Circuit Breaker Size Calculator
A circuit breaker protects wiring from current overload by tripping when current exceeds its rating. Sizing a breaker correctly requires knowing both the load current and whether that load is continuous (runs for 3 or more hours at a stretch) or non-continuous. The NEC requires that continuous loads not exceed 80% of the breaker's rating, meaning the breaker must be rated at 125% of the continuous load. Enter the load in watts or amps, the supply voltage, and whether the load is continuous to get the minimum breaker rating and the recommended standard breaker size.
Circuit breaker sizing formula
Load amps = Watts ÷ (Volts × power factor)
Minimum breaker (continuous) = Load amps × 1.25 (NEC 210.20)
Minimum breaker (non-continuous) = Load amps
Standard size = next standard breaker at or above minimum
For single-phase AC loads, current equals power divided by the product of voltage and power factor. Resistive loads (heaters, incandescent lights) have a power factor of 1.0. Motors and some electronics have lower power factors. After computing the minimum ampere rating, round up to the nearest standard breaker size listed in NEC 240.6(A).
Breaker sizing for common loads
- Lighting circuits (continuous): 20A breaker serves up to 16A (1,920W at 120V) of continuous lighting.
- 15A outlet circuits: use 15A breaker with 14 AWG wire; continuous load limit is 12A.
- Electric water heater (4,500W at 240V): 4,500/240 = 18.75A; continuous so 18.75 x 1.25 = 23.4A minimum; use 30A breaker with 10 AWG wire.
- EV charger (32A continuous at 240V): 32 x 1.25 = 40A minimum; use 40A breaker with 8 AWG wire.
- Always verify with the appliance manufacturer's nameplate and local code requirements.
Circuit breaker sizing: frequently asked questions
How do I size a circuit breaker for a load?
For non-continuous loads (less than 3 hours), the breaker must be rated at or above the load current. For continuous loads (3 hours or more), the breaker must be rated at or above 125% of the load current (NEC 210.20). After calculating the minimum ampere rating, round up to the next standard breaker size.
What are the standard circuit breaker sizes?
Standard residential breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 amps. The most common residential sizes are 15A, 20A, 30A, 40A, 50A, 60A, and 100A. Always use a listed breaker that matches your panel brand and the wire gauge installed.
What is the 80% rule for circuit breakers?
The NEC continuous load rule states that a branch circuit breaker must not carry more than 80% of its rating as continuous load. This is the inverse of the 125% sizing requirement. A 20A breaker should carry no more than 16A of continuous load. The breaker protects both the wiring and itself from thermal damage.
Do HVAC units need the continuous load factor applied?
HVAC compressors and similar motor loads have specific NEC rules under Article 440. The minimum circuit ampacity for an air conditioner is 125% of the nameplate rated load current (MOCP listed on the unit's data plate). Use the unit's Maximum Overcurrent Protection (MOCP) rating from the nameplate instead of calculating from watts.
Can I use a larger breaker to avoid tripping?
No. A breaker that is too large for the wire gauge will not protect the wire from overheating. The NEC (Article 240) requires breakers to be sized to protect the conductor, not just the load. If a breaker trips frequently, the solution is to reduce the load or install a dedicated circuit, not to install a larger breaker.
Official sources
- NFPA 70 (NEC): National Electrical Code Articles 210, 240.
- U.S. DOE: Home Electrical Systems.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.