Wiring a boat for shore power is a by-the-numbers project requiring more care than skill. Alternating current (AC) can be as dangerous as a cornered viper, so you must not be casual about AC wiring. Nor should you conclude that successfully completing some wiring jobs around your house qualifies you for this job. There are critical differences between AC wiring ashore and AC wiring on a boat.

Begin with a diagram

Before you do anything else, draw a map of exactly what you want. The more details you include in the drawing, the less likely you are to make a mistake during the wiring process. Although you could overlay your map onto an outline of your boat—showing outlets and/or appliance locations—a straight-line schematic drawing will be less confusing.

All electrical circuits require two connections to each appliance, one for the supply wire and one for the return wire. These are called positive and negative in 12-volt wiring, but alternating current changes polarity—120 times per second for the 60-cycle power that is standard in North America—so the two AC connections are called hot and neutral. Despite the reversing current, these wires are not interchangeable; the hot wire comes from the power source while the neutral wire leads, eventually, to a ground plate. Although an appliance will function normally if the connections are reversed, the switches, breakers, and fuses would then be on the “ground” side of the circuit. Turning off a switch or tripping a breaker interrupts the current flow so the appliance ceases to function, but because the interruption is not between the appliance and the power source, the appliance remains energized, which is a dangerous condition.

The insulation sheathing on AC wiring is colored black for the hot wire and white for the neutral wire. Label all the wires in your drawing black and white, and connect the black wire to the fuse or switch side of each appliance. As for the outlets, standard 120-volt outlets are polarized, with the larger of the two blade sockets being neutral. The neutral-side connection will have a silver terminal screw, and the hot side will have a brass, or dark-colored, terminal. Make it clear on the drawing that the white wire connects to the silver terminal and the black wire goes to the brass.

The third wire

Shoreside AC circuits in North America have a third wire, which is called the grounding wire. This is not a current-carrying wire, at least not in normal circumstances. Its function is to provide a low-resistance path to ground from an appliance’s metal housing or socket. If the current-carrying wire makes contact with the case, and there is no third wire, the case becomes energized. No fuse will blow because there is no circuit—until you touch the case. At that point your body completes the path to ground and you get a shock. But ground the case with the grounding wire and a short will blow a fuse, or at least bypass your body for the lower resistance provided by copper wire.

While a grounding wire ashore is typically bare, on a boat it should be insulated and green. Add this third green wire to your drawing and connect it to the grounding terminal on all appliances and outlets. Typically grounding terminals are either dyed green or are clearly labeled.

This is about all you need to know to draw up your shore-power map. Three wires come aboard via an inlet fitting, which is the beginning of your circuit. The black and the white wires lead from the inlet to a double-pole breaker—meaning that the breaker opens both sides of the circuit. From this main breaker the black and the white wires continue to the most distant appliance, and the remaining appliances on the same circuit are wired across these two wires like rungs on a ladder.

The green grounding wire, or third wire, bypasses the breaker panel entirely and connects the grounding terminals of each appliance in the circuit to the grounding terminal of the inlet fitting. This is different from wiring practices ashore, where you will find the grounding wire terminating into the same bus bar as the neutral wires inside the breaker box. On a boat, the green grounding wire must never connect to the neutral wire; rather, it must be connected to the main DC ground. The reason for this is that onboard AC power may leak into the DC wiring through a crossed wire, a moisture trail, or a dual-voltage appliance. A short circuit here would follow your DC ground circuit into the water, creating the same risk to nearby swimmers as dropping a hot wire into the water. Connecting the green wire to the DC ground terminal eliminates this possibility by providing a lower-resistance path to ground; make sure the grounding connection that goes back to shore is sound.

Install the pieces

Breaker size will be determined by the appliance(s) protected, not by the rating of the inlet fitting (except that the breaker rating must never exceed the inlet rating). If you are wiring a single circuit with outlets rated at 15 amps, the main breaker must be a 15-amp breaker. If you are wiring two circuits and both of them have 15-amp components, each of the branch breakers must be rated for 15 amps; the main breaker can be up to 30 amps.

Once the circuit (or circuits) is mapped out, you can begin installing the various components of your new shore-power system. The first element is the 30-amp weatherproof inlet fitting. This is adequate in most cases and minimizes compatibility problems, because the 30-amp locking receptacle is the norm at most U.S. and Caribbean marinas. If your AC requirements exceed 120 volts and/or 30 amps, you will have to install the appropriate inlet fitting. Considering the added risk higher voltage brings, you should leave 250-volt wiring to a marine electrician.

Locating the inlet fitting depends on the location of your main AC breaker. Because it is unprotected, the wiring run between the fitting and the breaker should be as short as possible and never longer than 10 feet. It is perfectly acceptable to install a small breaker box inside a locker near the inlet fitting instead of trying to locate the fitting within 10 feet (wire distance) of the main distribution panel. A single double-pole breaker is sufficient for a single AC circuit, but if you are wiring multiple circuits you must have a breaker for each branch circuit. A single-pole breaker, even on a branch circuit, is risky for AC wiring because it doesn’t protect the circuit if polarity is reversed; a double-pole breaker does. You can locate branch breakers in the same weatherproof box in the cockpit locker. You can also install a marine switch/breaker panel in the main cabin adjacent to the DC panel. Make sure a reverse-polarity warning device is a prominent feature.

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