Breaker size, for the load you have.

Short answer

Loads that run three hours or more count at 125% (the same as keeping them under 80% of the breaker). A 16 A continuous load needs a 20 A breaker; a 9,600 W continuous load on 240 V is 40 A, so 50 A. Round up to a standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and so on.

Worked example

An EV charger drawing 32 A for hours is a continuous load: 32 x 1.25 = 40 A, so a 40 A breaker. A 7.2 kW load on 240 V is 30 A; as a continuous load that is 37.5 A, so a 40 A breaker. A workshop tool that runs briefly at 15 A on its own needs only a 15 A breaker.

Formula, assumptions and sources

Design current = non-continuous load + 1.25 x continuous load, in amps (watts divided by volts when entered in watts). The result is rounded up to the next standard rating in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200 A. The 125% factor is the 80% rule seen from the load side: UL says a breaker not marked for 100% continuous use is for no more than 80% of its rating when the load runs three hours or more. Not checked: wire size, terminal temperature rating, the appliance's required breaker, multiple loads on one circuit, motor starting rules, GFCI or AFCI requirements.

Table values here follow the NEC as reproduced in the sources below (editions 2008 to 2020 era); fill percentages, tables and allowances can change between editions, so confirm against the edition your jurisdiction has adopted.

• UL, Circuit Breaker Guide (80% of rating for loads of three hours or more) (retrieved 2026-10)
• IAEI Magazine, Overcurrent Protection Part 2 (standard ratings in NEC 240.6(A)) (retrieved 2026-10)

Code and safety notes

This tool follows the U.S. National Electrical Code (NFPA 70). The Canadian Electrical Code (CSA C22.1) and local amendments differ in several tables and rules, so Canadian installations must be checked against the CEC and the authority having jurisdiction.

Planning aid only, not electrical advice. Electrical work can cause fire and shock; the adopted code edition, the equipment listing and a licensed electrician or inspector decide what is allowed.

FAQ

What size breaker do I need for a 3000 watt load at 240 volts?

3,000 W / 240 V = 12.5 A. If it runs three hours or more, 12.5 x 1.25 = 15.6 A, so a 20 A breaker; if it is not continuous, a 15 A breaker.

What counts as a continuous load?

A load expected to run for three hours or more, such as a space heater run all evening, an EV charger or store lighting.

Can I use a bigger breaker than the calculator says?

Only if the wire, devices and equipment label allow it. The breaker protects the wire, so a larger one on thin wire is a fire risk.

Does this work in Canada?

Standard ratings and the 80% principle are similar, but the CEC governs Canadian work and has its own rules, so confirm with the CEC and your inspector.