The correct wire size for a 20 amp circuit is 12 AWG copper wire. This wire is rated for 20 amps at 75°C and is the perfect size for a 20 amp circuit. Using a 14 gauge wire on a 20 amp circuit is not the correct size, as it is only rated to handle up to 15 amps in residential applications. Using a 10 AWG wire for a 20 amp circuit is possible and safe but not cost-effective. It is important to note that all conductors protected by a 20 ampere circuit breaker must be at least 12 AWG copper. It is also important to follow the National Electrical Code (NEC) NFPA 70 guidelines when selecting wire size for a circuit.