How Breaker Size Is Calculated
Sizing a circuit breaker starts with the current the load actually draws, then adds a safety margin before rounding up to a breaker your panel can actually stock. For a resistive or general AC load, current follows Ohm's-law-style power division: I = P ÷ (V × PF) for a single-phase (or DC) circuit, or I = P ÷ (√3 × V × PF) for a balanced three-phase circuit, where P is load power in watts, V is voltage, and PF is the power factor (1.0 for purely resistive loads like heaters and incandescent lighting; lower for motors and other inductive loads).
The 125% continuous-load rule
The National Electrical Code (NEC 210.19(A) and 210.20(A)) requires that a breaker and its conductors be rated at least 125% of the load current whenever the load is continuous — defined as expected to run at maximum current for three hours or more (think water heaters, EV chargers, or lighting left on all evening). Non-continuous loads only need to be covered at 100% of their current draw. This calculator applies the 1.25 factor automatically when you select "Continuous" load duration.
Rounding to a standard breaker size
Breakers are not sold in arbitrary sizes. NEC 240.6(A) defines a fixed list of standard ampere ratings — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and up. Once the minimum required rating is calculated, you always round up to the next value on that list; rounding down would under-protect the circuit.
Matching the wire gauge
The conductor feeding the breaker must be rated to carry at least as much current as the breaker itself, per NEC Table 310.16 (60°C copper column, which also governs the small-conductor limits in 240.4(D)). This tool reports the smallest standard copper AWG size whose ampacity meets or exceeds the recommended breaker — for example, a 20A breaker pairs with 12 AWG, and a 30A breaker pairs with 10 AWG.
Assumptions and limits
This calculator assumes a single load on a dedicated circuit with a balanced three-phase supply (if selected) and copper conductors at 60°C ambient rating — the most common residential/light-commercial scenario. It does not account for conductor bundling derates, long-run voltage drop, aluminum wiring, ambient temperature above 30°C/86°F, or local code amendments. Always verify the final breaker and wire selection against your local electrical code, panel schedule, and a licensed electrician before installation.