Grounds and Grounding

One of the least understood aspects of a boats electrical system, and the most troublesome, is the proper method of grounding. That we often get questions of whether AC or DC electrical equipment should be grounded to the boat’s bonding system is illustrative of this point. AC and DC grounding systems are two separate systems, for distinctly different reasons. If you don’t understand these systems, you run the distinct risk of creating a disaster. Actually, there are four separate ground systems: DC ground, AC ground, AC grounding (or bond), and the vessel’s bonding system. You can add to this lightning and HF radio grounds as well. Do you know the principles of each? Are you sufficiently confused to discourage you from doing your own wiring? Unless you understand each thoroughly, you’re headed for trouble.

The AC ground and grounding systems are “free floating,” meaning that they do not ground on the vessel, but only to shore. The ground, or neutral, is a current carrying conductor, and is the source of many troubles because people do not regard it as such. The grounding, bond or green wire is the “safety” intended to channel current safely to ground in the event of a short circuit. Both of these circuits are capable of conducting current and can be the source of electrolysis when there are system faults with the dock or marina wiring. This is very easy to test for.

There is only one point where the DC side is grounded, and that is at the battery. It, too, is a “free floating” system in which nothing is ever grounded to any metallic part of the vessel, most especially not the bonding system. Just like a car sitting on rubber tires, completely insulated from earth potential, the battery itself provides the negative potential.

The bonding system, also green wire, has nothing to do with electrical systems. Underwater metals are simply wired together to equalize differences in potential of different kinds of metal. Nothing should ever be grounded to the bonding system. Unfortunately, some people don’t understand this and use it to ground electrical equipment, occasionally with disastrous results.

Bonding Systems

Bonding simply means wiring all the boats underwater metals together. This is done because of the galvanism caused by the different metals. By wiring them together, the differing potentials are equalized. Bonding does not solve problems of galvanism or electrolysis, but it does spread the flow of current around over more metal, so that 1/4 volt or so won’t cause any damage. In other words, bonding lessens the effect of small amounts of current. On the other hand, it also spreads it around to all underwater metals so that higher currents end up damaging everything.

Bonding systems use wire and ordinary crimped ring terminals. After a while these get wet and corroded. Electricity doesn’t flow very well through corroded metal, so your bonding system after a while stops working. To maintain it, simply cut off the old terminals and install new ones. Do you have wires attached to sea cocks with hose clamps? Forget it. This is putting stainless and copper together, which are galvanic ally incompatible and it won’t work.

Bottom Paint

What does bottom paint have to do with electrical systems? Nowadays, with copper based paints, a lot. If, the next time your boat is hauled and you see large ugly burn patterns around all your underwater metals, you got a stray current problem. Copper-based bottom paints react severely to stray current, and serves as a great indicator.

Of course, the common wisdom is that the stray current “is from the marina.” Or it’s always the other guy’s boat that is causing your problem. Don’t bet on it. Most stray current problems are sourced on the boat in which they appear. Otherwise, everybody in the marina would have the same problem.

Electrolysis and Galvanism

Electrolysis is a word that is badly abused by boaters who don’t really know what it means, so let me correct this right now. First, understand that all boats have an electrical potential. That’s because of all the different metals on the boat which, themselves have differing electrical potentials. This is exactly the same principle that makes a dry cell battery generate electricity. This electrical potential is called galvanism and is the reason why we put zincs on boats.

Electrolysis is stray current escaping from the system and is most damaging. It is an abnormal condition. When this happens, it will eat up the zincs in no time, usually leaving that metal looking bright and shiny. Therefore: Shiny zincs = electrolysis. Dull

eroded zincs = galvanism.

I spent two years putting a meter on every boat that was hauled for survey. The average boat generates about 1/4 volt DC current and going as high as 1/3 volt without causing damage. But when it gets up to 1/2 volt, you got a problem. Zincs will erode rapidly and underwater metals begin to be affected.

Shore Power Cords

The single largest cause of problems with shore power systems results from failure to maintain the connectors on both the cord and the boat connectors. These devices are exposed to water and over time suffer from corrosion and general wear. High resistance caused by corroded, bent or worn connector’s results in high resistance which causes overheating and further amplifies the power drop. This not only creates conditions for a potential fire, but causes electrical equipment to work harder, resulting in reduced life span of equipment. IT PAYS TO MAINTAIN SHORE POWER CONNECTIONS.

You can perform a very simple check just by placing your hand on the shore cord near the connection to determine if it is heating up. Obviously, this should be done while you have a lot of equipment turned on. If it’s anything but slightly warm, not more than 110 degrees, suspect a problem.  Shore power connectors should be dismantled at least once per year, cleaned and repaired as necessary. Most of these connectors have replaceable parts. If you drop your shore power connector in the water, you must take it apart, clean and dry it. Otherwise, expect it to burn up.

