For a 20 amp circuit, the correct wire size is 12 AWG copper wire. This wire size is rated for 20 amps at 75°C and can handle a maximum continuous load of 20 amps. 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 a 20-amp circuit, protected by a 20-amp breaker or fuse, must be served by 12-gauge or 10-gauge wire. When selecting the appropriate wire size, factors such as distance, material and insulation type, load calculation, wire type, and voltage drop should be considered. It is always recommended to consult a qualified electrician to assist with specific wiring needs or to ensure compliance with local electrical codes and regulations.