The minimum wire size you can use with a 40-amp circuit breaker is 8-AWG. An eight-gauge wire is rated to handle a 40 amp breaker’s electric current. You can use a larger gauge 6-AWG wire for future-proofing, but a lower gauge wire will render the electrical system non-compliant with the NEC.

The recommended size wire for a 40 amp breaker is 8 AWG. This is the American Wire Gauge, and it is the standard that is used in the United States for determining wire size. 8 gauge wire is rated for 40 amps, and it can safely handle up to 50 amps. This 40 amp wire size can safely handle most needs.