By Don Casey
Revised by BoatUS editors in April 2012
However you bring shorepower aboard your boat, if you do it without the appropriate safeguards, you place yourself, your crew, and your guests at a terrible — and foolish — risk.
AC circuits use three wires, and it is essential to your safety to understand the function of each. Because the direction of current flow in an AC circuit is reversing 120 times per second (60 positive-to-negative-and-back-to-positive cycles), it is counter-intuitive to refer to one side of an AC circuit as positive and the other as negative. Instead, we call one side hot, the other side neutral. By code the hot wire in an AC circuit is black, red, or blue; the neutral wire is always white. Since the neutral side is connected to ground, it is the hot side of the circuit that is the most dangerous. However, you should always treat both sides with equal respect because a wire reversal to the outlet will make the neutral side hot.
The grounding wire changes all that. A short to the case instantly becomes a short to ground, probably opening the breaker in the circuit. Even if the short is insufficient to kick the breaker, or if it occurs while you are holding the appliance, the grounding wire still protects you by providing a lower-resistance path to ground than your body offers. Anything that degrades or opens the grounding circuit, such as a broken connection, a corroded or missing ground prong, or an improperly wired outlet leaves you dangerously vulnerable. Ashore the grounding wire is often bare, but in boat wiring it should be insulated and green. We will come back to this all-important green wire.
Inlet Fitting and Dock Cord
A safe shorepower system begins with a weather-tight inlet fitting. If the shorepower inlet on your boat isn't weatherproof — with and without the cord attached — you need a different fitting. The 30-amp locking receptacle has become the norm at most U.S. and Caribbean marinas, so equipping your boat with a 30-amp inlet minimizes compatibility problems.
Make sure the fitting is in a location that does not put it at risk for submersion or mechanical damage, and that the backside is well ventilated. The wire distance from the inlet to the main AC circuit breaker should never exceed 10 feet.
An extension cord from a builder's supply, even one labeled "Heavy Duty," is not an appropriate shorepower cord. To shrug off the exposure, submersion, abrasion, and strain of marine use, the cable should carry a hard-service rating — SO, ST, or STO. There must be no possibility of the cable pulling free of the plugs (dock cords sometimes become dock lines) or of the plugs pulling out of the receptacles. For 30-amp service, the conductors must be #10 AWG or larger.
Factory shorepower in older boats too often omitted a circuit breaker. The thinking was that a boat was just another "appliance" plugged into the marina circuit, which was already protected. But faulty marina wiring is too common to entrust your safety to an unknown breaker behind a dock office a quarter of a mile away. A safe AC system requires an onboard dual-pole breaker.
The breaker's rating must not exceed the service; i.e., a 30-amp inlet limits the main breaker to 30-amp. However, if the circuit directly supplies outlets in the cabin, the size of the breaker is limited to the outlet ratings — either 15 or 20 amps.
A single circuit offers the benefit of simplicity, but if you need multiple AC circuits, breaker panels are available with a main breaker and two or more branch breakers. Despite being commonplace, it is not a good practice to combine AC and DC in the same breaker panel.
Reversed polarity leaves your AC circuits unprotected if you have a single-pole main breaker since the breaker is now on the neutral side. Even with double-pole breakers, reversed polarity puts ON-OFF switches on the wrong side of the circuit, leaving OFF appliances fully energized. Be sure your AC panel is equipped with a polarity tester.
Where branch breakers are single pole, be sure they are on the "hot" side of the circuit.
Resist any temptation to use house wire (Romex) for the AC circuits on your boat. Common house wire is solid copper, fine for a quiescent life inside a wall but unsuitable for the marine environment. Boat wiring requires the flexibility of stranded wire, and it should be tinned for corrosion resistance.
Wire size for AC circuits is generally less a factor of voltage drop than of amperage capacity, but starting currents, particularly for compressors, can momentarily increase the current draw up to 5 times nominal. If the wire is too small, this momentary surge can actually cause supply voltage to drop below the level required to start the compressor. As a practical matter, 15 and 20 amp circuits should be wired with 12-gauge wire. The (unprotected) wire between the inlet fitting and the main breaker should be 10-gauge for a 30-amp inlet and 6-gauge if the service is 50-amp.
The green "grounding" wire in AC circuits provides a low resistance path to ground should any of the various metal cases enclosing your AC system become energized. But what if the leak is into the DC wiring, caused, for example, by crossed wires or a short in a battery charger, inverter, or other dual-voltage appliance? AC leaking into the DC system will seek ground, meaning it will automatically travel through the wiring to the ground connection on the engine and down the prop shaft to the water. This is essentially the same as dropping a hot wire into the water. In fresh water, this poses a real risk of electrocution for anyone in the water nearby. Electrocution is less likely in saltwater, but the current field can be enough to paralyze muscles and cause a swimmer to drown.
Connecting the green wire to the ground terminal on the engine offers AC leakage into the DC system a lower-resistance path to ground — through the grounding wire. This eliminates the risk to swimmers as long as the grounding wire connection to ground is sound. However, if corrosion on the ground prong of your dock cord or some other fault breaks the ground connection, all ground-fault current, not just AC to DC leakage, will flow into the water. It is essential to test the ground connection at the dock and to maintain cords and plugs in good condition.
In your breaker box at home the neutral wires and grounding wires all connect to the same terminal strip (or bus bar), but on a boat the AC grounding wire is connected to the DC ground. Also connecting the neutral wire to it makes underwater hardware a current-carrying path to ground, potentially lethal for anyone in the water nearby. On a boat the neutral (white) conductor and the grounding (green) conductor MUST NEVER BE DIRECTLY CONNECTED.
Outlets are all polarized, and the Lone Ranger's white horse, Silver, can provide the association necessary to remember which wire connects to which terminal. White goes to silver. So the black wire connects to the opposite terminal, usually brass, but sometimes dark. The green terminal is for the green grounding wire.
Electricians ashore simply tighten terminal screws onto a loop of wire or insert the stripped end into a spring clamp. Neither method is appropriate on a boat. Use a proper crimp terminal or install commercial-grade outlets with screw-tightened clamps in place of the more typical screw terminals. These may be used with stranded wire.
Ground Fault Interrupt
Regular circuit breakers are essentially fire-protection devices and offer no protection against electrical shock. A ground-fault circuit interrupter (GFCI), on the other hand, provides a high degree of shock protection. If you accidentally touch an energized wire or component and you are grounded, the GFCI disconnects the circuit in about 1/40 of a second, too little time for the current to build to a dangerous level.
In recognition of the increased risk of shock in damp environments, many municipal building codes mandate GFCIs in bathrooms and, increasingly, kitchens. Boats are damp environments, and every outlet on a boat should be protected by a GFCI.
GFCIs are inexpensive and easy to install. Simply replace the first outlet on a circuit (counting from the breaker) with a GFCI fixture, and all the outlets on that circuit will be protected. Make sure the terminals marked LINE are connected to the wires leading back to the breaker and those marked LOAD feed the remainder of the circuit. After you install a GFCI fixture, always test it.
The safest course is to leave AC system repairs to a qualified marine electrician, but if you opt to do the work yourself, never, ever work on an AC circuit hot. Disconnect the cord — and take it off the dock, lest some Good Samaritan send you to the next life. Also disconnect the inverter. Then religiously check all wires with a voltage tester. This is the electrician's version of "measure twice, cut once." Be absolutely certain that there is no voltage between any two wires or any wire and ground.
When it comes to AC power, what you don't know definitely can hurt you.
AC aboard raises a number of additional considerations not covered here. Sailors and powerboaters alike will find a more complete treatment in Sailboat Electrics Simplified by Don Casey.