages in a circuit.  The electro-physics of batteries, solar panels and fuel cells establishes permanent positive-voltage and negative-voltage terminals that never change with respect to one another.  But in AC systems, the voltage polarity changes in a repeating and rhythmic sequence, reversing once in every AC voltage cycle (“frequency,” measured in “Hertz,” with voltage reversing direction 60 times per second in North America).  Since the “polarity” for the voltage relationship changes each cycle in AC systems, the term “polarity” refers to the way the wires in the AC system are laid out; that is, case 2 above.

Just as in DC circuits, 120V AC circuits only “require” two wires to operate.  That is, an individual wire that transports power from the AC source to the power consuming device and a second individual wire that returns power from the device back to the source.  AC systems are attached to ground, as discussed earlier.  At the “designated source” of AC electricity on any premises, one of the source wires is intentionally, deliberately connected to the earth; “grounded.”  That specific connection to the earth is what establishes the electrical “polarity” of the AC system.  In the lingo of the electrician, that wire is from then on known as the “grounded current-carrying conductor” in the system, nicknamed the “neutral” conductor.

The neutral conductor will always and only have “white” insulation (in the North American color scheme; the European color scheme on some boats is different).  In that way, anyone, anywhere in North America working on a 120V/240V AC single phase system, will always be able to assume that the “white” wire is “grounded” at its source.  That also means that the working voltages of the not-grounded (ungrounded) conductors in the system (the “hot” conductors) are measured with respect to that “grounded neutral” conductor.  The colors used for the insulation of the various “hot” conductors communicates their roles in the circuit to humans working on the systems; that is, communicates their “polarity” as referenced to ground.  In North America, the colors “black” and “red” denote the “ungrounded current carrying conductors,” or the “hot” conductors, that transfer current from the source to the circuit, and the color “white” denotes the “grounded neutral” conductor that returns current from the circuit to the source. “Green” denotes the “Equipment Grounding Conductor” network, or “Safety Ground.”

From the above, we can appreciate the arcane lingo used in the electrical standards.  The ABYC definition of a “Polarized AC System” as: “a system in which the grounded and ungrounded conductors are connected in the same relation to terminals or leads on devices in the circuit.”  The above is also the language of the National Electric Code (NEC) in North American single phase systems.  This language is “enforced” by the insulation colors of the AC conductor(s).

The NEC and ABYC standards require that “hot” wires and “neutral” wires ALWAYS appear in the same relative positions on plugs and receptacles. Ubiquitous 120V utility outlets and plugs have blades of different width to denote “hot” and “neutral.” Marine 30A twist lock receptacles and plugs have blades that are also of different widths and shapes which make it possible to connect them together IN ONLY ONE ORIENTATION.  Again, this is because the electric codes REQUIRE that “hot” and “neutral” are ALWAYS maintained IN THE SAME RELATIVE PHYSICAL RELATIONSHIP.

In parts of the wiring system where wires are connected together permanently, with splices or via ring terminals at busbars rather than disconnectable plugs – as internally within appliances – the physical wiring relationship must also be maintained. That is, “hot” wires always connected only to other “hot” wires and “neutral” wires always connected only to other “neutral” wires.  It is WRONG and DANGEROUS to connect a circuit’s “black” wire to an appliance’s “white” wire, and a circuit’s “white” wire to an appliance’s “black” wire.   That is a “reversed polarity” condition, and particularly dangerous to living beings.

Reversed polarity will not prevent the connected appliance  from working, but if a fault occurs, reverse polarity elevates the metal cabinet of the appliance to a dangerous “touch potential” voltage compared to the case of the appliance that’s next to it, or nearby to it.  That is a potentially lethal combination.  Consider, for example, a washing machine wired with a reversed connection that is placed adjacent to a dryer wired correctly.  A fault to the metallic case of the washer would put 120V on the washing machine’s metal cabinet. The family member tasked with doing the laundry, while transferring wet clothes from the washer to the dryer, could get a shock; a potentially lethal shock.

So we see that the fixed relationship of wiring in an AC system MUST be maintained throughout the system. To accomplish that goal across society, NEMA standards define the blade locations on all plug and receptacle devices made/sold/used in the United States.  Similar standards associations do the same in Canada. The blade locations and orientations on household 15A/20A plugs and receptacles are standardized; 30A dryer and 50A range plugs and receptacles are standardized; the blade locations on all 30A marine twistlock receptacles are standardized; the blade locations on all 50A marine twistlock receptacles are standardized.

Why do we do all this?  Because the intent is to maintain the relative position and location of “hot” and “return” conductors to ensure personnel safety from lethal electric shock.

Understanding the above, it’s clear that “Reverse Polarity” wiring ONLY occurs from a man-made installation mistake in wiring a premises system; that is, incorrectly wiring plugs and receptacles or appliance internal wiring connections.  By definition, “Reverse Polarity” means the “hot” and “neutral” “return” conductors are “reversed” in the plug/receptacle or in the appliance.  When an appliance is plugged into a mis-wired outlet, or an appliance is wired backwards, that becomes a shock hazard that can kill.

Fortunately for all of us, “Reverse Polarity” is a relatively rare situation, but it does happen, and it has been responsible for deaths.  It can happen in a home, on a dock or on a boat.  It happens on docks if/when maintenance personnel make a mistake. When it happens on a 30A marina dock receptacle, the properly wired boat that plugs into that receptacle will have a potentially lethal “shock fault” between dock and boat. So when your crew member (self, spouse, guest, grandchild) steps off the boat with a properly grounded handrail and grabs the properly grounded metal dock railing, a very possibly lethal shock occurs.

On boats with 120V shore power cords, the ABYC requires that Reverse Polarity “Detector” circuits be installed.  These “detector circuits” sound an alarm or light a prominently displayed warning light, or both, to warn of a Reverse Polarity situation in dock wiring that reverses the polarity of circuits on the boat.   If this condition ever presents itself, boaters should immediately disconnect from that shore power source, and as a matter of personnel safety, NOT use it.  The presence of the fault should be immediately reported to facility management for URGENT investigation and correction.

