Boat Wiring Explained

​In any electrical system, on a boat or in a home, whenever a circuit is extended or rewired, or when any new circuit is installed, it is critical that the new wiring is made with wire conductors that are properly sized for the amperage rating of the circuit. The higher the amperage rating of the circuit, the larger the wires need to be in order to avoid excess heat that can melt wires and cause fires.

The proper circuit size, as indicated by amperage, is determined by several factors, including the planned load on the circuit, the number of outlets​ or components, and the length of the circuit. Once the proper amperage is determined, though, it is critical, that the wire gauge used in the circuit is appropriate for the amperage of the circuit breaker.

How Wires Are Sized

If you've shopped for electrical wire, you have likely noticed that there are many types and sizes of wire to choose from. There is specific wire approved for marine applications and we always recommend any wiring on a boat is completed with marine approved wire. ​

Wire is sized by the American Wire Gauge (AWG) system. Wire gauge refers the physical size of the wire, rated with a numerical designation that runs opposite to the diameter of the conductors—in other words, the smaller the wire gauge number, the larger the wire diameter. Common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire. The size of the wire dictates how much current can safely pass through the wire.

Electrical current is measured in ampacity, and each wire gauge has a maximum safe carrying capacity. For standard non-metallic (NM) cable, these amperage capacities are: 

Amperage Capacities for Standard Non-Metallic (NM) Cable
14-gauge wire15 amps
12-gauge wire20 amps
10-gauge wire30 amps
8-gauge wire40 amps
6-gauge wire55 amps
4-gauge wire70 amps
3-gauge wire85 amps
2-gauge wire95 amps

These ratings are for standard copper NM sheathed cable, but there are instances where these amperage ratings vary. ​

Why Wire Gauge Is Important

While circuit breakers or fuses offers good protection against overloading wires and overheating them, they are not absolute protection. Both these devices are designed to sense current overloads and to trip or "blow" before the wires can overheat to the danger point. But they are not foolproof, and it is still important to guard against exceeding the amperage rating of any given circuit by plugging too many appliances into them.

There is the potential for danger anytime a device or appliance tries to draw more power on a circuit than the wire gauge is rated for. For example, plugging a heater rated for 20 amps into a 15-amp circuit wired with 14-gauge wire poses a distinct danger. Should the circuit breaker fail to operate correctly, that heater will draw more current than the wires can safely handle, and could heat the wires to the point of melting the insulation around the wires and igniting surrounding materials. 

On the other hand, there is no danger whatsoever by plugging appliances with mild electrical loads into circuits with heavier gauge wires and a higher amperage rating. The circuit will draw the power asked for by whatever is plugged into them and no more. So, for example, running a laptop computer with a very small amperage demand on a 20-amp circuit wired with 12-gauge wire is perfectly fine. 

Wire UseRated AmpacityWire Gauge
Low-voltage lighting and lamp cords10 amps18-gauge
Extension cords (light-duty)13 amps16-gauge
Light fixtures, lamps, lighting circuits15 amps14-gauge
Kitchen, bathroom, and outdoor receptacles (outlets); 120-volt air conditioners20 amps12-gauge
Electric clothes dryers, 240-volt window air conditioners, electric water heaters30 amps10-gauge
Cooktops and ranges40-50 amps6-gauge
Electric furnaces, large electric heaters60 amps4-gauge
  

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