What size wire do I need?

[GlassWolf]

When running power and ground cable for car audio systems, it is strongly recommended that you replace all of the following lines with larger gauge cable:
motor to chassis ground
alternator to battery positive terminal
battery negative terminal to chassis ground
battery positive terminal to amplifiers or distribution block
amplifier(s) to chassis ground

power and ground cables:
For a reference chart to determine wire size based on distance run and current draw or wattage, see here:
Power and Ground cable chart
www.the12volt.com/info/recwirsz.asp

For actual wire diameter based on American Wire Gauge (AWG) see here:
American Wire Gauge Sizes
www.the12volt.com/wiring/awg.asp

These charts only go up to 2AWG, but the logical progression to 1/0, 0, and 00 gauge should be obvious.

For more information on wire and it's applications in car audio, see here:
Badic Car Audio and Electronics

Speaker wire sizing:
Speaker wire isn't as vital as power and ground, because you're dealing with high voltage, low current AC voltage for speakers, much like wall outlets in a house, as opposed to high current DC voltage for power and ground.
Generally, this is my rule of thumb:
for component speakers (midbass, midrange, or tweeters) 18-16AWG is fine.
For subwoofers, 14-12AWG is plenty.
Anything you choose to use of a larger gauge will really only be for looks, as there will be no electrical advantage to larger wire.