Breaker Size Calculator

How to Size a Circuit Breaker

Circuit breaker sizing starts with calculating load current from power, voltage, and power factor, then selecting the next standard breaker rating that safely carries that current.

Current Formulas

Single-phase:

$$I = \frac{P}{V \times PF}$$

Three-phase:

$$I = \frac{P}{\sqrt{3} \times V \times PF}$$

$P$ is real power in watts, $V$ is line voltage, and $PF$ is power factor (0 to 1). Example: 2400 W at 120 V, PF = 0.9 single-phase gives $I = 2400 / (120 \times 0.9) = 22.22$ A, so use a 30 A breaker.

Standard Breaker Sizes

Common residential and commercial breaker ratings are 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, and 200 amperes. Always round calculated current up to the next standard size.

Frequently Asked Questions

Should I round up or down for breaker size?

Always round up. A breaker must handle continuous load current without tripping under normal operation.

What power factor should I use?

Resistive loads (heaters, incandescent lights) use PF ≈ 1. Motors and fluorescent loads often use 0.8–0.9. Use the actual or estimated PF for accuracy.

Is line voltage the same for single and three phase?

For three-phase, use line-to-line voltage. For single-phase, use the supply voltage across the load (e.g., 120 V or 240 V).

Does this replace NEC code requirements?

No. This calculator provides a starting estimate. Follow local electrical codes, conductor sizing, and continuous-load derating rules.

Why is √3 used for three-phase?

Three-phase power distributes across three legs with 120° phase separation. Line current relates to total power through the factor √3.

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How to Size a Circuit Breaker

Current Formulas

Standard Breaker Sizes

Frequently Asked Questions

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