Breaker Size Calculator — Circuit Breaker Amperage Sizing
Find the right circuit breaker size for your electrical load: enter the total wattage, voltage and whether the load runs continuously, and the calculator applies the NEC 80% rule to recommend the correct standard breaker ampere rating.
W
Voltage
Load type
Next standard size above load ÷ 0.8 (NEC continuous-load rule)
- 1
Load current
2,400 ÷ (120 × 1) = 20 - 2
Minimum breaker rating (NEC 80% rule)
20 ÷ 0.80 = 25Continuous loads must not exceed 80% of the breaker rating (NEC 210.20A). - 3
Next standard breaker size
25
How does this calculator work?
Divide total watts by voltage (and power factor) to get load current. For continuous loads divide again by 0.80 (NEC rule), then round up to the next standard breaker size (15, 20, 25, 30 … A). Non-continuous loads skip the de-rating step. Always confirm with a licensed electrician before installation.
Formula
How this is calculated
Load current in amperes equals total watts divided by voltage times power factor (I = P ÷ (V × PF)). For purely resistive loads such as heaters and incandescent lights the power factor is 1.0; for inductive loads like motors and air-conditioning compressors it is typically 0.80–0.95.
NEC Article 210.20(A) requires that a continuous load — one that runs for three hours or more — must not exceed 80% of the breaker's rated ampacity. Dividing the load current by 0.80 gives the minimum breaker rating; then round up to the next available standard size (15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100, 125, 150, 200 A and up). Non-continuous loads may use 100% of the rated current, so no de-rating is applied.
This calculator implements NEC (US) sizing logic. IEC (EU/international) standards use different breaker rating series and terminology (Type B, C, D tripping curves) but the underlying current calculation is identical. Always verify final breaker selection with a licensed electrician and check local code requirements before installation.
Frequently asked questions
Breaker terminals and conductors generate heat proportional to current squared. At 100% rated current over many hours, thermal build-up can degrade insulation and cause nuisance tripping. The NEC 80% rule provides a safety margin that keeps the circuit within its rated thermal envelope for sustained operation.
Common NEC residential sizes are 15, 20, 25, 30, 40, 50, 60, 70, 80, 90 and 100 A; larger panels use 110, 125, 150, 175, 200, 225, 250, 300, 350 and 400 A. You must use the next size up from the calculated minimum — choosing a smaller breaker risks nuisance trips or worse, a breaker that fails to trip under fault conditions.
Add the nameplate wattage (or VA rating) of every appliance and fixture on the circuit. For appliances that list only amps and voltage, multiply them: W = A × V. For motor loads use the full-load ampere (FLA) figure from the nameplate rather than the starting current, which is much higher.
TG we-Calculate Editorial Team. (2026). Breaker Size Calculator — Circuit Breaker Amperage Sizing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/breaker-size-calculator
TG we-Calculate Editorial Team. "Breaker Size Calculator — Circuit Breaker Amperage Sizing." TG we-Calculate. 2026. https://we-calculate.com/calculator/breaker-size-calculator.
TG we-Calculate Editorial Team, "Breaker Size Calculator — Circuit Breaker Amperage Sizing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/breaker-size-calculator
@misc{wecalculate_breaker_size_calculator, title = {Breaker Size Calculator — Circuit Breaker Amperage Sizing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/breaker-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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