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Breaker Size Calculator

Find the correct circuit breaker size using the NEC 125% continuous-load rule.

Introduction

Breaker Size Calculator — Find the correct circuit breaker size using the NEC 125% continuous-load rule. Enter Continuous load (A) to get an instant, accurate result.

Formula

NEC 210.20(A)/215.3: for a continuous load (running 3 hours or more), the overcurrent device must be rated at least 125% of the continuous load current, rounded up to the next standard NEC 240.6(A) breaker size.

Step-by-step

  1. Enter the Continuous load (A).
  2. Click Calculate to see your result instantly.

Real-world example

Example: With Continuous load (A) = 16, the Breaker Size Calculator gives Breaker Amps: 20, Required Min Amps: 20.

Frequently Asked Questions

Does the 125% rule apply to non-continuous loads too?
No — the 125% factor specifically applies to continuous loads. For non-continuous loads, the breaker generally only needs to be sized at 100% of the load (subject to wire ampacity and other NEC limits), so mixed continuous/non-continuous loads use a combined calculation.
Does this list every standard breaker size?
It covers the most common residential/light-commercial sizes (15A–100A). NEC 240.6(A) also lists additional standard sizes such as 35A, 45A, 110A, and 125A that aren't included in this simplified lookup.
What inputs does the Breaker Size Calculator need?
You'll need to provide: Continuous load (A). All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Breaker Size Calculator?
The Breaker Size Calculator applies the formula exactly as calculated — NEC 210.20(A)/215.3: for a continuous load (running 3 hours or more), the overcurrent device must be rated at least 125% of the continuous load current, rounded up to the next standard NEC 240.6(A) breaker size. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Breaker Size Calculator free to use?
Yes, the Breaker Size Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Breaker Size Calculator

The Breaker Size Calculator uses a real, verifiable formula — NEC 210.20(A)/215.3: for a continuous load (running 3 hours or more), the overcurrent device must be rated at least 125% of the continuous load current, rounded up to the next standard NEC 240.6(A) breaker size. — so results are accurate every time, not an approximation.