Circuit Breaker Size Calculator
Determine the correct circuit breaker size for residential and commercial electrical circuits based on NEC 80% continuous load rule and wire ampacity ratings.
Formulas Used
Continuous Load (NEC 210.19(A)(1) & 215.2(A)(1)):
Design Current = Load Current × 1.25
Breaker Size = Next standard size ≥ Design Current
Non-Continuous Load:
Design Current = Load Current × 1.00
Breaker Size = Next standard size ≥ Design Current
Power (Single Phase):
P = V × I
Power (Three Phase):
P = √3 × V × I ≈ 1.732 × V × I
Assumptions & References
- Wire ampacity values are based on NEC Table 310.15(B)(16) for copper conductors at 75°C in conduit (most common residential/commercial installation).
- The 125% continuous load factor is required by NEC 210.19(A)(1) and 215.2(A)(1) for loads energized for 3 or more hours.
- Standard breaker sizes follow 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, 400A.
- The breaker size must not exceed the wire ampacity per NEC 240.4 to protect the conductor from overcurrent.
- This calculator assumes copper conductors. Aluminum conductors have lower ampacity ratings and require larger gauge wire.
- Ambient temperature derating and conduit fill derating factors are not applied here — consult NEC 310.15(B)(2) and (3) for those adjustments.
- Always verify calculations with a licensed electrician and local Authority Having Jurisdiction (AHJ) before installation.
- Reference: NFPA 70: National Electrical Code (NEC), 2023 Edition.