Just wondering if there is a formula to figure out the size of power wire you need to power an amp.Or is there justa comon size used for all applications?
Thanks

 

http://www.the12volt.com/info/recwirsz.asp

This chart is a little on the high side for wire (you definately don't need 4 guage to run 100 watts 20 ft.)

As for a formula, (Amplifier Watts)/14 will give you a rough estimate of maximum current draw. This current value can be referenced to the chart in the bottom right to get the minimum wire that can be used safely. However, using a bigger wire will always result in less voltage drop which results in more power at the amp, so for longs runs it is a good idea to go up a size or two.

 

http://www.canadiancaraudio.com/online/car-audio-technical-discussions/23538-wiring-information-cable-selector.html

Doh!