We recommend that you buy only the highest quality power cords, as these will last longer and have the advantage of replaceable connector parts. Cheap connectors usually can’t be taken apart. We also advise against ever using the three-pronged household type adapters as this type of connector is highly unreliable and prone to causing system faults and fires. Only the twist-lock type connector is suitable.

One more thing: If you are not turning off the dock breaker before disconnecting the power cord, start doing it now. Not only do you risk getting electrocuted, but disconnecting an energized connector damages the contacts. Also consider what happens if you drop the energized cord in the drink!


This is an issue only with 125 VAC systems since 250 VAC systems will not function with wrong polarity. Since you have three terminals on a shore connection, wrong polarity can mean that any of these wires are in the wrong position. Not only should you pay attention to the polarity indicator on your boat, we recommend that you keep a plug in polarity indicator aboard and use it every time you hook up to shore power at a different location.

Reverse polarity is not only an electrocution hazard, but can also damage electrical equipment. It is most often found with the three prong spade connectors (household type), but occasionally twist lock connectors as well, particularly in marinas with dilapidated equipment. Never trust the power supply at strange docks, but always check the polarity. When hooking up to strange docks, always check your volt meters to make

sure you have adequate voltage. Low voltage is very damaging to electrical equipment. Turn on the stove or water heater and watch what happens to the meter.

Main Circuit Protection

Many people think that the circuit breakers on the dock protect their boat. They do not; they only protect the dock wiring. Your main circuit breaker protects your boat’s systems. What about that section of wiring and connectors between your main panel and the dock breaker? Well, the fact is that it is unprotected is why so many fires occur. Check out all the top end boats and you will find that they have circuit protection located directly at the shore connectors. Having slow blow cartridge fuses installed directly at the connectors can go a long way toward preventing fires and burned up shore cords, particularly if you are a traveler and frequently rely on uncertain power supplies. Circuit breakers should NEVER be installed on the exterior of the boat. Only gasket, water proof cartridge holders should be used.

Circuit Breakers

Circuit breakers wear out, and when they do they work less well, or not at all. If you are using circuit breakers as ON/OFF switches, you are helping them wear out that much faster. It also damages breakers when you shut off equipment via the breaker. This causes arcing at the contact points which damages the points. When connecting and disconnecting shore power, you should always turn OFF equipment at the appropriate equipment switch, then shut the main breaker off. Do not ever simply throw the main breaker off to shut down equipment that is operating. The circuit breaker arcs and damages it.

Also be aware that any equipment run by a motor, such as air conditioning and refrigeration equipment will start up with initially much higher amperage than the normal running amperage. An air conditioner that runs at 14 amps may have startup amperage of 20 amps, so that if you just go and turn all the equipment on at once, it overloads the system. Then the circuit breaker gets hot and won’t stay engaged until it cools down. Ergo, start up heavy equipment one item at a time, allowing it time to cycle into its normal operating voltage before turning something else on. For example, don’t turn the AC, refrigerator and icemaker all on at once and not expect the breaker to pop.

Chronic Breaker Popping

Breakers that pop frequently are signaling that there is a problem, which could either be the breaker, or something in the circuit. Yet most people will keep on attempting to make the breaker engage. This can be dangerous because you may cause the contact points of the breaker to fuse together from arcing, in which case it will never trip again. DO NOT ATTEMPT TO ENGAGE A BREAKER THAT IS OVERHEATED BY FORCING IT. You must allow it to cool down.

If you are experiencing chronic problems with circuit breakers popping, first check how much current draw is involved. A single 30 amp circuit is not much when you’re running things like air conditioners, water heaters and battery chargers. One very simple way to check whether you’re dealing with an overload problem is to add up the amperage draw of each piece of equipment. List both the start up and run amperages. You will usually find the amperage right on the equipment label. By making a list of the total power demand, you’ll get a good idea of what you can and cannot operate simultaneously, particularly when starting the equipment. If you have an ammeter on your panel, check it against the amperage tally you made. Ideally, you should try to hold power consumption at 80% or less than the line rating.

Check the breaker by allowing it one hour (or whatever it takes) to cool down. Turn the equipment off and, after it is cool, reengage the breaker. Now turn the equipment back on. Place your finger on the front of the breaker and note its temperature. If it does not heat back up again, then the problem was probably a start-up overload. If the temperature rises again, there is a fault in the circuit or the breaker. (Note: when the breaker contact points become eroded, the breaker itself can overheat).


