What happens if you put a 20 amp breaker on 14 gauge wire?

A 20 amp breaker on 14 gauge wire creates a circuit that can overheat without ever tripping. Fourteen gauge is limited to 15 amps by NEC 240.4(D); the breaker exists to protect the wire, and this pairing defeats that.

At 18 or 19 amps the breaker holds while the undersized wire runs past its rating, cooking insulation inside walls over months. That is the actual fire mechanism, not an instant failure.

Inspectors treat it as a straightforward violation, and it is a common find after amateur panel work.

A GFCI makes no difference here; ground-fault protection is not overload protection. If a circuit needs 20 amps, the fix is 12 gauge wire, not a bigger breaker.

The 20 amp wire size calculator pairs every breaker size with its minimum gauge, citing the rule.