By Don Casey
Revised by BoatUS editors in April 2012
Whether adding new electrical gear or rewiring something already aboard, the requirements for a reliable and safe electrical connection are the same. You need the right wire, the right terminal fittings, and a couple of inexpensive hand tools.
A wire could hardly find a more hostile environment than aboard a boat. On-board wiring is salted and doused, shaken and whipped, crushed and abraded, exposed to sunlight, subjected to heat, and coated with petroleum. Neither lamp cord nor house wiring will long endure these conditions.
Choose your wire carefully. Never use solid wire on a boat. Wave or motor induced oscillations eventually fracture solid wire. Boat wiring must have the flexibility stranding provides. Boatbuilders save a few dollars using Type 2 wire, but a boatowner should use only the most flexible wire, called Type 3.
The wire must be copper, but even copper corrodes in the marine environment, and corrosion is the primary cause of electrical failures on a boat. Plating each strand of the wire with a thin coat of tin--called tinning--dramatically improves corrosion resistance. The additional cost of tinned wire is nominal, the benefits substantial. Under normal circumstances use only tinned wire.
Stranded single-conductor wire is called hook-up or primary wire. Since most after-construction wiring requires two wires, duplex wire is more convenient and provides the added safety of a second layer of insulation. The best choice for most 12-volt wiring projects is duplex safety wire, where the twin conductors are red (positive) and yellow (ground). Making the ground wire yellow rather than black reduces the likelihood of confusing a DC ground wire with an AC hot wire, also black.
In recent years wire designed for the marine environment has become widely available to boatowners. This wire is known as Boat Cable. Unfortunately the Underwriter's Laboratories standard that defines Boat Cable, UL 1426, is less stringent than commonly thought. Boat Cable can be Type 2, tinning is not a requirement, and the heat rating of the insulation can be quite low. When you select Boat Cable, Type 3 is better, tinning is essential, and you want the highest heat rating--designated on the jacket as BC5W2 (105°C in a dry environment, 75°C wet). Tinned Boat Cable from a reputable supplier is your best choice for all wiring needs.
As with water through hose, electricity flows more easily through larger wire. It is essential to size wire for the maximum current flow you expect it to carry. If the wire feeds a single item, the current requirements will be shown on a label or plate on the appliance, or in the accompanying manual. If the rating is in watts, divide it by 12 (assuming a 12-volt electrical system) to convert the rating to amps. When the wire is part of a circuit that supplies several appliances, the potential current through the wire is the sum of the current requirements of every appliance on the circuit. For example, if a circuit is comprised of six 25-watt cabin lamps, the wire will be carrying about 12 amps ([25 watts ÷ 12 volts] x 6) when all the lights are on.
To size wiring, you need the wire length from the power source to the appliance(s) and back to the power source. Doubling the straight line distance to the battery or electrical panel is not adequate. You must determine the actual length of the wire by measuring along the path it will follow-up, over, and around. It is not unusual for a wire run to be more than twice the straight-line distance.
Some voltage is used up pushing the current through the wire. This loss, called voltage drop, should not exceed 3%. The table shows what size wire is required to deliver adequate voltage to the other end. Use the potential current draw to select the row, the round trip wire length to select the column. The number where these two intersect is the wire size you need. Electrical wire appropriate for marine use will have the gauge designation printed on the insulation. The smaller the gauge number, the larger the wire diameter. Always buy wire at least a foot or two longer than your measurement. You can easily shorten the wire after it is installed, but lengthening it requires a highly undesirable splice. Each wire should be a single continuous run between terminals.
Other than chafe or lying against hot metal, wires rarely experience failures in the middle of a wire run. Almost all wiring problems occur at the connections. Never twist wires together to make a connection, and never wrap a bare wire around a terminal screw. You will minimize wiring problems if you terminate all wire ends with crimp connectors.
Selecting the proper connector requires that you match it to the wire gauge and to the size of the terminal screw. Ring terminals are your best choice unless the terminal screw is captive. In that case, use flanged spade connectors.
Use butt connectors for appliances supplied with wire leads instead of terminals. Step-down butt connectors let you connect heavy supply wires to lighter leads. To simplify servicing, it can be a good idea to make the connection with blade or snap connectors instead of butt connectors. Three-way connectors are useful for tapping into an existing circuit.
Terminals used on a boat must always be copper, never steel or aluminum, and like the wire, they should be tin-plated to resist corrosion.
Nine times out of ten, stripping insulation with a pocket knife results in nicked wire, which opens the door to corrosion. And you simply cannot make a dependable crimp connection with a pair of pliers. Every boatowner should own a wire stripper and a good-quality crimper. The cost is nominal. Strippers sold by auto supply and hardware stores are for SAE wire, which is about 10% smaller than AWG, so these strippers will nick AWG wire. Be sure the wire stripper you buy is intended for AWG wire.
An inexpensive pliers-type crimper is more than adequate for a few connections, provided you make a few practice crimps first. But if you will be making a lot of connections, a ratchet crimper offers the benefit of consistency, even in the hands of an amateur.
Remove only enough insulation for the wire to reach the end of the barrel of the terminal. Grip the terminal in the correct die in the crimper, fully insert the wire into the terminal, and squeeze. If the barrel has a seam, the crimp indent should be opposite.
Better terminals feature a brass sleeve that can be crimped over the insulated wire to add mechanical strength. This type of terminal is usually installed with a double crimp tool. If your crimper doesn't have a double-crimp die, crimp the terminal to the wire first, then reposition the crimper and crimp the sleeve to the insulation.
There is only one accepted field test for a crimp terminal-pull on it. Test every crimp terminal this way. Without using any tool, grip the terminal and the wire and try to separate them. If they come apart, the crimp was bad.
Be sure to run your wire as high in the boat as possible-to keep it dry-and support it at least every 18 inches.