Return to TOC


Adapters are physical devices that change the manufacturer-installed blade configurations of electrical cords while maintaining polarity consistency.  Adapters allow plugs of one NMEA blade configuration to be changed to use outlets of a different NMEA blade configuration.

For cruising boaters, there are many cases and conditions where adapters are necessary and can be extremely useful.  Many facilities have marine 20A outlets, which are very similar, but not identical to, Marine 30A outlets.  An “adapter” can be used to access the 20A pedestal power source from a 30A or 50A shore power cord.  Often, only a conventional 15A/20A utility outlet is available ON A DOCK, but with a proper “adapter,” can be accessed to charge batteries and keep a refrigerator running. “Adapters” make it possible for 30A shore power twistlock cords to access 50A pedestal outlets.  These cases often mean not being able to run everything aboard the boat at the same time, but can provide for essential, if minimum, electrical needs.

“Splitters” (also called “Wye Adapters”) are a form of electrical adapter.  There are two common types:

  1. One 240V/50A plug (fits into 50A pedestal outlet) to two 120V/30A outlets (fits 30A shore power cords to boat)
  2. Two 120V/30A plugs (fits into two 30A pedestal outlets) to a single 50A outlet (fits 50A shore power cord to boat)

Case one above is called a “splitter” (or “wye adapter”); case two is called a “reverse splitter” (or “Reverse Wye Adapter”)

The use of adapters in electrical systems ALWAYS REQUIRES VIGILANCE on the part of the user, because the adapter often creates a connection that is under-protected against electrical overload.  When an adapter is used to obtain power for an electric tool built for use on a 15A circuit from a 30A source circuit, conventional extension cords and the tool cord itself are not properly protected for overload. When an adapter or splitter is used to obtain power for a 30A circuit from a pedestal source that is protect at 50A, the 30A cord is not properly protected for overload.  So the use of adapters can create some net additional risk. It is ALWAYS better to use commercially-made adapters from reliable electrical equipment manufacturers than homemade adapters of lesser-known quality materials and assembly safety.

🎓More information in 50A Power From 30A Sources

Return to TOC

Isolation transformer
Polarization transformer

The ABYC defines an “Isolation transformer” as: “a transformer installed in the shore power supply circuit on a boat to electrically isolate the normally current carrying AC system conductors from the normally current carrying conductors of the shore power supply. NOTE: The shore power grounding conductor connection to the onboard grounding conductor is also isolated.

The ABYC defines a “Polarization transformer” as: “a transformer installed in the shore power supply circuit on a boat to electrically isolate the normally current carrying AC system conductors from the normally current carrying conductors of the shore power supply. NOTE: The shore power grounding conductor connection to the onboard grounding conductor is maintained.

The only difference between these definitions is the grounding connection of the boat. In one case, it’s isolated from the shore power grounding system, in the other, the connection to the shore power grounding system is fully maintained.

Shore power transformers can be very useful, but SHOULD NOT BE USED to mask “ground fault” issues on the boat. Transformers DO NOT “fix” wiring problems in the boat’s system, and if there are fire safety or shock safety issues on the boat, they will still be there after a shore power transformer is installed unless they are first found and fixed.

Return to TOC

Battery “Technologies”

In 2021, there are two types of battery chemistry options available to boaters:

  1. Lead-Acid
    •          $ – Flooded Wet Cells
    •     $$$ – Absorbed Glass Mat (AGM)
    •     $$$ – Gel
    •   $$$$ – Carbon Foam
  2. Lithium Ion
    • $$$$$ – Lithium Iron Phosphate, LiFePO4, Lithium Ferrophosphate or LFP (all these terms are used in articles and advertising, and all mean the same thing in reference to the chemistry of the battery.)

Lead-Acid Batteries: Flooded Wet Cells, AGM, Gel and Carbon Foam are all Lead-Acid chemistry battery manufacturing techniques that all have identical electro-physics.  The individual names (wet cell, AGM, Gel, etc) reference the composition, suspension and retention techniques used in managing the cell’s electrolyte. Flooded wet cells have liquid, weak-concentration sulphuric acid electrolyte.  AGMs retain the sulphuric acid electrolyte in fiberglass “mats” sandwiched between lead plates.  Gel batteries have a gelatinized sulphuric acid electrolyte int the space between adjacent lead plates.  The details of their manufacture give them slightly different operational capabilities and characteristics, but all are fundamentally lead-acid chemistry batteries.

🎓More information in Battery Replacement
🎓More information in Batteries: Questions and Answers
🎓More information in Batteries: Charging and Care
🎓More information in Battery Bank: Separate vs Combined

Lithium Chemistry Batteries: LFP batteries are relatively new arrivals to the boating market.  Other lithium chemisrty batteries have been around for years in miniature electronic equipment and commercial applications in different forms.  Lithium chemistry batteries are fundamentally different than lead-acid batteries, and require different control and safety systems.

🎓More information in Lithium Chemistry Batteries On Boats
🎓More information in Lithium Batteries On Boats, Part 2/

Return to TOC

Battery “Service Types” 

Lead-Acid batteries are manufactured in two service application categories:

  1.          $ – Start Service
  2.     $$$ – Deep Cycle Service

“Start Service” batteries have thin lead plates, and are built in packages that contain many pairs of thin plates. These batteries do not store much total energy, but they can give up the energy they do store very quickly. They are very good for starter motor applications that require many, many hundreds of amperes to turn the motor and crank the engine, but they do not have the total storage capacity that allows that energy to be delivered for very long. The Battery Industry rates start service batteries in “Cranking Amps” (CA/CCA/MCA) and “Reserve Capacity.”

