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.

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    I'll just comment that you are NOT going to be happy working with #10 in device boxes. Your sanity will be worth the cost of a roll of 12/2. Also, be careful with regard to box fill. You can go over the limit with #10 real fast. – Speedy Petey Apr 6 '15 at 19:43

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.

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    There is a limit on the max gauge a receptacle/breaker can be connected to, which should be listed somewhere on it or in the sheet with it. – diceless Apr 6 '15 at 15:37
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    Which can be addressed with a splice or wirenut to a pigtail of appropriate size, if the wire is too large for the breaker. 10Ga will be fine in most 20A breakers. – Ecnerwal Apr 6 '15 at 16:06

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