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?

Explication

The breaker protects the wire, not the load, so the conductor must be able to carry the breaker's rating continuously without overheating. For a 20 ampere branch circuit the smallest copper conductor permitted is AWG 12; AWG 14 is limited to 15 amperes.