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Why is the size of an automotive wire so important? When electricity passes through a wire, The wire resists the electricity and some of it is converted into heat that is absorbed by the wire. This lost energy is realized in the form of a voltage drop at the end of the wire. In its simplest terms, you can think of 12 Volts as during 12 units of work. If you're wire has a 2% voltage drop, then the wire is using up 0.24 volts, or units of work. That leaves 11.76 volts, or units of work left. So if you put a volt meter on your battery and read 12 volts, when you put your volt meter on the end of the wire, you will read 11.76 volts. If you are running 20 amps through your wire then your wire is absorbing 4.8 watts of power. Now let's say your wire has a 20% voltage drop. The voltage reading on your volt meter at the end of the wire will only be 9.6 volts, and the wire is absorbing a whopping 48 watts of power!

Automotive Wire Recommended Amperage Capacity Chart - 2% Voltage Drop
Recommended Length and Amperage for Automotive Wire while maintaining a 2% or less voltage drop at 12 volts
Automotive Wire Size 5 Amps 10 Amps 15 Amps 20 Amps 25 Amps 30 Amps
20 Gauge Wire (AWG) 4.5 ft 2.2 ft 1.6 ft .
18 Gauge Wire (AWG) 7.3 ft 3.7 ft 2.4 ft 1.8 ft
16 Gauge Wire (AWG) 11.5 ft 5.8 ft 3.8 ft 2.9 ft 2.3 ft 1.9 ft
14 Gauge Wire (AWG) 18.4 ft 9.2 ft 6.1 ft 4.6 ft 3.7 ft 3.1 ft
12 Gauge Wire (AWG) 29.4 ft 14.7 ft 9.8 ft 7.4 ft 5.9 ft 4.9 ft
10 Gauge Wire (AWG) 46.8 ft 23.4 ft 15.6 ft 11.7 ft 9.4 ft 7.8 ft
8 Gauge Wire (AWG) 74.4 ft 37.2 ft 24.8 ft 18.6 ft 14.9 ft 12.4 ft