PROTECTION · CALCULATOR

Breaker size calculator

Free breaker size calculator: get minimum rating (125% continuous + noncontinuous) from continuous load current, noncontinuous load current and continuous load.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Load entered asContinuous load current

Noncontinuous load current

Conductor ampacity after correction and adjustment (0 to skip)

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Imin = 1.25 × Icontinuous + Inoncontinuous; I = W / (k × V × PF); next standard rating per 240.6(A)

US (NEC) only. The minimum rating applies 125% to continuous load (210.20(A), 215.3) for a device not listed for 100% operation. The next-size-up allowance of NEC 240.4(B) applies only to overcurrent devices rated 800 A or less, and not to multioutlet branch circuits supplying receptacles for cord-and-plug-connected portable loads; above 800 A, 240.4(C) requires the conductor ampacity to be at least the device rating. Enter conductor ampacity explicitly; 240.4(D) small-conductor limits and 110.14(C) terminal ratings still apply. Engineering arithmetic, not installation approval.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Load entered as: Amps · Continuous load current: 24 A · Noncontinuous load current: 10 A · Conductor ampacity after correction and adjustment (0 to skip): 0 A

Minimum rating (125% continuous + noncontinuous): 40 A · Next standard breaker rating: 40 A

This example uses the stated inputs; results for the entered values appear above.

Explore the protection branch →See the related electrical concepts →Record breakers and loads in a panel schedule →Size the wire by ampacity and voltage drop →See the standard breaker sizes →Read the AWG wire gauge chart →All wire sizing pages, 20 A to 200 A →

Estimating and invoicing tool

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