Can I use 10 gauge wire off of a 20 amp breaker for outlets, lights or a window unit? I am wiring a barn and have a lot of 10/3 wire I would like use up.
Yes, you can use 10 AWG copper conductors with a 20 ampere breaker. The smallest size conductors you could use with a 20 ampere breaker, are 12 AWG copper conductors. There's no problem using larger conductors, other than cost to you, and difficulties associated with working with thicker conductors.