Circuit Breaker Size Calculator
Enter load wattage and voltage to estimate the minimum breaker amperage under the NEC continuous-load rule; verify conductor ampacity and local requirements with a qualified electrician.
Use the Circuit Breaker Size Calculator
Circuit Parameters
Continuous = on for 3 hours or more (NEC 100). Applies the 80% derating rule.
Enter load wattage above to see the required breaker size.
Recommended Breaker
Amps
Calculation Breakdown
Standard Breaker Sizes
Summary
Enter load wattage and voltage to estimate the minimum breaker amperage under the NEC continuous-load rule; verify conductor ampacity and local requirements with a qualified electrician.
How it works
- Enter the total load wattage (or watts per circuit).
- Select the supply voltage: 120 V, 240 V, or 208 V.
- Select load type: continuous (3+ hours) or non-continuous.
- Choose single-phase or three-phase for commercial circuits.
- The calculator shows the minimum breaker amps and the nearest standard size.
Use cases
- Size a breaker for a new kitchen appliance circuit.
- Verify an existing breaker is not undersized for added loads.
- Plan sub-panel branch circuits for a home addition or workshop.
- Size breakers for commercial HVAC, lighting, or motor loads.
- Check NEC 80% compliance for continuous-duty circuits.
Frequently Asked Questions
Last updated: 2026-09-22 ·
Reviewed by Nham Vu