Circuit Breaker Size Calculator
Determine the correct circuit breaker size for continuous, non-continuous, or mixed loads per NEC requirements.
Results
Visualization
How It Works
Circuit breakers protect wiring from overcurrent. NEC requires continuous loads (running 3+ hours) to be sized at no more than 80% of the breaker rating, meaning the breaker must be 125% of the continuous load.
The Formula
Continuous: Breaker ≥ Load × 1.25
Non-continuous: Breaker ≥ Load
Mixed: Breaker ≥ (Continuous × 1.25) + Non-continuous
Non-continuous: Breaker ≥ Load
Mixed: Breaker ≥ (Continuous × 1.25) + Non-continuous
Variables
- Load — The actual current draw of the circuit in amperes
- 1.25 — NEC 210.20 multiplier for continuous loads (80% rule inverse)
- Breaker — Next standard breaker size above the adjusted load
Example
A 16A continuous load: 16 x 1.25 = 20A adjusted load. The next standard breaker size is 20A. Required wire: 12 AWG copper.
Tips
- Continuous loads run for 3 hours or more (lighting, HVAC, electric heaters).
- Standard breaker sizes are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A.
- Always match wire gauge to the breaker size, not just the load.
- Some 100% rated breakers can carry continuous loads at full rating but are more expensive.
- When in doubt, size up to the next breaker and wire gauge for added safety margin.