Circuit Breaker Size Calculator
Calculate the correct breaker size with NEC 80% continuous load rule. Add multiple loads.
Loads
NEC 80% Continuous Load Rule
NEC Article 210.20 requires that branch circuit overcurrent devices not be loaded to more than 80% of their rating when serving continuous loads (loads that operate continuously for 3 hours or more). This means the breaker must be rated at 125% of the continuous load current.
Formula: Breaker size = continuous current ÷ 0.80 — then round up to the next standard breaker size.
Standard breaker sizes (amps): 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200.
For three-phase circuits: I = W ÷ (V × 1.732). For single-phase: I = W ÷ V.