Compared to “Start Service” batteries, “Deep Cycle” batteries have pairs of lead plates that are relatively thicker, but fewer of them in their finished package. “Deep Cycle” batteries can store relatively much larger amounts of energy, but because the plates are thicker, they cannot give up their energy at as fast a rate as their “Start Service” cousins. “Deep Cycle” batteries are rated in “Ampere Hours” (aHr).

“Start Service” batteries will not be suitable for powering “house” and “inverter” loads, but a “Deep Cycle” battery of sufficient capacity will easily start a diesel engine. Batteries of the same type can be placed in parallel for long periods without degrading their service life, and many boaters leave separate battery banks switch-connected together “for the long haul” (although the defeats the supposed redundancy of having multiple banks). “Start Service” batteries are much less expensive than “Deep Cycle” batteries, and have long service lives. It’s a matter of personal preference and personal economics how individual boaters select and mix-‘n-match lead-acid battery use.

Return to TOC

Battery Monitor

A device that provides “overwatch” on the operational conditions and charge state (SOC) of a battery or battery bank. It is not, itself, a battery charger, but it does “watch over” the charging process in real time and does report the battery terminal voltage (V), the amount of current (A) entering (or leaving) the battery, and the remaining charge level (aHr) of the battery at the present time. Think of this as equivalent to the fuel gauge of a car. It allows the boater to know when to recharge and to avoid over-discharge.

EVERY BOAT – CERTAINLY EVERY CRUISING BOAT – SHOULD BE EQUIPPED WITH A BATTERY MONITOR. It is not sufficient to simply monitor battery terminal voltage. Battery terminal voltage is a late indication of charge and discharge condition, and it’s way too easy to over-discharge batteries, which impairs their potential service life.

There are two different types of battery monitors. One is a “Coulomb Counter” and the other is a “Conductance” technology. I personally prefer the former type. Installing a battery monitor is a one-time investment, and costs are similar. There are feature alternatives and marketing choices that determine the number of battery banks that can be monitored and the types of batteries that can be monitored. Monitoring lead-acid (today) and lithium batteries (in the future) is a feature consideration for a new buyer.

Return to TOC

Battery “Watering”

“Watering” is a term that applies only to lead-acid Flooded Wet Cells.  All lead-acid batteries contain a liquid electrolyte, but only Flooded Wet Cells function in a way that the electrolyte is accessible to owners and must be periodically refreshed. Lead-acid battery electrolyte is a dilute mixture of water and sulfuric acid.  As the battery discharges and recharges, the liquid electrolyte interacts chemically with the lead plates to release (discharging) or absorb (recharging) electrons.  In the process, hydrogen gas can be given off.  Lost hydrogen amounts to lost water, and distilled (pure) water must be used to periodically replace what may have been lost in operation.

The process of replacing lost electrolyte volume is known as “watering” the battery.

Return to TOC

“Best practice”

As in all things in life, there are many ways to accomplish most electrical tasks. Many self-taught DIY electricians can “get things to work.” But, “things” on a boat are different than “things” in a residential dwelling unit, and even paid, professional electricians often do not understand marine requirements.  “Doing things” on a boat as they would be done in a single family residence creates MISTAKES and DANGEROUS CONDITIONS on boats, and often results in unexpected inconvenience for the unsuspecting boat owner.

There are materials “best practice” and operational “best practice.  For materials “best practices,” ADHERING TO SAFETY CODES IS THE ABSOLUTE MINIMUM REQUIREMENT FOR WORK ON ANY BOAT.  The NEC and the ABYC Electrical standard are MINIMUM INSTALLATION AND PERFORMANCE STANDARDS.

Work done exactly to ABYC standards in an installation will be safe, but not necessarily “best practice.”  For example, single-pole breakers are allowed in 120V branch circuits, but double-pole breakers protect from “Reverse Polarity” and are more safe, so would be “best-practice.”   Use of Class T fuses in high current circuits vs other fuse types would be “best practice.”  Inverters rated to UL458 over units not rates to UL458 would be “best practice.”  Marine rated appliances made with double insulation, DC compressors and corrosion resistant components would be “best practice” compared to household appliances. Type THHN untinned copper wire with 90℃ insulation is allowed, but Type BC5W2 tinned Boat Cable with 105℃ insulation would be “best practice.” “Best-practice” is often virtually always “more expensive.”

“Best Practice” in operations is also important to service life of electrical equipment.  It’s always best to apply power “from source to load:” i.e., “outwards.”  In that model, we assume that EVERYTHING starts out “off.”  The sequence is, first, attache the shore power cords to the boat at both ends.  Then, turn the pedestal breaker “on,” next, turn the Main AC Panel breaker on the panel “on,” and then, last, branch circuit breakers can be turned “on.”  When preparing for departure from a dock, do it all in reverse; “from the away end back into the source.”  Turn all branch circuits “off,” then turn the Main AC Panel breaker “off,” and finally turn the Pedestal breaker “off.”  DO NOT disconnect the shore power cord until power if “off” at the pedestal.  Is all this “necessary?”  Well, it extends the service life of all of the switching devices in the entire circuit, and minimizes the chances of sparks and arcs.

People take shortcuts.  Homemade electrical adapters are never “best practice.”  Homemade stuff is virtually always made from components that come from big box or hardware stores.  Those components ARE NOT RATED for marine use, and are not rated to be installed in high-UV or wet locations. Most of them do not meet the requirements for marine use at all, and are certainly not “best-practice.”

Always employ professionals who are fully trained for boat and marine installations. Whenever possible, make “best-practice” choices.

Return to TOC


“Alternator” being a word derived from “Alternating Current,” an “alternator” is a device that creates AC electricity.    The term “Alternator” has two significant and very different meanings to boaters, so context is important.

