Overcurrent Protection of Conductors

The breaker protects the conductor. Fourteen-gauge copper gets a 15-amp breaker, twelve gets a 20, and the small-conductor rules in 240.4(D) are not negotiable no matter what the load is.

I still catch people who think a bigger breaker fixes nuisance tripping. All it does is remove the protection the wire was counting on.

When it applies

Every conductor gets protected at its ampacity. Pick your wire size for the load, then the breaker or fuse ahead of it can never let more current flow than that wire can carry safely.

Common mistakes

• Protecting 14 AWG with a 20-amp breaker. That is a fire waiting to happen.
• Forgetting the small-conductor rules that cap 14, 12, and 10 AWG regardless of the ampacity table.
• Ignoring derating from temperature and conduit fill, then sizing the breaker to a number the wire cannot actually hold.

In the field

I got called to a strip mall where breakers kept holding while a receptacle circuit cooked. Someone had upsized the breaker to stop nuisance trips instead of fixing the overloaded circuit. The wire was the fuse at that point. I put the correct breaker back and split the load. The breaker protects the wire, not the other way around. Get that backwards and you burn a building down.