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In order for your automotive electrical system to perform properly, your wire must be sized properly based on the amount of current (amperage) the wire must carry, and the length that the wire needs to be. For example, you want to add some flood lights to your vehicle. The floodlights draw 15 amps of current. The length the wire needs to be to connect to the floodlights is 9 ft. Based on the chart above, you would want to use 12 gauge automotive wire for your floodlights.

 

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 Amps10 Amps15 Amps20 Amps25 Amps30 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

 

Is it better to solder or to crimp connectors to automotive wire? Soldering is electrically superior to crimping since it creates a maximum amount of surface contact between the wire and the terminal or connector. However, if soldering is not done properly, it can actually cause problems such as cold solder joints or too much solder wicking up into the wire making it susceptible to breakage from vibration.  In 95% and more cases, a good crimp will work just fine. The key is to make sure that there is good electrical contact between the copper strands of the wire, and the barrel of the terminal or connector. Basically, if you crimp your wire and terminal together, and you cannot pull them apart, the connection will most likely suffice. It is always a good idea to cover the terminated ends of your automotive wire with some heat shrink tubing.