Ground fault current interrupter service outlets are required to be installed in wet locations such as the galley or head. In reality, there’s little chance of being electrocuted inside a boat because you are not grounded within the boat. A greater risk is from service outlets being located in places that get wet, such as below leaking windows, hatches or close to doors. Three pronged plugs are prone to shorting across the terminals when wet, so having all your service outlets changed to GFCI’s is a good idea.

Use only the highest quality devices from a reliable manufacturer like GE. Service outlets located anywhere on the exterior of the vessel are an invitation to trouble for reasons that should be obvious.

DC Systems

The same advice about jury-rigging wiring applies to DC systems as well AC systems. While you’re not going to create an electrocution hazard, it is very easy to take a faultless system and create faults in it. A typical problem starts like this:  The owner wants to add a new piece of equipment, but the electric panel is way over there, and the place he wants to install the equipment is way over here. Besides, there are no extra breakers in the panel, and no space in the panel to add another one. To make matters worse, the panel is located in such a way that he couldn’t string new wires into it even if he wanted to. So what he does is to find a place where he can tap off an existing circuit, and maybe adds an in-line fuse, stringing wires all over the place in the process. Or maybe he is replacing a piece of equipment that has a faulty circuit, but instead of trying to locate the fault, he just clips off the old wires and strings new ones. This happens a lot, and by the time the boat has a few years on it, it’s got cut wires all over the place, many of which are still hot!

In many cases, he will just go and take new leads off the batteries, bypassing the panel altogether. Now when he goes to turn off the main power supply, all that new stuff added remains energized. In addition to which in-line fuses have also been added all over the place, so when something craps out he’s got to go tearing through the boat to find that hidden fuse.

The worst of the problems with DC system add-ons comes with improperly installed wiring and the use of wire splices of all sorts. Typical of these are the use of electrical taped connections which, when the tape gets warm (as in the engine room) the tape glue gets soft and the tape falls off. Or the use of wire nuts or crimped butt connectors in locations that get wet. Wire nuts (those twist-on cones) are not approved for marine use. When connections get wet, the wire corrodes, creates high resistance, usually resulting in equipment damage or failure for reasons which the owner will never discover. He’ll think just that damned lousy piece of equipment crapped out; when in fact the lousy wiring job is the culprit.

Principles of Wiring

I would venture to say that half the electrical problems on boats result from improperly installed wiring. After the boat is built, there’s no convenient way to route new wiring. But we need to understand that systems on boats are subject to high G-forces due to pounding, rolling and vibration. Connections get stressed and wires rub and chafe against abrasive or sharp objects. It doesn’t take much damage to wire insulation before you have a condition where stray current may develop. And the chance of finding a little bit of damage on one wire is about nil.

1. Must be routed in a suitable, dry area and be well secured. They should not be lying in bilge or in areas that get wet.

2. Must not be routed with pipes or hoses of any kind, and not be in contact with fuel tanks or fuel lines.

3. Splicing circuits should be avoided. If splicing is necessary, it should employ a proper terminal block, and not butt connectors. Every splice in a circuit creates additional resistance, and the potential for the connection to come apart. Taped connections and wire nuts should not be used.

4.    Wiring must be firmly secured and in locations where it won’t get damaged. Should not be dangling or strung across open spaces. Use only plastic, not metal, clips to secure the wiring.

5.    Must have chafing protection or conduit at vibration points around machinery.

6.    Must not be in contact with, or proximity to machinery exhaust systems.

7.    Wiring should be neat. A boat full of tangled wiring demonstrates unprofessionalism and the inability to fix something that goes wrong. An electrician can’t trace a plate of spaghetti, and when something does go wrong, the cost of fixing it goes way up.

Circuit Protection

Adding in-line fuses to a newly installed piece of equipment is a terrible way to add circuit protection. First of all, these devices trap water and corrode internally. Secondly, you end up with two more splices in a wire circuit that shouldn’t have any. Third, you usually forget where they’re located. A boat full of in-line fuses is a boat full of short cuts and amateur installations.


Batteries are a constant source of aggravation to many boat owners, almost always for reasons that are preventable. These are: low quality batteries, poor or nonexistent maintenance, and improper installation and wiring. As a general rule, batteries perform consistent with the price you pay for them. Good batteries are expensive, and shopping for price will only lead to momentary satisfaction. Cheap batteries have thin plates and poorly insulated casings and therefore cannot give long service.

Secondly, batteries have to be installed correctly. That means in a clean, dry location that can be reached. If you can’t reach them, if you have to kill yourself to get at them, then you will not maintain them. If installed in an inaccessible location, you should consider having them moved to a better location.