The most familiar use of the word alternators refers to a device mounted on gasoline and diesel engines as a DC device that charge batteries.  Nothing there to suggest AC, right?  Well, the alternator on engines, as a machine, is actually an AC device (a 3∅ AC device at that), but inside its case, it uses a “diode” circuit to convert the generated AC into a DC output suitable to charging batteries.  So the first and most common use of the term is to refer to the belt-driven machine on the engine that charges batteries.

Now since the word “alternator” derives from the term “alternating current,” consider the boat’s AC generator.  The generator has two major internal components: one, it’s drive motor and two, the alternator, which is the mechanical part that creates AC electricity.  The drive motor is usually a gasoline or diesel engine, but the electrical end is called the “alternator.”  That alternator, like its on-engine cousin, is a repairable, replaceable electrical device component.

So context is important.  The device to which the word “alternator” refers depends on the topic under discussion.  The word “alternator” is used correctly in either context.

Return to TOC

Zener Diode
Surge Suppressing Device

A “Diode” is an electronic component device that conducts electricity in only one direction.  Diodes are widely used in circuits that convert AC into DC electricity and in anti-corrosion circuits.  This latter use is because when a diode does conduct, it creates a known, fixed voltage drop across it’s input and output terminals.  That voltage is very small, but so are the natural corrosion potential voltages of metals.  So the internal diode junction will block small currents that cause undesirable corrosion in underwater metals, but it will pass larger currents that are desirable or necessary to the operation of the circuit of which they are a part.

A “Zener Diode” is a special kind of diode.  Zener diodes will conduct, but not until a specific voltage threshold is reached.   So think about an outlet strip used to power your computer, printer, monitor, router, TV, and stereo equipment.  If there is a voltage surge on the line powering that equipment, all of that expensive electronics can be damaged.  But if that line is “clamped” with a surge suppressor, then the spike energy will be diverted to ground through the Zener diode and the attached equipment saved from damage.  That is the purpose of Zener Diodes and other “avalanche-type” diodes.

As a “best practice,” Surge Suppressor diodes should be connected to the DC output of engine alternators.  If the output line of an engine alternator is accidentally disconnected when the device is working, the magnetic field in the device at that time will collapse, and create a very large electrical spike.  The spike can damage the alternator diodes and also the electronics of devices attached to the DC buss of the boat.  A Surge Suppressing Device can mitigate, maybe prevent, serious damage.  It just sits there, maybe for years, maybe forever, until it is needed. just waiting to protect against a serious and expensive event that may never happen.

Return to TOC

Voltage Drop

“Voltage Drop” is usually not a “primary complaint.” “Voltage Drop” is usually a secondary consideration arising from discussions of some other equipment performance or reliability symptom. Typically, this will be an issue that has emerged over time in the operation of a particular appliance or device, and has reached the point of being an annoying operational problem; ex:

  • a slow running or stalling water pump, circulator pump, thruster motor, windlass motor or davit crane,
  • autopilot dropping out
  • gauges reporting a lower voltage at one helm station than is reported at another helm station,
  • random alarms from electronic equipment, for no apparent reason,
  • lights dimming or flickering when the water heater comes on.

“Voltage drop” affects the electrical performance of equipment.  “Voltage drop” can be an issue in all electrical circuits, both AC and DC. “Voltage drop” is ALWAYS associated with either:

  1. the quality (electrical integrity) of electrical connections between power source and attached appliance, or
  2. the gauge (diameter) of the wiring between the power source and the attached appliance.

“Voltage Drop” is often caused by poorly made electrical splices, screws or nuts/bolts that are not tight, or corrosion of a splice connection. It can also be associated with circuit breakers having weak internal springs or cycle-life wear-and-tear to the internal electrical contacts. Often, there will be signs of overheating at the site of connections that are the cause of voltage drop.

All electrical wiring has the physical property of “electrical resistance.”  Small-diameter wire has more resistance-per-foot of length than large-diameter wire. Aluminum has more resistance-per-foot than copper. When installing electrical equipment, it is extremely important to match the type of cable and its diameter to the requirements of the attached device. Included in that calculation is the length of the round-trip wiring from power source to appliance AND back. It is always OK to use larger wire than needed.  It is NEVER OK to use smaller wire than is needed. ONLY copper wire should be used on boats. Preferably, wire rated as Type BC5W2 Boat Cable.

Special attention and consideration should be given to the requirements of bilge pumps. Bilge pumps are frequently small motors that do not draw greatly excess current when mechanically stalled.  Bilge pumps are particularly susceptible to debris floating in the bilge. Debris can block the impeller of a bilge pump, effectively creating a stalled rotor condition. ABYC requires that motors be fused at a level that will prevent overheating of the pump in a stalled rotor condition in cases where the stall condition lasts for seven hours or longer. Per ABYC: Motors and motor operated equipment, except for engine cranking motors, shall be protected internally at the equipment, or by overcurrent protection devices suitable for motor current. The protection provided shall preclude a fire hazard if the circuit, as installed, is energized for seven hours under any conditions of overload, including locked rotor.  So if the manufacturer of a bilge pump calls for a 10A fuse, installers MUST NOT simply use larger diameter supply wire to the pump and protect that circuit at too high a current.  True, there won’t be a voltage drop issue, but a stalled motor fused too high CREATES a fire hazard.  I wonder, do you suppose a surveyor would catch such an issue on a boat?

Return to TOC

Amps vs Amp Hours

Amp Hours and Amps ARE NOT the same thing.  These two terms are very frequently confused and used incorrectly.

“Amp Hours” (abbr: “aHr”) is a measure of quantities of energy storage capacity, or a statement of energy requirements.  That can be either the storage capacity of a battery or battery bank (ex: a fuel tank able to hold some fixed number of quarts or liters or gallons of fuel; its “capacity”) or the amount of energy needed to support the operation of an appliance over a period of operation (ex: it takes 500 gal of diesel to move Sanctuary from Baltimore to Punta Gorda; it takes 250 Amp Hours of 12V DC to power Sanctuary overnight at anchor).  This is about quantities of energy.

