Tools you might need next
Select AWG wire size from ampacity and circuit load. Calculate conductor size for copper and aluminum per NEC ampacity tables with temperature rating.
Calculate total electrical load for residential service sizing. Sum appliance watts and apply demand factors for panel and service entrance capacity.
Calculate receptacle circuit load for NEC 210.21 compliance. Enter outlet count and appliance watts to verify 15A and 20A circuit capacity limits.
Breaker protects wire — cannot exceed ampacity of smallest conductor in circuit.
Max Breaker ≤ Conductor Ampacity (NEC 240.4)Load on 3+ hours limited to 80% of breaker rating. 20A breaker max 16A continuous.
Breaker ≥ Load ÷ 0.80 (continuous loads)Next standard size above calculated requirement. Motor circuits use NEC Table 430.52 for max breaker.
15, 20, 30, 40, 50, 60 A — NEC 240.6(A)Updated: July 2026
12 AWG copper → 20A breaker maximum. Do not protect 12 AWG with 30A breaker — fire hazard.
40A continuous needs 50A breaker (40÷0.8=50) and 6 AWG wire minimum per NEC 625.
Use manufacturer Minimum Circuit Ampacity and Maximum Overcurrent Protection on nameplate — overrides general calc.
Select appropriate circuit breaker size based on wire gauge, load type, and NEC rules. Enter conductor AWG, continuous vs non-continuous load, and motor/special loads to verify breaker does not exceed conductor ampacity.