Circuit Breaker Sizing Calculator
Calculate the minimum required circuit breaker size for your electrical circuit based on load current, wire ampacity, and NEC safety factors.
Total continuous or non-continuous load current
Continuous loads require 125% derating per NEC 210.20
Typical: 0.85–0.95 for motors, 1.0 for resistive loads
From NEC Table 310.16 — leave blank to skip wire check
Formulas Used
NEC Design Current (NEC 210.20(A)):
- Non-Continuous: Idesign = Iload × 1.00
- Continuous (≥3 hrs): Idesign = Iload × 1.25
- Mixed: Idesign = (Icontinuous × 1.25) + (Inon-continuous × 1.00)
Apparent Power:
- Single Phase: S = V × I (VA)
- Three Phase: S = √3 × V × I (VA)
Real Power: P = S × PF (W)
Breaker Selection: Next standard size ≥ Idesign per NEC 240.6(A)
Assumptions & References
- NEC 210.20(A): Branch circuit overcurrent protection must be rated at 125% of continuous loads plus 100% of non-continuous loads.
- NEC 240.6(A): Standard ampere ratings for fuses and inverse time circuit breakers: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and above.
- NEC 240.4: Conductors must be protected against overcurrent in accordance with their ampacity as specified in NEC 310.15.
- NEC 310.16: Ampacity of conductors rated 0–2000V based on conductor size, insulation type, and ambient temperature (60°C/75°C/90°C columns).
- A continuous load is defined as a load where the maximum current is expected to continue for 3 hours or more (NEC Article 100).
- Power factor of 0.85 is a typical assumption for mixed commercial/industrial loads; use 1.0 for purely resistive loads (heaters, incandescent lighting).
- This calculator does not account for ambient temperature correction factors, conduit fill derating, or harmonic distortion — consult NEC 310.15(B) for full derating.
- AFCI protection is required for most 15A and 20A, 120V branch circuits in dwelling units per NEC 210.12.
- GFCI protection is required in bathrooms, kitchens, garages, outdoors, and other locations per NEC 210.8.
- Always consult a licensed electrician and your local Authority Having Jurisdiction (AHJ) before installation.