“Amps” (abbr. “A”) is the measurement quantify of electrons moving in an electric circuit at a particular moment in time; i.e., the rate at which energy is being consumed in the moment (ex: currently consuming 4 gallons per hour, against an operating range of 2 gallons per hour at idle, 6.0 gallons-per-hour at “hull speed” and 32 gallons-per-hour at “planing speed”; or, drawing 15A because the water heater is “on”).

Return to TOC

Ampere Interrupt Capacity

“Ampere Interrupt Capacity” is the amount of current that a switching type device (circuit breaker, fuse, switch, relay, solenoid) can safely stop.

All electrical switching devices of all sizes have a rating for the amount of current that they can safely switch “on” and “off.” Actually, it’s the amount of current they can switch “off” when they are “opened;” i.e., “turned off.”

Whenever an electric current is interrupted by a switching device, and particularly current flowing to any kind of electric motor or transformer containing coiled windings, there is an electric arc at the contacts as the contacts part (open up). These arcs produce extremely high spot temperatures and erode the conductive metal contact surfaces of switches and relays. To be effective in a circuit over time, switching devices must be able to withstand the spot temperatures that occur while the contacts are parting and until they are fully open. Equally important, the gap of the open contacts must be able to actually stop the flow of current. Arcs are composed of a “plasma” of flowing electrons. If the switch contacts don’t open far enough, the arc will not be quelled, and the electric current will continue to flow. In fact, we are all familiar with this phenomena; it called “welding.” In any kind of application OTHER THAN welding, that arc will melt the switch and start a fire in nearby combustible materials.

Return to TOC

Watt Hours

Like “Amp Hours,” “Watt Hours” is a measure of energy; specifically, the energy consumed by equipment.  “Watt Hours” refers to the amount of energy consumed over a period of one hour. AC electrical equipment is rated in Watts.

Boaters may find it helpful to convert “Watt Hours” into “Amp Hours” when estimating the amount of stored battery energy an item of 120V AC equipment may demand from the house battery bank when running via an inverter.  The conversion calculation is: (aHrDC) = (WhAC) / (VAC/VDC).

Not including inverter efficiency, a 100W/120V lightbulb would take 100W / (120VAC / 12VDC) = 100W / 10 = 10 aHr from the batteries per hour of operation.  Running for 4 hours, that’s 40 aHr just for that one lightbulb.  (Makes the case for LEDs, eh?!)  A 50W electric blanket running for 8 hours would need (50W / (120VAC / 12VDC)) * 8hr = (50 / 10) * 8 = 40 aHr for overnight sleeping comfort on a 45℉ overnight.

Return to TOC

Voltage regulator

An electronic device, typically associated with an on-engine alternator of 12V or 24V. A voltage regulator can be “internal” to the alternator it controls or can be a discrete, “externally-mounted” device. The Voltage Regulator adjusts the output voltage of an engine-mounted alternator in a manner that is pre-programmed to properly charge batteries of the various different types discussed above.

Return to TOC

Solar controller

An electronic device that adapts the “raw voltage” produced by solar panels to voltages that are compatible with 12V or 24V boat battery systems.  The “raw voltage” that comes from a bank of solar panels can vary to voltages upwards of 60V DC as the sun moves through its daily arc through the summer sky.  The solar controller adapts that “raw voltage” to 12V or 24V for boat DC system use.

Return to TOC


The term “Relay” is a generic term that applies to a class of electrical switching devices having a wide range of physical sizes and electrical capacity ratings.  They range from tiny control circuits mounted on printed circuit boards and installed inside electronic equipment all the way up to multi-megawatt grid-switching applications in power-grid transformer substations.

Specifically to typical cruising boats, “relays” and “solenoids” are used to control both high current draw, high power appliances and low current draw devices like those found in lighting circuits.

“Solenoids” are widely used as a technique to enable “user friendly,” convenient, low space-occupying switches to control very high powered appliance circuits that require large gauge (large diameter) cable between battery and appliance.  Solenoids can be implemented with multiple activation points, so they can control high power devices from several locations that are all relatively distant from the heavy duty device itself.  Solenoid activation (control) circuits do not require large gauge (diameter) cabling, so are much easier and more economic to install.  The most common example of these applications is the “engine starter motor.”  Starter motors draw many hundreds of amps from battery banks, and require very large feeder cables. In circuits of that kind, “voltage drop” along the feeder cable length is always an engineering constraint, so reducing the length of power cables is a very high priority to reliable device performance. “Solenoid devices” enable that capability.

Relays and Solenoids have a magnetic coil that operate the contacts for the high-current (controlled)t circuit. Starter solenoids are usually mounted directly on the starter motor.  Only small gauge wiring is needed to operate the starter solenoid. So its typical for the ignition key to pick a relay, and for that relay to, in turn, pick the starter solenoid to start the engine. That minimizes cable length losses, minimizes materials costs and minimizes the size of switches that occupy space on control panels and at operator console stations.

Because of relays, it’s also easy for switches at multiple locations to activate a single heavy duty Solenoid. This is a technique that allows operation of larger motor-operated appliances from multiple operator locations.  Examples of solenoids used for high power equipment include thrusters, anchor windlasses, davit hoist motors and power capstans.  Solenoids are also widely used to parallel battery banks, and to provide safety disconnect functions for starter motors. and inverter/chargers.

As mentioned above in the “Ampere Interrupt Capacity” section, whenever an electric current is interrupted by a switch, and particularly to any kind of electric motor or transformer, there is an electric arc at the contacts as the contacts part (open up). These arcs cause symptoms of overheating and voltage drop across the relay, necessitating replacement.  In similar manner, circuit breakers used as “on”/”off” switches in power distribution circuits have a “nominal” 15 – 20 year service life.  Use of relays as a control circuit technique allows for physically larger metallic contacts, which lengthens service life and allows for modestly-sized, more delicate user controls. The modern car has probably 30-50 small relays that control all functions of the car, from windshield wipers to horns to marker and turn signal lights to tank sensors to windshield washer levels. The relays are inexpensive electrical commodities, and produce service lives in the 15 to 25 year range.

