Determining Wire Sizes and Fuse Sizes

By: Dusty Arlia
Published on November 17, 2011
Last Updated on Tuesday, July 07, 2015 at 9:39 PM
Total Updates: 5

To determine what wire size you have is pretty simple. Check the plastic sheathing for a stamped or printed identification, which includes the gauge of the wire. If you're installing wire and are trying to determine which gauge wire to use in your installation, you will need to figure this out based on what devices will be attached to this circuit (your load), the voltage of your power source, and the appropriate current to power this device.

Cars run on just 12 volts. To operate correctly, the current running to all your devices must be large for things like headlamps to work. A high current, which has 10s of amps running through it, requires having thick wires. The higher the current, the thicker the wire. You need to make sure you know exactly how much current is running though a circuit and select the correct fuse and wire size for it, plus a little margin for error. Here is the formula for calculating the current/amps:

I = W / V
(I is amps, W is watts and V is volts)

For example, if you have a 48 watt headlamp, you need a current that is 4 amps:

4 amp current = (48 watt headlamp) / (12 volt battery)

Most of the time there's two headlamps in the same circuit. Therefore the current is 8 amps:

8 amp current = (96 watt headlamps) / (12 volt battery)

In this last example, you need to select a 10-amp fuse. A lot of the time the same circuit also powers the parking and running lights. If this is the case, you might need a larger fuse. 15A to 20A will work in most cases, but do the math and be sure. Too large a fuse might allow too much current to flow through small wires and cause damage. Use caution and double check your numbers. Extra attention to details can save you time and hassles down the road.

When you're doing electrical wiring in a home, follow these specifications:

A 15-amp circuit should have a 14-gauge wire or larger
A 20-amp circuit should have a 12-gauge wire or larger
A 30-amp circuit should have a 10-gauge wire or larger