What size circuit breaker do I need?

Calculate total amps (watts ÷ volts), then apply the NEC 80% rule: divide by 0.8 and select the next standard breaker size up (15, 20, 30, 40, 50, 60, 100A).

What is the NEC 80% breaker rule?

NEC 210.20 requires that continuous loads (running more than 3 hours) not exceed 80% of breaker rating. So a 20-amp breaker should not carry more than 16 amps continuously.

What is a standard breaker size?

Standard residential breaker sizes are: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 100A, 125A, 150A, and 200A.

Can I put 15-amp outlets on a 20-amp breaker?

Yes, 15-amp outlets (NEMA 5-15R) are permitted on 20-amp branch circuits per NEC 210.21(B). The outlets are rated for the connected devices, not the circuit capacity.

What size breaker for a 240V dryer?

Electric dryers typically require a 30-amp double-pole breaker with 10 AWG wire. Some larger dryers may require a 40-amp breaker — check the nameplate.

Circuit Breaker Calculator — Correct Amp Size

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Quick Answer

To size a circuit breaker, add up the total wattage of all devices, divide by voltage to get amps, then divide by 0.8 (NEC 80% rule) and choose the next standard breaker size up. For example, 1600W at 120V = 13.3A, requiring at least a 20A breaker.

Use the Circuit Breaker Calculator below to get instant results.

Circuit Breaker Calculator

Add up to 10 devices. The calculator applies the NEC 80% continuous load rule to recommend the correct breaker size.

Circuit Voltage:

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Cite This Tool

Circuit Breaker Calculator. (2026). Electrical Calculators Online. Retrieved from https://electricalcalcpro.com/circuit-breaker-calculator/

Circuit Breaker Calculator

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