Breaker Size Calculator

Determine the minimum overcurrent protection device (OCPD) size for a given load.

Inputs
Results
Minimum Breaker Size
20 A
Adjusted Load (after 125% factor)20.0 A
Minimum Breaker Size20 A

· Standard breaker sizes from NEC 240.6(A). Verify conductor ampacity, derating, and system voltage before specifying.

How It Works

The NEC requires that continuous loads (loads energized for 3 hours or more) be calculated at 125% of their actual ampere draw when sizing the overcurrent protection device. Non-continuous loads are calculated at 100%.

  Adjusted load = (continuous portion x 1.25) + non-continuous portion

The next standard breaker size at or above the adjusted load is the minimum acceptable breaker.

Standard OCPD sizes are defined 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, 600A and above.

Example

16 A continuous load. Adjusted load = 16 x 1.25 = 20 A. Minimum standard size = 20 A breaker.

Frequently Asked Questions

What counts as a continuous load?

Any load expected to operate without interruption for 3 hours or more. Examples include store lighting and HVAC units on timers.

Can I use a 20A breaker on a 15A circuit?

No. The breaker size must match the conductor ampacity. Oversizing a breaker is a wiring violation and a fire hazard.

What if my calculated load falls between standard breaker sizes?

Always round up to the next standard size, never down.

These numbers are for reference only. We are not liable for any decisions you make or how you use these results in the real world. Check the current NEC and your local codes, and have a licensed electrician sign off before you install or buy. Full disclaimer