Automotive wiring is one of those seemingly black arts left only to the electronic necromancers of Mordor (aka "Detroit"), but it's actually pretty simple if you just remember to take everything one step at a time. Proper wiring starts with knowing how many amps you're dealing with and what the distances are like; from there, it's just picking a wire color you like, stripping some insulation and crimping a few connectors.
The right wire for any given application has more to do with the distance from the powered accessory's power source to the accessory than with the power draw itself. If you're new to electrical wiring, then the first thing you need to learn is Ohm's Law, which states that current through a conductor between two points is directly proportionate to the potential difference across the two points, and inversely proportionate to the distance between them. That's a very long way of saying that you can use a very small-diameter wire -- one with more ohms of resistance -- to carry current for a short distance, but that you'll need a larger-diameter wire if you want to carry the current farther away.
00-Gauge to 8-Gauge Wiring
Wiring sizes measure from smallest to largest on a numerical scale that starts with a zero for the largest sizes and gets smaller as the numbers go up. Very large wires may measure with numerous zeroes; a 000-gauge wire is bigger than a 00-gauge wire, which is bigger than a 0-gauge wire. The largest wire you're likely to find in an automotive application is 00-gauge, which is used in big trucks for carrying current from multiple batteries. You'll normally find 0-gauge and 1-gauge wire used to wire the starters of high-compression gas and diesel engines. More commonly, you'll find starters and batteries wired with 4-gauge and 6-gauge cables; and sometimes 8-gauge for very small-displacement, low-compression motors.
10-Gauge to 14-Gauge Wiring
You'll typically find the thickest "normal" wires -- 10-gauge -- running from the alternator or generator to anything that it powers, or in very long circuits that run from a front-mounted battery to a high-draw accessory at the rear. Cars using very powerful amplifiers or trunk-mounted compressors will generally use 10-gauge or even 8-gauge wiring. Smaller 12- through 14-gauge wiring generally carries power from the battery to high-draw accessories such as headlights, the main fuse block, horn to relay, windshield wiper, gauges, the 12-volt "cigarette lighter" outlets in your cab and the fuel pump. The latter may use heavier-gauge wiring not because it draws a great deal of power, but because it's so far from the battery.
16-Gauge Wiring and Smaller
Most of the systems in your car use 16- or 18-gauge wiring. You'll typically find 16-gauge wiring carrying current to the ignition coil, from the generator to the starter -- on cars so-equipped -- interior lights, parking lights and tail lights. Almost everything else will use either 18-gauge or smaller, with the smallest being your non-subwoofer speakers.
Wiring Guide: 2-Gauge to 10-Gauge
The above are generalizations about the wire sizes generally used, but you need always select your wiring based upon the amp draw of your accessory. Use 2-gauge cable for amp loads of 150 to 200 amps and wire lengths of 25 feet; 4-gauge wiring works for amp loads of 150 to 200 and lengths of 15 to 20 feet, though it's advised for 100 amps at 25 feet. Some 6-gauge wiring works for 100 amps draw and 15 to 20 feet. Use 8-gauge for 150- to 200-amp loads more than 3 feet in length, and 40- to 50-amp loads of 25 feet. The 10-gauge wiring is appropriate for 150 to 200 amps at 3 feet, 100 amps at 7 to 10 feet, 30 to 50 amps at 15 to 20 feet and 20 to amps at 25 feet.
Wiring Guide: 12-Gauge to 18-Gauge
Some 12-gauge wiring is appropriate for 100 amps at 3 to 5 feet, 75 amps at 5 to 7 feet, 50 amps at 7 to 10 feet, 40 amps at 10 feet, 20 to 24 amps at 15 to 20 feet and 15 to 18 amps at 25 feet. The 14-gauge wiring works for 50 amps at 5 feet, 40 amps at 7 feet, 30 amps at 10 feet, 15 to 18 amps at 15 to 20 feet and 11 to 12 amps at 25 feet. The smaller 16-gauge wire is very versatile, working for 50 amps at 3 feet, 30 to 40 amps at 5 feet, 18 to 30 amps at 7 to 10 feet, 8 to 12 amps at 15 to 20 feet and 8 to 10 amps at 25 feet. If you don't see your application listed here -- your distances are shorter or amps lower -- and you're pumping 40 amps or less, then 18-gauge wire is probably your safest bet. Remember: when in doubt, go a gauge bigger.
- Auto Electricity and Electronics: Principles, Diagnosis, Testing, and Service of All Major Electrical, Electronic, and Computer Control Systems; James E Duffy
- How to Diagnose and Repair Automotive Electrical Systems; Tracy Martin
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