Circuit Breaker Size Calculator
Determine the correctly sized circuit breaker for any electrical load, accounting for continuous versus non-continuous use, ambient temperature, and safety margins. Use it before wiring a new circuit or replacing a breaker.
About this calculator
Choosing the right breaker size prevents both nuisance tripping from undersizing and fire hazard from oversizing. The formula is: Breaker Size = ceil(totalLoad × (1.25 if continuous, else 1) × safetyFactor / ambientTemp). Continuous loads—those running for 3 hours or more—must be derated to 80 % of breaker capacity per NEC 210.20, which is equivalent to multiplying the load by 1.25. The safety factor adds an additional design margin. Ambient temperature affects breaker ratings: breakers are calibrated at 40 °C (104 °F), and higher temperatures reduce the breaker's ability to carry current safely—represented here by the ambientTemp divisor. The ceil() function ensures the result is always rounded up to the next whole amp, because you must never install a breaker smaller than the calculated requirement.
How to use
You're wiring a 20 A continuous load (e.g., office lighting) with a safety factor of 1.0 and an ambient temperature correction of 1.0 (standard 40 °C conditions). Step 1 — apply continuous-load multiplier: 20 × 1.25 = 25 A. Step 2 — apply safety factor: 25 × 1.0 = 25 A. Step 3 — apply ambient correction: 25 / 1.0 = 25 A. Step 4 — round up: ceil(25) = 25 A. Select a standard 25 A breaker (or the next available size, typically 30 A, depending on your jurisdiction's available breaker ratings).
Frequently asked questions
What is the difference between a continuous and non-continuous electrical load?
The National Electrical Code defines a continuous load as one expected to operate for 3 hours or more at a time—examples include lighting circuits in commercial spaces, EV chargers, and HVAC equipment. Non-continuous loads, such as toasters or power tools, run for shorter periods and generate less sustained heat in conductors and breakers. Because continuous loads cause more thermal stress, the NEC requires breakers and conductors to be sized at 125 % of the continuous load current (the 80 % rule applied from the breaker's perspective). Mixing continuous and non-continuous loads on the same circuit requires calculating the continuous portion at 125 % and the non-continuous portion at 100 %, then summing both.
How does ambient temperature affect circuit breaker sizing?
Circuit breakers are factory-calibrated to trip at their rated current when the surrounding air is at 40 °C (104 °F). In hotter environments—such as attic panels, industrial facilities, or outdoor enclosures in hot climates—the breaker's thermal trip mechanism is already partially heated by the surroundings, causing it to trip at a lower current than its rating. Manufacturers publish derating tables; for example, a 20 A breaker in a 50 °C ambient may effectively carry only about 17–18 A continuously. The ambient temperature factor in this calculator lets you account for that derating. In normal conditioned indoor environments, the factor is 1.0.
What standard circuit breaker sizes are available and how do I choose the right one?
Standard residential and light-commercial breaker sizes in North America are 15, 20, 25, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200, 225, and 400 amps. After calculating your required minimum breaker size, round up to the next available standard size—never round down, as that would undersize the protection. NEC 240.4(B) permits rounding up one standard size for conductors, provided the load is not a motor circuit. For motor circuits, special rules apply and you may size the breaker significantly above the motor's full-load current to handle inrush. Always verify the breaker size matches the wire gauge: a 20 A breaker requires at least 12 AWG copper wire.