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
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.
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.
Always mount an outlet in an outlet box, which typically will extend into a locker. I prefer a plastic box because it doesn’t corrode. The wires should enter and leave the bottom of the box because this avoids creating a path for water to enter.
Do not use push-in terminal holes in the outlet to make the wire connection. Those are for solid wire, which you must never use to wire a boat. Similarly, looping the stripped end of the wire around the terminal screw, which is a common practice ashore, is verboten on a boat. Commercial-grade outlets may have screw-tightened clamps in place of the screw terminals, and you can safely connect these units with a stripped wire end. Otherwise, every outlet and appliance connection must be made with proper crimp terminals. If the terminal screws are captive, use locking spade terminals rather than ring terminals.
Standard circuit breakers are primarily fire-protection devices and do not protect against electrical shock. For this you need a ground-fault circuit interrupter (GFCI). If you accidentally touch an energized wire or component and you are grounded, the GFCI disconnects the circuit in about .025 second, which is too short a period of time for the current to build to a dangerous level. Because of increased shock risk in a damp environment, every AC outlet on a boat must be protected by a GFCI.
When installed as the first outlet on a circuit—counting from the breaker—a GFCI outlet provides shock protection for all other outlets on that circuit. GFCI outlets are inexpensive, and they are wired exactly like a regular outlet except that some models have wire leads rather than screw terminals. That means they will use butt crimp connectors rather than ring or spade terminals. Be sure the terminals or leads marked LINE are connected to the wires that lead back to the breaker. Those marked LOAD feed the remainder of the circuit.
Pressing the test button on a new GFCI outlet should cause the unit to interrupt and de-energize all the outlets on the circuit. Remember, though, that the GFCI senses only a short to ground. If you make contact with the hot and the neutral wires of a circuit, a GFCI won’t protect you.
Use Proper Wire
Solid electrical wire is intended for use inside a motionless building wall. Aboard a boat it will flex until it breaks, like paper-clip wire. Always use stranded and tinned wire for your AC system, preferably triplex boat cable—three insulated Type 3 wires (black, white, and green) inside an insulating jacket. As with all electrical wiring, route AC wiring as high in the boat as possible and across the top of a locker rather than across the bottom. Support long wire runs with continuous conduit or cable clamps every 18 inches.
A 30-amp inlet requires 10-gauge wire at least to the main breaker. For 15-amp circuits, you could use 14-gauge wire, but you will be ahead of the game if you do not use anything smaller than 12 gauge. This is because you might later decide to add another appliance or change to 20-amp outlets or plug in an air conditioner, which can require start-up current (called inrush) as much as five times the compressor’s nominal draw. Larger wires also run cooler, so an oversized wire is almost always money well spent.
Connecting the green grounding wire to a boat’s DC ground does have an undesirable side effect; it can create a circuit between your boat and other nearby boats that have their own green wires grounded. This opens the door to both galvanic and stray-current corrosion. The best solution is to install an isolation transformer. Although these are heavy and relatively expensive, they do eliminate the direct connections between your boat’s wiring and the shore. In most situations, though, a less-expensive galvanic isolator limits corrosion just as effectively.
An isolator consists of a pair of diodes that are connected in parallel to a second pair that is conducting in the opposite direction. These opposing diodes pass current in both directions, allowing both AC and DC to flow freely through the isolator once the diodes become conductive. It takes about 0.6 volts for that to happen, so two diodes in series will block all current flow unless the voltage exceeds about 1.2 volts. Galvanic voltages between underwater metals are lower than this, so no current flows. The voltage of most stray currents, by the time it reaches your boat through the water, will be too low to cause the diodes to pass a current. Locate the isolator in the green wire and put it as near to the inlet fitting as possible.
Once you have installed all the various components and wired them according to your circuit drawing, the only thing left to do is connect a hard service–rated locking shore-power cord to your inlet fitting. Then, I might suggest, testing your blender would be a nice way to try out your new onboard AC power system.
If you make contact with both wires in a 12-volt circuit, you are unlikely to be injured because the current flow will be less than 0.01 amp—12 volts divided by the resistance of your body, which typically is at least 1,200 ohms. But when the voltage goes up to 120 volts, the current becomes 0.1 amp. Combine that with the pulsating nature of AC and you have more than twice the level needed to interfere with your body’s natural electrical impulses and perhaps even fatally disrupt the heart’s rhythm. That’s why it’s so important to take the necessary precautions.
- Never work on energized wiring.
- Religiously check all wires with a voltage tester before making contact with them.
- Never leave the power cord on the dock; a helpful passerby might decide to plug it in.
- Always work in rubber-soled shoes.
- Never work alone.
Don Casey’s latest revision of This Old Boat has just been published by International Marine.