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Electrical Work Rules

Lights and other electrical appliances require proper wiring to remain effective.
I have done some bad boat wiring in my time. I've used bell wire, lamp cord, and Romex—whatever was at hand. After all, any wire capable of handling 120 volts must be 10 times as capable of carrying 12 volts, right?

As a matter of fact, boat appliances connected with almost any kind of electrical wiring will work perfectly—at least at first. It is this seeming "success" that makes inappropriate boat wiring so common—and so dangerous.

In my case, after a couple of near disasters caused by bad wiring, I became more cautious and more selective when performing maintenance or enhancements that involved electrical equipment. But even though the quality of my wiring vastly improved, I remained uneasy, always thinking of the wiring—my wiring—as the potential weak link in the completed project.

I didn't finally settle those self doubts until last year when I spent several months researching this subject in preparation for writing a book on boat electrics. As it turns out, the list of requirements for safe, efficient, and reliable 12-volt wiring is reassuringly short.

So this month I thought I'd try a little experiment. What follows are the basics for trouble-free boat wiring. Print this column and stow it on board with your terminals and crimpers, and you will always have a ready reference at hand.

Use only the most flexible multistrand wire, called Type 3.
"Boat wiring must have the flexibility that stranding provides."
Solid wire has no place on a boat. Wave and/or motor-induced oscillations eventually fracture it. Boat wiring must have the flexibility that stranding provides.

Use only tinned wire.

It should go without saying that all wiring must be copper, but even copper corrodes in the marine environment. Corrosion is the primary cause of onboard electrical failures. Tinning—plating each strand with a thin coat of tin—dramatically improves corrosion resistance.

Use duplex safety wire with red (positive) and yellow (ground) conductors.
Neat, color-coded wiring makes future repairs or additions much easier.
American Boat and Yacht Council (ABYC) standards designate a variety of wiring colors for specific circuits, but unless you are willing to hunt down light blue wire for your new oil-pressure gauge, you should stick with red wire for the positive side of the circuit and yellow for the ground side. The existing ground wires on your boat may be black, but because of the risk of confusing a DC ground wire with an AC hot wire—also black—yellow has replaced black as the color of choice for DC grounds. Since most after-construction wiring requires two wires, duplex wire is more convenient and provides the added safety of a second layer of insulation.

Select wire with BC5W2 stamped on the insulation.
Wire designed for the marine environment—known as boat cable—has become widely available to boatowners. Unfortunately the standard that defines boat cable (UL 1426) is less stringent than it should be. Type 2 wire satisfies the standard, tinning is not a requirement and the heat rating of the insulation can be quite low. The boat cable you want is tinned Type 3 with the highest heat rating (105 C in a dry environment, 75 C wet). This wire is designated as BC5W2.

Make sure the wire diameter is large enough for the load.
Wiring that is too small will overheat, perhaps catastrophically. It can also affect the performance of the connected appliance. It is essential to size wire for the maximum current flow it will carry. Assuming a 12-volt electrical system, divide the rating of the appliance in watts by 12 to determine the current requirement in amps. Where there will be more than one appliance on the circuit, the potential current through the supply wire is the sum of the current requirements of every appliance on the circuit.
"Wiring that is too small will overheat, perhaps catastrophically."
For example, if a circuit is comprised of six 25-watt cabin lamps, the wire will be carrying about 12.5 amps when all the lights are on (25 watts x 6 lamps = 150 watts/12 volts = 12.5 amps).

Determining the appropriate wire diameter also necessitates knowing the length of the wire from the power source to the appliance(s) and back to the power source. Twice the straight line distance from the appliance 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, down, over, and around.

