According to the National Electrical Code, what is the minimum wire size that may be used safely for wiring with a 20-ampere circuit breaker?
Why The National Electrical Code matches conductor size to breaker rating so the wire can carry the fault current without overheating before the breaker trips. For common copper branch circuits the pattern is AWG 14 for 15 amps, AWG 12 for 20 amps, and AWG 10 for 30 amps. So a 20-amp breaker requires at least AWG number 12 copper wire.
Watch out AWG 16 and AWG 20 are thinner wire (larger AWG number means smaller conductor) and would overheat well before a 20-amp breaker opened; AWG 8 is heavy enough but far larger than the minimum the question asks for.
14-15, 12-20, 10-30: wire gauge counts down as amps count up.