Electrician Prep

Breaker Size Calculator

NEC 240.6(A)

Determine the correct breaker size for your circuit. Accounts for continuous and non-continuous loads per NEC 240.6(A).

About this calculator

Picking the right breaker means matching the load type to the NEC sizing rules and then snapping to a standard breaker size from NEC 240.6(A). For continuous loads (any load expected to run for three hours or more), the breaker has to be sized at 125% of the load. Non-continuous loads use 100%. Mixed loads add the two together. The result is then rounded up to the next standard breaker size.

Formula

For a circuit with both continuous and non-continuous loads, multiply the continuous portion by 1.25, add the non-continuous portion, and round up to the next standard breaker.

Breaker size ≥ (1.25 × continuous load) + (1.00 × non-continuous load), rounded up to a standard size from 240.6(A).

Reference: NEC 210.20(A), 215.3, 240.6(A), 422.10

How to use

  1. Enter the continuous load current in amps.
  2. Enter any non-continuous load current.
  3. The calculator multiplies the continuous part by 1.25 and adds the non-continuous part.
  4. It then rounds up to the next standard breaker size from NEC 240.6(A).
  5. Verify the conductor ampacity supports the breaker, since 240.4(B) lets you go up to the next standard size only when the conductor ampacity does not match a standard.

Worked example

Setup

A commercial lighting circuit pulls 16 A continuously. No non-continuous load.

Calculation

16 A × 1.25 = 20 A. The next standard breaker size at or above 20 A is 20 A itself.

Answer

Use a 20 A breaker. Verify the branch circuit conductors are at least #12 copper THHN at 75°C (rated 25 A, derated as needed) so the breaker is properly protected.

Frequently asked questions

What counts as a continuous load?
NEC Article 100 defines a continuous load as "a load where the maximum current is expected to continue for three hours or more." Office lighting, store displays, sign circuits, and most commercial fixtures qualify. Receptacles in a dwelling do not.
What are the standard breaker sizes?
NEC 240.6(A) lists: 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, 1,000, 1,200, 1,600, 2,000, 2,500, 3,000, 4,000, 5,000, and 6,000 amps.
When can I round up vs. round down?
Always round up to the next standard size when the calculated value falls between standards. NEC 240.4(B) lets you use the next standard size up when conductor ampacity does not align with a standard breaker, but only for circuits 800 A and below and only when the conductor is not part of a multi-outlet branch supplying receptacles for cord-and-plug-connected portable loads.