Some devices on cars and boats have relays/solenoids that are actually incorporated into them. One typical example is the fuel solenoid, which opens a mechanical valve to allow fuel to flow and closes the valve to stop fuel from flowing. The valve is moved by the magnetic field of the solenoid.  Solenoid-operated valves that require power to open them are actually fairly sophisticated devices.  A fuel solenoid that requires DC power to open it has to be able to operate reliably while the battery is cranking, and perhaps cranking for a prolonged period of time.  Battery terminal voltage falls during engine cranking, typically from 12.5V to 10.5V, sometimes less, and the fuel solenoid must operate with that lower available system voltage.  So the solenoid is built with two coils; one to “pick” it and another to “hold” it open.  The “pick” coil will close the solenoid, but takes a great deal of power to do so, and produces a lot of heat.  The “hold” coil will kick in after the solenoid valve has shuttled into the open position, and “hold” the valve open.  There is an internal microswitch that energizes the “hold” coil and disconnects the “pick” coil when the valve is fully open.  Fuel solenoids can be found in models that require 12V/24V DC power to keep them open and in models that require 12V/24V power to close them.

Relays/solenoids provide the electrical system designer with a great variety of ways to implement electrical system functional flexibility for boat owners.

Return to TOC

Galvanic isolator

A passive electronic device that is installed in the boat’s main AC Safety Ground Conductor where the ground conductor enters the boat at the shore power inlet. (Note: this does not apply to boats fit with an isolation transformer.)  The Galvanic Isolator contains a diode pack that blocks small DC voltages and DC currents formed by the interaction of underwater metals of the boat in salt water. The purpose of the device is to prevent metal corrosion by “galvanic” currents.

Galvanic Isolators are after-market add-on components for boats.  GIs have been available for many years.  In that time, they have gone through several technical design iterations (generations).  In 2021, the ABYC-required version is called “Fail Safe.” If buying for a new or replacement installation, make sure to only buy the “Fail Safe” model.  This device is intended to maintain the integrity of the ground connection in cases where spikes in the shore power system may have damaged the diode pack.  The “Fail Safe” rated devices are “more likely” to survive a nearby lightening strike. Nothing will survive a direct hit lightening strike.

🎓More information in Galvanic Isolators

Return to TOC

  Sacrificial Anodes

In Chemistry, “zinc” is one of the earth’s fundamental metallic elements; chemical symbol “Zn.”  The word “Zincs” is also generic “lingo” that refers to a corrosion control component more properly called a “Sacrificial Anode.”  Zinc is one metal element that is used as a “sacrificial anode.” Zinc is most appropriate for use in Salt and brackish waters.  Aluminum is used as a sacrificial anode material and it’s suited to all waters.  Magnesium is used as sacrificial anode material in fresh waters.  Sacrificial anodes protect more valuable metals by participating in self-destructive electro-chemical reactions so that the more valuable underwater metals of the boat do not deteriorate.

🎓More information in href=””>Metal Corrosion and Zinc Wasting

Return to TOC


A mechanical device that converts mechanical energy into electrical energy.  Usually on boats, a device with a fossil fuel engine driving a rotating electrical generator.  The generating end can be either a device that produces AC (single phase or three phase) or a device that produces DC.

AC generators must spin at speeds that result in 60Hz AC output (two-pole machines spin at 3600 RPM, four-pole machines spin at 1800 RPM).  Generally, the slower the machine spins, the quieter and less vibration is likely.  Typical output voltages from boat AC generators are 120V/240V, which is directly usable by conventional 120V/240V appliances.  AC generators are typically less efficient than their DC cousins because they must spin at that fixed high rotational speed regardless of load. That requirement also leads to the mechanical complexity of the speed governor.  Safe AC generator installations on boats require sophisticated electrical switching techniques.

DC generators are generally used as battery chargers.  Used in that way, Inverters are used to create 120V/240V AC for use on the boat. DC machines are generally more efficient because they do not have the high constant rotational speed requirement or the need for precise RPM regulation.  DC generators are often not made in the large output (kW) sizes found in AC generators because in battery charging applications, that is not necessary.

Both AC and DC generators are viable for powering boat electricity needs.

Return to TOC


An electronic device that converts DC to AC; commonly, 12V or 24V DC from battery banks to 120V or 120V/240V AC electricity.  Inverters allow boaters to use 120V home appliances on boats.  Most typically, refrigerators, freezers, ice makers, mixers, microwaves, crock pots, toasters, TVs, SOHO computer equipment and iGadgets.  Inverters are available in a wide range of capacities to support 120V/240V appliances.  These devices usually require large capacity battery banks.

🎓More information in href=””>Inverters On Boats

Return to TOC


A propulsion device that causes a boat to move in the water. A single bow or stern thruster causes the boat to pivot around the natural center-of-rotation of the hull. Pairs of thrusters (bow and stern) cause the boat to move “sideways” in the water. Thrusters are used as an aid to maneuvering in close quarters.

Thrusters can be electrical, powered from batteries, or hydraulic, powered by the propulsion engine or an auxiliary engine. Electric thrusters are made with reversible DC motors having the mechanical style and electrical demand profile of engine starter motors. To produce needed thrust, they require large amounts of electric current, large diameter feed cables, and short cable runs. An electric solenoid is used to reverse the polarity of the DC power applied thruster in order to reverse its direction of rotation.

