Breaker & Fuse Size Calculator
Breakers and fuses come only in standard sizes (NEC 240.6), so the job is to find the next size up from the calculated current. For continuous loads — running 3 hours or more — the circuit is sized at 125% of the load; non-continuous loads use 100%. Enter your load and this calculator returns the standard size and the reasoning.
Method: continuous → required = load × 1.25; non-continuous → required = load; then the next standard size at or above the requirement (NEC 240.6)
Worked example: a 32 A continuous load: 32 × 1.25 = 40 A required, and 40 A is itself a standard size — so a 40 A breaker. A 36 A continuous load needs 45 A, which is also standard. A 38 A continuous load needs 47.5 A, so the next standard size is 50 A. The conductor must be sized for the same current — check it with the wire size calculator.
Motor circuits are a special case: short-circuit protection may be set much higher than FLA so the breaker rides through starting current, while overload protection stays tight — see the motor FLA calculator for the FLA side.
Frequently asked questions
What counts as a continuous load?
A load expected to run at maximum current for 3 hours or more — lighting in a shop, a heater, a continuously running motor. The 125% rule gives the breaker and conductor thermal margin for that sustained heat.
Can I use the next size down if it is closer?
For general branch circuits, no — the standard rule rounds up to the next standard size (with specific exceptions, such as the NEC 240.4(B) next-higher rule for conductors under 800 A in defined cases). When in doubt, round up and check the conductor.
Does this replace the breaker curve or type?
No. This gives the ampere rating only. The breaker type (B, C, D curves), interrupting rating, and ground-fault or arc-fault requirements depend on the installation and must be selected separately.