Use the Breaker Size Calculator
Enter your load current to find the NEC-compliant breaker size and suggested wire gauge automatically.
Open Breaker Size Calculator →How to Size a Circuit Breaker — Step by Step
- Determine the load current in amps. For appliances, use the nameplate amperage or calculate from watts ÷ volts. For subpanels, calculate the total connected load or use the demand load per NEC 220.
- Determine if the load is continuous. A load running 3 or more hours at a steady state is continuous. Examples: HVAC, commercial lighting, EV chargers. Plug-in appliances like toasters or microwaves are typically non-continuous.
- Apply the 125% rule if continuous (NEC 210.20). Multiply the continuous load current by 1.25. If mixed, add 125% × continuous portion + 100% × non-continuous portion.
- Round up to the next NEC 240.6(A) standard size. Never round down — the breaker must be at least the calculated minimum.
- Verify the breaker does not exceed the conductor ampacity. Per NEC 240.4, the breaker must not exceed the ampacity of the associated conductor at the applicable temperature column.
- Check equipment MOCP. For motor loads, appliances, and HVAC, check the nameplate Maximum Overcurrent Protection (MOCP) — the breaker must not exceed this value regardless of calculation.
NEC 240.6 Standard Breaker Sizes
Per NEC 240.6(A), standard ampere ratings for overcurrent devices are:
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 · 600
When your calculated minimum falls between two standard sizes, always round up to the next standard size. You may not round down (that would result in a breaker smaller than the required minimum).
Breaker Sizing for Common Residential Applications
| Application | Breaker Size | Pole | Min. Wire (Cu) | NEC Reference |
|---|---|---|---|---|
| General 15A receptacles | 15A | 1-pole | #14 AWG | 210.19 |
| Kitchen / bathroom / garage receptacles | 20A | 1-pole | #12 AWG | 210.52 |
| Electric dryer (240V) | 30A | 2-pole | #10 AWG | 422.10 |
| HVAC / heat pump (check nameplate MOCP) | 40–60A | 2-pole | #8–#6 AWG | 440.22 |
| Electric range (240V) | 50A | 2-pole | #6 AWG | 422.10 |
| EV charger 32A EVSE (continuous) | 40A | 2-pole | #8 AWG | 625.42 |
| EV charger 48A EVSE (continuous) | 60A | 2-pole | #6 AWG | 625.42 |
| 100A subpanel feeder | 100A | 2-pole | #1/0 AWG | 215.2 |
| 200A service / main breaker | 200A | 2-pole | #4/0 AWG or 350 kcmil Al | 230.79 |
The Continuous Load Rule — NEC 210.20
NEC 210.20(A) requires that branch circuits supplying continuous loads have breakers and conductors rated at 125% of the continuous load current. A continuous load is defined as a load expected to run for 3 or more continuous hours.
The 125% rule accounts for the additional heat generated by sustained current flow. Without it, breakers and conductors operating at their rated limit for hours on end would overheat.
Single-Pole vs. Double-Pole Breakers
- Single-pole breaker: Controls one hot conductor on a 120V circuit. Standard residential 15A and 20A circuits use single-pole breakers.
- Double-pole breaker: Controls two hot conductors simultaneously on a 240V circuit. When it trips, both legs open at the same time. Required for all 240V appliances: dryers, ranges, HVAC, EV chargers, hot tubs, subpanels.
- GFCI breaker: A single- or double-pole breaker with built-in GFCI protection. Required for bathrooms, kitchens, garages, outdoor circuits, and other locations per NEC 210.8.
- AFCI breaker: Required for most bedroom, living room, and other dwelling unit circuits per NEC 210.12. Provides arc fault protection in addition to overcurrent protection.
Frequently Asked Questions
Determine load current, apply 125% if continuous, round up to next NEC 240.6 standard size, and verify it doesn't exceed the conductor's ampacity. Use the breaker size calculator for automatic NEC sizing.
Per NEC 240.6(A): 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, 600 amps.
No. NEC 240.4 prohibits a breaker from exceeding the conductor's ampacity. If more current is needed, both the breaker and the wire must be upsized together.
30A double-pole breaker, per NEC 422.10. A double-pole breaker is required because a dryer circuit is 240V and needs simultaneous disconnection of both hot conductors.
Per NEC 625.42 (continuous load, 125% rule): 32A EVSE → 40A breaker. 40A EVSE → 50A breaker. 48A EVSE → 60A breaker. See the EV charger wire size guide for full sizing table.
A load expected to run steadily for 3 or more hours. Examples: HVAC, commercial lighting, EV chargers. These require a 125% breaker rating per NEC 210.20(A).