Return to TOC

Terminal Block / Bussbar

A component in an electrical distribution system, either AC or DC.  Bussbars are specifically designed and used as a means of connecting conductors together.  Buss bars provide great flexibility for electrical system designers to achieve cost effective and safe electrical connections in boat electrical distribution systems.Return to TOC

“Dielectric Grease”

A “dielectric” is a substance or material that DOES NOT conduct electricity; an “insulator.”  The way all good electrical connections are made is to tightly compress together the current carrying elements of the connection; usually, wire ends.  It is the firm, intentional, mechanical contact of the metal conductors that makes an electrical connection of high-integrity. Strong mechanical connection can be accomplished by securing ring terminals under a screw, bolting connections together, crimping a sleeve over conductor ends, or of course, with soldering techniques.

Dielectric grease is NON-CONDUCTIVE.  As a liquid, it can be sprayed (Corrosion-X, Boeshield T-9) directly into 120V/240V AC receptacles.  As a grease, it can be applied into terminal barrels and used to coat conductor ends before assembly.

When coated plug blades are inserted into a receptacle, they make mechanical contact.  As they are inserted, the dielectric is “wiped away” at the points of contact, creating a clean electrical connection. The dielectric then surrounds the area of mechanical contact, and prevents moisture and oxygen-bearing air from getting to the mechanical connection and causing oxidation (metal corrosion).

I use and recommend Dow-Corning “High Vacuum Grease” (developed for NASA for Space Shuttle electrical connections) for assembling 2/0 and 4/0 wire-to-terminal connections. I pack the barrel of the terminal connector with dielectric grease, and coat the wire with grease.  The electrical connection is formed at the crimp, where the conductors have been compressed mechanically together.  At that joint, all of the dielectric will have been pushed away and clear of the metal contact areas of the mechanical compression joint.  The crimped assembly will consist of a water tight, air tight, sealed chamber that will never corrode.  There will be waste in the form of grease “squeeze out.”

Return to TOC

Bulk / Absorb / Float

The names of the three stages of charging for lead-acid batteries, based on the most energy efficient manner of charging lead-acid cells and the best manner of charging for maximizing in-service life of the batteries.

A lead-acid battery that is significantly discharged will take charge quite rapidly. During that period, the charger operates in “Bulk” mode to give the battery the largest amount of charge (in Amps) that the battery can accept.  As the battery charges, the rate at which it can accept charge goes down and the terminal voltage rises. At about 85% State-Of-Charge (SOC), the battery electrolyte will begin to break down and produce hydrogen gas. At that point, the charger changes into “Absorb” mode. Absorb mode slows the rate of current flow to eliminate the hydrogen off-gassing, and allows the battery to become more completely charged. Finally, when the rate of current flow is less that 2% of the battery’s charge acceptance rate, the charger will drop into “Float” Mode. Float Mode is designed to offset the effects of “Self-Discharge.”

🎓More information in Batteries: Charging and Care

Return to TOC

Boat monitor

An electronic device that has the capability to monitor the state of various boat systems and report real-time status back to absentee owners. The unit I have installed on Sanctuary monitors for the presence of Shore Power, monitors Salon Temperature, monitors Bilge Pump Cycles and High Bilge Warning Alarm, and monitors Refrigerator, Freezer and Engine Room Temperatures.  The device contains a GPS, and so it knows where it is and reports to me when the boat moves more than about 100 feet (“Geofence;” programmable option).  The device is web-based.  It reports via cellular connection to a central server.  When there is an alarm or change-of-state that I have told the system I want to be notified about, I get a text from the host manufacturer’s server farm with the information I requested.  I also get a daily summary email from the system if I ask for that.

The device has “saved our bacon” several times.  When traveling away from the boat, we know if shore power has been lost, and I can call to have the facility investigate and take corrective action.  I know if important temperatures are out-of-spec. Under way, I can see bilge pump cycles as they occur and I can monitor bilge pump run frequency.  This is a huge “Peace of mind” item.

Return to TOC

Wire Gauge

A term that refers to the diameter of wire conductors. There are two “international” scales with which boaters come into contact:

  1. American Wire Gauge (AWG)
  2. Society of Automotive Engineers (SAE)

Both scales specify the safe amount of electric current that diameter wire can carry, called “Ampacity.” Both scales are widely used in boating.  SAE wire sizes are about 10% smaller than AWG wire sizes, so it’s important to be certain to know which scale is being cited, and adjust if larger wire is needed.  Wire “ampacity” ratings are also affected by the temperature rating of the insulation protecting the wire.

Return to TOC

Voltage Sensing Relay (VSR)
Automatic Charging Relay (ACR)

Terms used to describe electrical devices, each different types of “Solenoids.” Combiners interconnect two circuits in parallel. Often, they connect battery banks. In battery banks, in automatic operation, when one bank is being charged, at a certain point in the charging cycle, a second bank is added in by the “combiner” to be charged by the same battery charging equipment. These devices can also be set up to allow the boater to manually interconnect the battery banks. Depending on the manufacturer’s ratings of the specific device, the rated amperage capacity may be limited.

These devices are “dumb” devices that connect two circuits and allow current to flow in either direction.  The current flow is uncontrolled and dependent on conditions in other parts or the electrical system.

Return to TOC

DC-to-DC converter

An electrical device; can be used as a “combiner” in a “master”/”slave” relationship, can be used as a regulated DC power supply to change voltage (up from 12V to 24V, or down from 24V to 12V, or other combinations), or can be used simply as a regulated DC power supply (for instruments, for example) that is independent of the regulation quality of the source of its input power. DC-to-DC converters are “smart” devices (programmable, automatic). When use to transfer power, they transfer that power in only one direction (from a “master” source to a “slave” circuit or component).

DC-to-DC converters are preferred over VSRs for interconnecting battery banks of different chemistries.

Return to TOC

Heat Pump
Reverse Cycle

Terms that relate to HVAC equipment (Heating, Ventilation and Air Conditioning).

