Breaker Size Calculator

Size a breaker from continuous and non-continuous load per NEC 210.20(A), rounded up to a standard 240.6(A) rating, with the arithmetic shown.

How to use it

Split the load on the circuit into two buckets and enter each in amps. The continuous load is everything expected to run at its maximum current for three hours or more — an EV charger, commercial lighting, a sign circuit. The non-continuous load is everything else. If you are not sure which bucket something belongs in, the test is the clock, not the equipment: three hours or more at max current makes it continuous.

The calculator shows two numbers, and the gap between them is the useful part. The computed minimum is what NEC 210.20(A) says the overcurrent device must be rated for. The selected breaker is the first standard rating from Table 240.6(A) that covers it — because you cannot buy, or install, a breaker rated at your arithmetic. The code fixes the list; you round up to it.

The method

Two steps, two code sections:

minimum = (continuous × 1.25) + non-continuous
breaker = next standard 240.6(A) rating ≥ minimum

The 125% factor on continuous load comes from 210.20(A), and it exists because a standard breaker is not listed to sit at 100% of its rating for hours on end — the factor keeps a continuously loaded device at or below 80% of its rating, which is the territory it was tested for.

The standard ratings run 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 and on up to 6000 A. The list has gaps that surprise people — there is no 55, no 65, no 130 — and it ends at 6000 A: a computed minimum above that has no standard rating at all, which is why this calculator refuses rather than answers there. 240.6(A) also names a few small fuse-only sizes (1, 3, 6, 10) that have no breaker equivalent; this tool sizes breakers.

This rule is not for motors. A motor branch circuit sizes its short-circuit device from the 430.52 multipliers on table full-load current, and the result can legitimately land far above the conductor ampacity, because the overload relay — not the breaker — protects the motor. Run a motor through the 125% rule and you get a breaker that trips on starting inrush.

Worked example

A circuit carrying a 24 A continuous load and 8 A of non-continuous load.

(24 A × 1.25) + 8 A = 38 A minimum

There is no 38 A breaker. The next standard rating up is 40 A, so that is the device — with 2 A between the minimum and the rating, which is normal and fine. The minimum is a floor, not a target.

Now a case that lands exactly: a 48 A hardwired EV charger, continuous by definition. 48 × 1.25 = 60 A, and 60 is a standard rating, so a 60 A breaker with zero headroom — which is exactly why 60 A is the breaker every 48 A charger’s instructions call for.

One more with awkward numbers: a 4500 W water heater at 240 V draws 18.75 A, treated as continuous. 18.75 × 1.25 = 23.44 A, and the next standard size is 25 A. Nobody stocks a 23 A breaker; the list decides.

Where it goes wrong in the field

The classic failure is sizing the breaker and calling the circuit done. The breaker’s other job is protecting the conductor under 240.4, and that check runs the opposite direction: the wire’s ampacity — after any derating — has to support the breaker you picked. A 40 A breaker on the worked example above needs a conductor good for the breaker, and the 240.4(D) caps mean 10 AWG stops at 30 A however healthy its column ampacity looks. This page does the load side only; size the wire separately and let the stricter answer win.

The second failure is miscounting the continuous bucket. A load that runs 2 hours 45 minutes is non-continuous; move it to three hours and the same amps need a bigger breaker. Storefront lighting, heat tape and parking-lot circuits get misfiled as non-continuous constantly, and the undersized breaker then nuisance-trips in exactly the season the circuit works hardest.

The third is going the wrong way at a gap in the list. A 42 A minimum rounds up to 45, never down to 40 — down is a nuisance-trip machine. And the mirror mistake: treating the rounded-up breaker as permission to add load. The headroom between 38 A and 40 A belongs to the arithmetic, not to the next appliance someone plugs in.

Code references

• NEC 240.6(A)Standard ratings for fuses and inverse-time circuit breakers2023 edition
• NEC 210.20(A)Continuous loads taken at 125%, non-continuous at 100%2023 edition

Section numbers and table values move between editions, and jurisdictions amend them. Confirm against the edition your authority having jurisdiction enforces.

Common questions

What size breaker do I need for a 40 amp load?

It depends on whether the load is continuous. Forty amps of non-continuous load needs a device rated at least 40 A, and 40 is a standard 240.6(A) rating, so a 40 A breaker works. Forty amps of continuous load — running three hours or more — is taken at 125%, which is 50 A, so you need a 50 A breaker. Same load, different answer, and the three-hour question is what decides it.

What is the difference between a continuous and a non-continuous load?

A continuous load is one where the maximum current is expected to run for three hours or more — EV chargers, most lighting in commercial spaces, water heaters under recovery. Everything else is non-continuous. Continuous load is counted at 125% when sizing the overcurrent device; non-continuous at 100%. The clock, not the equipment type, is the test.

Why do continuous loads get multiplied by 125 percent?

Heat. A standard breaker is not listed to carry 100% of its rating indefinitely — held at full rating for hours, it and its terminations heat up and it can trip below its rating or cook its enclosure. Loading it to 80% of rating for continuous duty, which is what the 125% factor achieves from the other direction, keeps the device inside what it was tested for. Assemblies listed for 100% continuous operation exist, but they are the exception you have to buy on purpose.

Can I use a 45 amp breaker?

Yes — 45 A is on the 240.6(A) standard list, sitting between 40 and 50, even though supply houses rarely stock it. What you cannot do is invent a rating that is not on the list: the code fixes the standard sizes, and the device you install must be one of them. If your computed minimum is 42 A, the answer is the next standard size up, which is 45.