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.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
The minimum breaker size starts from the load current. Continuous loads (3 hours or more) must be multiplied by 1.25, because NEC 210.20(A) requires the breaker to be rated at least 125% of the continuous load; motor loads also use 125% here (NEC 430.22 sizes motor branch conductors at 125% of full-load current). An optional extra design margin can be added, and for high ambient temperatures the requirement is divided by the NEC Table 310.15(B)(1) correction factor for 60 °C-rated conductors (0.91 at 31–35 °C, 0.82 at 36–40 °C). That keeps the conductor sized to the breaker adequate after derating: choose a conductor whose table ampacity is at least the breaker rating. The result is the next standard breaker rating from NEC 240.6(A) (15, 20, 25, 30, 35, 40, 45, 50, 60, 70 A and so on). Formula: Breaker = next standard size ≥ load × (1.25 if continuous or motor, else 1) × margin / temperature factor. Motor branch circuits may use a larger breaker (up to 250% of full-load current for inverse-time breakers, NEC 430.52) to ride through starting current; the conductor still follows 125%.
How to use
You are wiring a 20 A continuous load (for example office lighting) with no extra margin in a normal ambient. Step 1 — continuous-load multiplier: 20 × 1.25 = 25 A. Step 2 — margin: 25 × 1.0 = 25 A. Step 3 — ambient correction: 25 / 1.0 = 25 A. Step 4 — next standard size: 25 A. Use a 25 A breaker with a conductor rated at least 25 A (10 AWG copper). The default 16 A non-continuous load needs 16 A, so the next standard size, 20 A. The same 16 A continuous load in a 38 °C attic: 16 × 1.25 / 0.82 = 24.4 A, so 25 A.
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?
Breakers are calibrated at 40 °C (104 °F) under UL 489, so in normal indoor conditions they need no derating; manufacturers publish derating data for hotter enclosures. The bigger effect of a hot location is on the wire: NEC Table 310.15(B)(1) reduces conductor ampacity above 30 °C (to 0.91 at 31–35 °C and 0.82 at 36–40 °C for 60 °C-rated conductors). This calculator divides the requirement by that factor, so the breaker and the conductor you pick to match it both stay adequate after derating. In a normal conditioned space, use 1.0.
What standard circuit breaker sizes are available and how do I choose the right one?
NEC 240.6(A) lists the standard ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300 A and up. Round the calculated minimum up to the next standard size, never down. The calculator already does this. NEC 240.4(B) permits the next higher standard breaker above a conductor’s ampacity only for circuits up to 800 A where the conductor ampacity is not a standard size and the circuit does not supply multi-outlet receptacle branch circuits. Motor circuits follow Article 430 instead. Always match the breaker to the wire: a 20 A breaker requires at least 12 AWG copper.