Breaker Size Calculator

Find the minimum standard breaker size for a load using the NEC 125% continuous-load rule and the standard breaker list — a planning estimate to confirm with a licensed electrician.

Your load
A
Results update as you type — no submit
ResultsLIVE
Minimum breaker size
20 A
Continuous 16 A load · needs ≥ 20 A
Required device rating20 A
Next standard size20 A
Formula
required = 16 A × 1.25 = 20 A
next standard NEC 240.6(A) size = 20 A
The conductor must be rated for at least this breaker's ampacity. Pair this with the wire-size tool and confirm the circuit with a licensed electrician.
How it works

A breaker protects the wire, so its size follows the load and the code rules, not the other way around. This calculator applies NEC 210.20(A): the overcurrent device must be rated for `100%` of a noncontinuous load plus `125%` of a continuous load. A continuous load is one expected to run at its maximum for three hours or more, per NEC Article 100 — think of lighting, EV charging, or a long-running motor. It then rounds up to the next standard breaker size. Treat the result as a planning estimate and have a licensed electrician confirm it against the full circuit.

The `125%` factor exists because breakers and conductors heat up under a sustained load, so the code builds in a `25%` margin. Enter the load current and whether it is continuous, and the calculator multiplies by `1.25` when needed to get the minimum device rating. A `16 A` continuous load, for instance, needs a device rated for at least `20 A`.

Breakers only come in standard sizes, listed in NEC 240.6(A): `15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100` amps and up. The calculator picks the smallest standard size that meets or exceeds your requirement. This is where round numbers surprise people — there is no `65 A` breaker, so a `62.5 A` requirement rounds up to `70 A`. The breaker must also match the conductor's ampacity, which is why wire size and breaker size are always chosen together.

Worked example
Load `50 A` · continuous
required = `50 A` × `1.25` = `62.5 A`
standard sizes near `62.5`: `60 A` (too small), `70 A`
result = `70 A` breaker (there is no `65 A` standard size)
Questions about breaker sizing
Multiply the load by 1.25, then round up to the next standard breaker size. A 16 A continuous load needs 16 × 1.25 = 20 A, which is a standard 20 A breaker. The rule comes from NEC 210.20(A); the conductor must be sized to match, so confirm the pair with an electrician.
SOURCES — Continuous-load 125% rule NEC 210.20(A) · standard OCPD ratings NEC 240.6(A) · continuous-load definition NEC Article 100
NEC-referenced estimate for planning only — electrical work must meet local code and be done or inspected by a licensed electrician; local code governs.
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