Breaker Size Calculator — The 125% Rule

NEC Article 100 defines it as a load where the maximum current is expected to continue for three hours or more. Commercial lighting, EV charging, electric water heating on a long recovery, and most commercial refrigeration qualify. Household receptacle circuits, ranges, and dryers generally do not — they cycle. When in doubt, treating a load as continuous is the conservative choice.

Because a breaker carrying its full rating continuously runs hot, and heat is cumulative. NEC 210.20(A) requires the overcurrent device to be rated at not less than 100% of the non-continuous load plus 125% of the continuous load. The inverse of 1.25 is 0.80 — which is where the informal "80% rule" comes from. They are the same requirement stated from opposite ends.

No. NEC 240.4(D) caps 10 AWG copper at a 30 A overcurrent device regardless of what Table 310.16 says its ampacity is, and caps 12 AWG copper at 20 A and 14 AWG copper at 15 A. For aluminum the caps are 25 A on 10 AWG and 15 A on 12 AWG. The narrow exceptions are motor and air-conditioning circuits, where Articles 430 and 440 permit the device to exceed conductor ampacity because it is protecting against short circuits rather than overload.

NEC 240.6(A) lists them: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, and 600 amps, continuing upward from there. Anything not on that list is a non-standard rating — available as a special order but not something to design around.

You round up to the next standard rating. A 51 A calculated requirement goes to a 60 A device, not 50 A. This surprises people because it feels like over-protection, but the breaker is sized to carry the load without nuisance tripping — the conductor then has to be large enough for that breaker, which is why rounding up can push the wire up a size too.

Primarily the wire. That is the core logic of NEC 240.4: overcurrent devices protect conductors from carrying more current than their ampacity allows. Equipment protection is a separate matter handled by the appliance's own internal protection, or by the nameplate's maximum overcurrent protection rating for HVAC equipment. This is why the breaker can never simply be sized to "whatever the appliance wants."

Several specific cases. 240.4(G) lists them, and motor circuits (Article 430) are the big one — the branch-circuit device there provides short-circuit and ground-fault protection while a separate overload relay handles sustained overcurrent, so a 60 A breaker on conductors rated 40 A is normal. Air-conditioning equipment (Article 440) works the same way, which is why nameplates give a minimum circuit ampacity and a separate maximum overcurrent protection value. For HVAC, the HVAC Amp Draw Calculator works out MCA and MOCP from BTU capacity.