“Heat Pumps” transfer heat from one type medium to another; on boats, back and forth from water-to-air or air-to-water.  The heat transfer occurs through a chemical refrigerant in a heat exchanger.  The chemical refrigerant is forced by a compressor to change state from gas-to-liquid or liquid-to-gas.  the change of state is what determines the direction of flow of the heat that is transferred.  Heat pumps have two heat exchangers:

  • Condenser
  • Evaporator

In “Cooling” mode (air conditioning mode), heat pumps transfer heat from warm air in living spaces to circulating raw water. The heat that is transferred into the circulating raw water is “dumped” into the boat basin.  In “Heating” mode, heat is transferred from the basin water to the air in interior living spaces.  When they are operating in space heating mode, they are in their “Reverse Cycle” mode.  “Reverse Cycle” units contain valves that change operating mode by changing the direction of circulation of the refrigerant in the unit.  This also reverses the functional relationship of the “coils” in the unit between the “cooling” cycle and the “heating” cycle.  In the “cooling” cycle, air is cooled at the coil functioning as the refrigerant’s “evaporator.”  In the “heating” cycle, the air is heated by the same physical coil, but this time functioning as the refrigerant’s “condenser.”

Not all marine air conditioning equipment is designed to operate as described above; i.e., in “reverse cycle” mode. Some boat HVAC units have single-mode refrigerant systems that perform the “air conditioning” cooling function only, and are fit with electric heating elements that perform the “heating” function. The electric heating elements perform the function of an “electric furnace.” Installation ventilation requirements for these units can differ from units that are operated by refrigerant and compressors only. This is simply a design alternative.

Return to TOC


Self-discharge is a phenomena mostly of lead-acid batteries. Self-discharge is what happens as batteries stand unused (usually disconnected) “on the shelf,” out of service. Electrons and ions migrate within the battery and the result is loss of charge. This process is affected by temperature, much increased as temperature increases. The process occurs at a greater rate with flooded wet cells, less so for AGM and Gel, and least for Carbon Foam.

Battery chargers must be sized for the bank they will support. A medium-sized bank – say 700 amp hours – will have a larger self-discharge load than a smaller bank, and a 1200 amp hour bank of the same battery type will have greater self-discharge.

Self-discharge rates range from 3.5%/month for wet cells at ambient temps to lesser numbers for other technologies. Curves of self-discharge characteristics are available on the Internet.

Chargers use the “Float” stage of battery charging to manage self-discharge to keep lead-acid cells at full charge. Float-charging currents are 1.5% to 2% of Charge Acceptance Rates (CAR), so for flooded wet cells, maybe 2A for an 8D battery of 220 amp hours. Three 8Ds in parallel would be 660 amp hours, and “Float” current would range upwards from 6.5A, ±. Self-discharge percentage increases with age and battery condition, so could easily exceed 15A for a moderately sized bank that’s been in service 3 – 4 years.

Return to TOC

Charge Acceptance Rate (CAR)

Term that describes the rate at which batteries can accept charge from a charging source. Usually described at the ratio of battery capacity, in “Amp Hours,” to the nominal rate of charge acceptance current, in “Amps.” Batteries of different construction technique accept charge from charging sources at different rates. Nominally, flooded wet cells can accept a charge of 25% of the rated Amp Hour Capacity of the battery, in Amps. For example, a 100 Amp Hour wet cell can accept ~ 25A from the charger. Nominally, AGM and Gel cells can accept 40% of the rated Amp Hour capacity of the battery in Amps, so a 100 Amp Hour AGM could accept ~ 40A.

The discussion of Charge Acceptance Rate applies to partially discharged batteries. The maximum Charge Acceptance Rate is generally considered to occur when the battery is substantially discharged; i.e., 50% or less, State-of-Charge. As the battery State-of-Charge increases, the rate of charge acceptance decreases. Understanding Charge Acceptance Rate helps select battery charging equipment for best compatibility for the capacity of the batteries being charged.

Return to TOC

Power Quality

This term refers to the AC sinusoidal voltage waveform in an AC system being both 1) free of distortion at 2) within the rated voltage and frequency (60 Hz throughout North America) specified for the system.

The following are common forms of voltage and frequency events that lead to customer equipment failures:

The following are forms of voltage waveform distortion “anomalies,” which can be present one-at-a-time or in random combination of several-at-a-time.  These waveform anomalies can cause customer equipment to fail to operate correctly or to fail to operate at all.

Return to TOC

Switch Mode Power Supply (SMPS)

A switch mode power supply is a power “converter” that utilizes solid state switching components to continuously switch internal circuit power “on” and “off” at high frequency. The switched power is fed to energy storage devices (capacitors and inductors) to supply power during the non-conduction state of the switching device.

SMPS power supplies have efficiencies approaching 90%; much “better” than traditional linear mode power supplies which run around 40%-45%.  SMPS supplies are small in size and have become widely used in computers and other sensitive electronic equipment.

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

  • AC to DC – DC power supply
  • DC to DC – Converter
  • DC to AC – Inverter
  • AC to AC – Cyclo-converter or “frequency changer.”

The main components of an SMPS are:

  • Input rectifier and filter
  • Inverter consisting of a high frequency signal and switching devices
  • Power transformer
  • Output rectifier
  • Feedback system and circuit control

Following is a block diagram of the main components of an SMPS. The red circles highlight the voltage waveforms that pass from block to block.  The pulses fed to the transformer are created by a “Pulse Width Modulation” (PWM) circuit in the bottom, center block.  

Advantages of SMPS:

  • More compact and use smaller transformers. Smaller size and lighter weight is an advantage for electronic device with limited space
  • Regulated and reliable outputs regardless of variations in input supply voltage
  • High efficiency: 68% to 90%
  • The transformer-isolated supplies have stable outputs independent of the input supply voltage
  • High power density

Disadvantages of SMPS:

  • Generates EMI and electrical noise (waveform distortion and harmonics)
  • Complex design
  • More components → expensive compared to linear supplies

🎓More technical information/description: SMPS

Return to TOC