Conductor Size for 3 Percent Drop in Voltage

Minimum Wire Size (AWG) Selector Table




Round-Trip Length of Conductor (Feet)
  10 20 30 40 60 80 100 120 140
1 16 16 16 16 16 14 14 14 12
2 16 16 16 14 14 12 10 10 8
5 16 14 12 10 10 8 6 6 6
10 14 10 10 8 6 6 4 4 2
15 12 10 8 6 4 2 2 1 0
20 10 8 6 6 4 2 2 1 0
25 10 6 6 4 2 2 1 0 2/0
30 10 6 4 4 2 1 0 2/0 3/0
40 8 6 4 2 1 0 2/0 3/0 4/0
50 6 4 2 2 0 2/0 3/0 4/0  
60 6 4 2 1 2/0 3/0 4/0    
70 6 2 1 0 3/0 4/0      
80 6 2 1 0 3/0 4/0      
90 4 2 0 2/0 4/0        
100 4 2 0 2/0 4/0        

The table shows what size wire is required to deliver 97 percent of the supply voltage to the other end. Use the potential current to select the row and the round-trip length to select the column; the wire size you need is shown where these two intersect. Most electrical texts also include a 10 percent voltage drop table for sizing wires in less critical applications, but larger wire (per the three-percent table) will be more durable and provide capacity for future appliances—benefits that eclipse saving a few pennies per foot on smaller wire.

Note that the smaller the gauge number, the larger the wire diameter. Note also that the wire size in this table is AWG (American Wire Gauge). Wire from hardware and auto supply stores is most often SAE (Society of Automotive Engineers), which is about 10 percent smaller than AWG wire of the same gauge. The complete designation, e.g., 12 AWG, will be printed on the insulation. Don't use wire smaller than the table calls for.

Each wire should be a single continuous run between terminals.

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.

Terminate wire ends with crimp connectors.
Don't twist wires together to make a connection. Don't use wire nuts. Don't wrap a bare wire around a terminal screw. Nearly all wiring problems occur at the connections. You will avoid most of these with crimp connectors.

Crimp connector must match the wire gauge and the terminal screw.
For connections to terminal screws, ring terminals are your best choice unless the terminal screw is captive. In that case, use flanged spade connectors. For appliances supplied with wire leads instead of terminals, use butt connectors for a permanent connection, or snap connectors if you want to be able to disconnect the connection.

Avoid steel or aluminum connectors.
Terminals used on a boat must always be copper. Like the wire, they should be tin-plated to resist corrosion.

A wire stripper and a good-quality crimper are essential.
Stripping insulation with a pocket knife invariably results in nicked wire, which opens the door to corrosion. And you simply cannot make a reliable crimp connection with a pair of pliers.

Be sure the wire stripper you buy is intended for AWG wire. Strippers sold by auto supply and hardware stores are for smaller SAE wire, so they will nick 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.

Learn proper crimping techniques.
Strip 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. If your crimper doesn't have a double-crimp die for this type of terminal, crimp the terminal to the wire first, then reposition the crimper and crimp the sleeve to the insulation.

Test every crimp terminal.
There is only one accepted field test for a crimp terminal—pull on it.

Solder or not.

If you want to do it, soldering crimp terminals is a terrific way of perfecting the electrical connection between the wire and the terminal, but never depend on solder for the mechanical connection. If the wire gets hot, the connection can fall apart—a hazardous possibility. A crimp terminal should give several decades of reliable service without soldering.

The less wiring or terminal exposed, the lower the opportunity for shorts or corrosion.
Wrap exposed connections with electrical tape or enclose them in adhesive shrink tubing. Some electricians paint connections—terminal screw and all—with so-called liquid electrical tape.

Support wire runs with clips or ties every 18 inches.
The finished product is reliable and has a professional look.
Allowing the wire to flex with the motion of the boat leads to fatigue fracture, typically where the wire enters the connector. It is likewise a good practice to route wiring as high in the boat as possible to keep it dry.

In the end, dependable and long-lasting 12-volt wiring comes down to three essentials:

 Size the wire correctly.
 Use only BC5W2 wire and top-quality terminals.
 Equip yourself with an AWG stripper and a ratchet crimper with a double crimp die.

The title of the book, by the way, is Sailboat Electrics Simplified—in case your specific electrical concerns go beyond 12-volt wiring.


Don Casey is offline  
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