Wire uses the measurement of gauge to determine the size of the wire. The larger the wire, the smaller the gauge number. To find the required wire gauge for a specific application, you must know the current draw of the accessory on the circuit and the total wire length between the accessory and the power source. More current draw (higher amperage) requires a larger wire gauge to safely power the accessory.
Automotive Voltage is not 12 Volts
Automotive wiring is not exactly 12 volt. The actual resting voltage of a fully charged, 6-cell automotive lead acid battery is about 12.7 volts or about 2.1 volts per cell. 6.4 volts for a 6-volt lead-acid battery. Older batteries will probably show a lower voltage. When the vehicle is running, the alternator will increase the automotive voltage up to about 13.8 volts. 13.8 volts is a better value to use for wire gauge calculations though it will generally result in about the same gauge as 12 volts.
Resistance = Voltage Drop
With cable length comes resistance. All wire has inherent resistance and the longer the wire, the greater the resistance and the greater the voltage drop over the length of wire. For that reason, it is important to factor in the length of wire when determining the wire gauge. A 3-foot wire length will have less resistance than a 20-foot wire and therefore a longer length of wire may require an increase in the wire gauge to provide adequate voltage to the accessory. Installing too small a wire gauge decreases performance and may create a potential safety hazard. Alternately using an oversized wire gauge does not have a downside and may provide better performance of the accessory, however, overkill in itself has the downside of wasted money and valuable space. But when choosing between two potential gauge sizes in the gray area of estimated gauge calculations or references, it’s always better to go with the larger gauge.
Choosing the Wire Gauge
To choose an adequate wire gauge, determine the amp draw (amperage) that the wire circuit will carry. Then measure the distance that the wire will travel (length) including the length of the return to ground (the ground wire running to the chassis or back to a ground block or battery. Using these two numbers, Amps and length, locate the nearest gauge value in the chart below. For 6 volt automotive systems typically a wire gauge 2 sizes larger than what is shown should be used.
| Amps |
@ 13.8 Volts
| LENGTH OF WIRE|
American Wire Gauge (AWG)
|0-4 ft.||4-7 ft.||7-10 ft.||10-13 ft.||13-16 ft.||16-19 ft.||19-22 ft.|
|35-50||10-ga.||10-ga.||10-ga.||8-ga.||8-ga.||8-ga.||6 or 4-ga.|
|50-65||10-ga.||10-ga.||8-ga.||8-ga.||6 or 4-ga.||6 or 4-ga.||4-ga.|
|65-85||10-ga.||8-ga.||8-ga.||6 or 4-ga.||6 or 4-ga.||4-ga.||4-ga.|
|85-105||8-ga.||8-ga.||6 or 4-ga.||4-ga.||4-ga.||4-ga.||4-ga.|
|105-125||8-ga.||8-ga.||6 or 4-ga.||4-ga.||4-ga.||4-ga.||2-ga.|
|125-150||8-ga.||6 or 4-ga.||4-ga.||4-ga.||2-ga.||2-ga.||2-ga.|
|150-200||6 or 4-ga.||4-ga.||4-ga.||2-ga.||2-ga.||1/0-ga.||1/0-ga.|
SWG and AWG are wire gauge systems used to represent the thickness of the wire. Different numbering systems are used depending on the type of metal. Non-ferrous metals (not containing iron) use American Wire Gauge System or AWG. Ferrous metals generally use Standard Wire Gauge (SWG).
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