Enter your load type, system phase, power or current, voltage, and power factor into the Breaker Size Calculator to get the recommended circuit breaker amperage. The tool applies the appropriate safety factor based on your load type and returns the minimum breaker size in amps — so you can select the correct breaker without guessing.
Recommended Breaker Size
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Load Current
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Safety Factor Applied
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Nearest Standard Breaker Size
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A correctly sized breaker protects your wiring and equipment from damage caused by overloads and short circuits. If the breaker is too small, it trips unnecessarily under normal operation. If it's too large, it won't trip during a fault, allowing excess current to flow — which can damage equipment insulation or cause a fire.
The safety factor is a multiplier applied to the calculated load current to determine the minimum breaker size. It accounts for startup surges, continuous operation limits, and NEC code requirements. For example, resistive loads use S.F = 1.25, motors use S.F = 2.50 because they draw much higher inrush current on startup.
First, calculate the load current using the formula I = P / V for DC, I = P / (V × pf) for single-phase AC, or I = P / (√3 × V × pf) for three-phase AC. Then multiply by the appropriate safety factor for your load type. Round up to the nearest standard breaker size (15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200 A, etc.).
A 15 A breaker on a 120 V circuit can handle up to 1,800 W (15 × 120). However, the NEC recommends loading a breaker to no more than 80% of its rating for continuous loads, so the safe continuous limit is 1,440 W. On a 240 V circuit, a 15 A breaker could handle up to 3,600 W.
Yes. You can enter any supply voltage — 120 V, 240 V, 208 V, 480 V, or any other value. For three-phase systems, enter the line-to-line voltage and the calculator uses the √3 factor automatically.
Generally, no — not without verifying that the wiring can handle the higher current. Breakers are sized to protect the wire, not just the device. Upgrading a breaker without upgrading the wire can create a fire hazard. Always consult a licensed electrician before replacing a breaker with a larger one.
A correctly sized breaker can still trip if there are too many devices on the same circuit, if a motor or appliance draws excessive inrush current, if there is a ground fault or short circuit, or if the breaker itself is worn or faulty. Adding a dedicated circuit or using a breaker with a higher safety rating for motor loads can help.
Standard residential and commercial breaker sizes in North America are: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, and 400 amps. This calculator automatically rounds up to the nearest standard size for your convenience.