what gauge wire for 40 amp

The recommended wire gauge for a 40 amp breaker is 8 AWG copper or 6 AWG aluminum. The minimal wire gauge for a 40 amp circuit breaker is 8 AWG. Using 10 AWG or 12 AWG copper wires is not suitable for 40-amp circuit breakers. If the wire for a 40 amp breaker runs for a long distance, a larger wire gauge should be used to accommodate voltage drop. For example, 6-gauge copper wire or 4-gauge aluminum wire should be used for cables running over 100 feet, and 4-gauge copper wire or 2-gauge aluminum wire should be used for cables running over 200 feet. The 80% Rule increases the safety of the electric installation, and for a short 40 Amp wire, one should use a 6 gauge wire (AWG 6).