Automotive Wiring and the Tao of Wire Size

Wire conductor size:

The things to consider when selecting a wire size and insulation are the ambient temperature, the current the wire will be carrying, the total length of the wire, and how much voltage loss is acceptable. (There is always some loss.) The longer the wire, or the smaller the wire, the more the loss. As a general rule sensitive circuits and headlights can tolerate 3%, and most everything else can tolerate 10%. Headlights are sensitive because the light output varies as the square of the voltage or more, so if you want the best light output you can get, go heavy on the wire and use a relay for your lights. Also, EFI, ECU, and Stereo systems have a high peak current requirement that demands a larger wire size than the average current drawn suggests. If you don't have a manufacturer's recommendation, see note 3 below.

Temperature considerations

Keeping the non-engine wiring out of or at least at the edges of the engine compartment reduces the heat on the wire. Given that, for many uses the 85°C wire will be OK, but if you have any doubts, go ahead and use the 125C wire; it is cheap insurance. Also, remember that the 3% or 10% loss we are tolerating above shows up as heat in the wire. This is a factor in bundled wire cables. If bundling more than four wires, and two or more are running a 10% loss, just go up one gauge on the wires, to reduce heating.

Wire Current Load vs Wire gauge

Here are two ways to select the right gauge. The first is quick and dirty but not unreasonable, (a table of circuits vs typical currents and wire gauges). The second, using current vs. wire table, is more precise and analytical and also requires knowledge of the current load of each component in the vehicle. The current vs. wire table is located here. Of course, you can combine the two.

Typical Automotive Circuits, Currents and Wire gauges

* The power circuit wire indicated is for connecting from a battery or switch to a relay, and from the relay to the component. The relay coil wire can be 18 gauge.

NOTES:

1. The above table assumes typical wire lengths as found in a typical car. If the wire is going to be more than about 10 feet long, such as in a trailer, RV or Boat, go to the next larger gauge.

2. If the wires are to be bundled in to a bundle of 4-5 wires, then move up one gauge for all the wires carrying a current above about 80% of that shown in the table. For 6 or more wires in the bundle, move up 2 gauges. Your headlights are the likeliest to need this adjustment.

3. A way to check voltage drop at peak currents after installation is to put a small light bulb or a light type circuit tester in the circuit at the load end of the circuit while the load is active. If you can see any variation in the light's brightness, you probably need a larger wire.

After you've got everything working, it is a good idea to turn on all the electrical loads, lights, heater/AC, etc, then let it all run for 15-20 minutes, and then go around your vehicle and put your hand on the various wires and wire bundles. Carefully, of course. If anything is uncomfortably warm, investigate the cause of the heating. Keep in mind that on a long trip, with heater and lights running for hours, things could get hotter. It could save some pushing, or blown fuses, or even a fire later.