Place a fully charged battery on the concrete floor of your garage. Then come back two weeks later and check the charge. That battery will have completely discharged, and it will have done so right through the plastic casing. Now you understand my point about proper installation and dryness. On many small boats, I usually find the batteries sitting in uncovered plastic boxes that are full of water. Or they’re sitting in bilge water or on wet decks. If that’s the case, you needn’t look any farther for at least part of your problem. If you want your batteries to be reliable, they must be kept clean and dry. That includes the top surface, particularly between the terminals.

Proper Lugs

It won’t do to make your cable connections with threaded studs and wing nuts. These afford inadequate contact surface that can cause high resistance and is one of the major causes of engine starting motors burning out. Your starting motor cables should be attached only with swaged lead lugs, not the ring terminal kind smashed with a hammer to make the connection. Small boats are usually the worst offenders in this regard.


Batteries develop heat when charging, as well as hydrogen gas. For that reason, they need to be in a well-ventilated area. Gel cells are particularly vulnerable to overheating damage. Putting gel cells in covered, plastic boxes has proved to be a problem, and for this reason they are falling out of favor. Sometimes improved technology isn’t an improvement after all.

Electrical Devices Exposed to Weather

Why it is that there so many builders that install instruments, panels and switches in locations that are going to get wet is something I’ll never understand. Oh, I know, you look at those switches and think that they’re water proof. Well, you just go look at the back side of the panel and see if you still think so. See if you don’t see a lot of corrosion back there. What happens when the back side gets wet? Well, water being a conductor means that these devices will short small amounts of current across the terminals, or to any available ground. This is one of the reasons why you have so many engine instrument failures, and boats have so much of a hardware corrosion problem, and why they have stray current problems, never mind equipment failures. Electrical equipment exposed to weather that is not absolutely water proof is just asking for trouble.

Locating Internal Equipment

Just because it’s inside the boat doesn’t mean that electrical equipment will stay dry. Boats leak, and stuff located under those leaks are going to get wet. That means that you have to pay attention to where you put stuff. Many people mount various types of pumps low in the bilge, assuming that the bilge water is never going to rise. Believe me, the bottom of the boat is the last place you ever want to mount something. Sooner or later your bilge pump will fail, and when it does that expensive equipment is going to get ruined.

If you have exposed panels you need to keep them covered and dry. The vast majority of small boats that I see don’t even have panel covers, yet alone having covers that are used. Having a small cover made up is a small price to pay to avoid serious damage and other problems.

Engine Wiring

Due to vibration and high temperatures, damaged wiring on and around engines is one of the most common causes of stray current damage, i.e. true electrolysis. It is extremely important to consider the routing of the wiring so that it is not in contact with hot manifolds, or vibrating on sharp edges or rough surfaces. Engine wiring should be inspected periodically for signs of damage.

Adding New Equipment

Most boats, particularly smaller ones, are unfortunately not designed with the idea of adding equipment, even though nearly everyone does.  The primary reason why systems get so messed up results from the attempt to force something in where there is no provision for it.

The best way to deal with this is to simply add a new panel, whether AC or DC. Doing this is neither very difficult nor expensive, especially considering the cost to repair all the problems you’re likely to create when you try to cram something onto a system not designed to handle it. A six breaker DC panel will cost less than $200 and is a whole lot better than scattering inline fuses all over the place, and with installation will run less than $500. Plus, the new panel can be added at any location where it will be convenient.

Adding a second shore power circuit is especially useful for those boats with single 30 amp shorelines where the addition of new equipment will tax the system. It’s not very convenient to have to turn the water heater off to turn the air conditioning on, or constantly be managing equipment as breakers keep popping off. You have two options here, the first of which is to increase your shoreline to 50 amps. To do that you have to change the main breaker, power receptacle and the shore cord, which is a lot bigger and heavier.  The other option is to add a second 30 amp circuit, which means adding a new panel, receptacle and shore cord, which costs a bit more.

Adding a second circuit has the advantage of separating the air conditioning onto a separate line, which is the way all good systems are designed. That’s because of the high power demand; combined with frequent dock power faults makes it desirable to separate it from the main service.

Multiplex Systems

A multiplex system is a multiple power source system which permits switching from one line to another, from one power source to another. For travelers, this is extremely useful to deal with unreliable dock power or sudden faults in your own system. It allows you to choose which line you want to run equipment on, and particularly if you have a generator, it provides a great deal of flexibility.  Let’s say a circuit breaker on the dock goes out while you’re cooking dinner and it’s very hot this day. Instead of only being able to turn the AC off and go on cooking in the heat, you can fire up the generator to run the failed circuit simply by switching it over. The better larger boats invariably have this kind of system.