Circuit Breaker Size Calculator
A properly sized circuit breaker protects your wiring from overload and fire. This calculator determines the correct breaker size based on your load type, current draw, and NEC requirements.
Enter Your Measurements
Results
How to Use This Calculator
- Enter the total load in watts.
- Select the circuit voltage.
- Indicate if the load runs continuously (3+ hours).
- Review the recommended breaker and wire size.
How Breaker Size Calculation Works
The calculator divides watts by voltage to get amps, applies the NEC 125% continuous load factor if applicable, then rounds up to the next standard breaker size per NEC 240.6. It also recommends the minimum wire gauge that matches the breaker rating. The breaker must never exceed the wire's ampacity — they are sized as a matched pair.
Formula
Load amps = Watts ÷ Volts. Required capacity = Load amps × 1.25 (if continuous). Round up to next standard breaker size (15, 20, 25, 30, 40, 50, 60, 70, 80, 100A).
Frequently Asked Questions
What size breaker for a 1,500-watt heater?
Can I put a larger breaker to stop tripping?
What size breaker for an EV charger?
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