300 watts and under - 8 gauge
300 - 1200 - 4 gauge
1200 - 1800 - 2 gauge
1800 and over - 0/1 gauge
If the run of wire is much over 20 feet, bump it up to the next step. If in doubt, go larger. Don't use 10 gauge, it's glorified speaker wire. Extra-large wires almost never hurt anyone (Except...