Wire Sizes

Wire size is very important for the correct operation of electrical circuits. Selecting a wire gauge that is too small for an application will have an adverse effect on the operation of the circuit. This will cause voltage drop and poor performance, or, in extreme cases, the wire will get hot enough to melt the insulation. Selecting a wire gauge that is too large increases costs and weight and the size of wiring harnesses.

Table 36-1: Metric Wire Size
Comparison and American Wire
Gauge (AWG)
 Metric Wire Sizes AWG Wire Sizes 0.22 24 0.35 22 0.5 20 0.8 18 1.0 16 2.0 14 3.0 12 5.0 10 8.0 8 13.0 6 19.0 4 32.0 2
Table 36-2: AWG Wire Sizes Based on Amperage and Wire Length*
 Circuit Amps Wire Length from Battery to Load 2 Feet 5 Feet 7.5 Feet 10 Feet 15 Feet 20 Feet 25 Feet 2 20 20 20 18 18 18 16 5 18 18 18 18 16 14 14 8 18 16 16 14 14 12 12 10 16 16 16 14 12 12 10 12 16 16 14 14 12 12 10 15 16 16 14 12 10 10 8 18 16 14 12 12 10 8 8 20 14 14 12 10 10 8 8 25 14 12 12 10 8 8 6 30 12 12 10 10 8 6 6 *Chart is based on a maximum 0.4-volt drop per wire size; shorter distances are less than a 0.4-volt drop.

The resistance of a wire affects how much current it can carry. Even good conductors have a slight amount of resistance. The resistance of a wire is determined by its length, diameter, construction material, and temperature. The longer the wire and the smaller the diameter, the higher the resistance. The shorter the wire and the larger the diameter, the lower the resistance.

There are two scales used to measure the sizes of wires: the metric wire gauge and the American wire gauge (AWG) table 36-1. The metric system measures the cross-sectional area of the conductor in square millimeters. The AWG system uses a rating number; the larger the rating number, the smaller the wire and the lower its current-carrying capability. Most countries use the metric scale. American manufacturers are split, with some using AWG and others using the metric scale.

To select the correct wire gauge for any given application, it is best to refer to a wire chart. Manufacturers and standards bodies use wire gauge charts to define how much current each wire gauge can carry safely and efficiently. A vehicle uses a variety of wire sizes depending on the requirements of each particular circuit.

The correct wire size for an application can be looked up on a wire size chart if you know the amperage of the circuit and the length of the wire. But be careful of the chart you use, since many of them allow up to a 10% voltage drop over the length of the wire, which is way more than is allowed in most automotive circuits. For example, if a 12-volt circuit is designed for a maximum current flow of 10 amps and is approximately 20´ (6.1 meters) long, using the AWG table as a reference, you can determine that the correct wire gauge to use is 12 AWG table 36-2.

There are two different methods of describing the conductor size within these standards. A wire may be described in metric size as 5.0, indicating it has a cross sectional area of 5.0 millimeters squared (mm²). It can also be expressed as 10/0.5, indicating there are 10 strands of wire, each with a cross-sectional area of 0.5 mm². The same system can be applied to the AWG rating.

Length Versus Resistance

Copper is used to conduct electrical current because of its low resistance value. However, it does offer some resistance, and as the length of the wire increases, so, too, does the resistance within the wire. To overcome the effect of resistance, the greater the length of the wire, the larger the cross-sectional area needs to be. Increasing the cross-sectional area overcomes the resistance and maintains the current-carrying capacity of the circuit. Refer to Table 36-2 for information on wire size and current-carrying capacity.