Breaker Size Calculator
Calculate the correct circuit breaker size for a given load. Follows NEC 210.20 and 240.6 standard breaker sizes.
How it works
Per NEC 210.20, the overcurrent device (breaker) must be rated at 100% of the non-continuous load, plus 125% of the continuous load. This calculator applies that factor and then selects the next standard breaker size per NEC 240.6(A). Standard sizes include: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 500, and 600A.
Example
A commercial LED lighting circuit draws 18A continuously. Minimum breaker = 18 × 1.25 = 22.5A. The next standard size above 22.5A is 25A. Use a 25A breaker and AWG 10 wire rated for 35A at 75°C.
Frequently asked questions
Can I use a smaller breaker to save money?
No. Undersizing a breaker creates a fire hazard. The breaker protects the wire, not the load. If the breaker is smaller than required, it will trip during normal operation. If the wire is too small for the actual load, it can overheat and cause a fire before the breaker trips.
What is a GFCI breaker?
A GFCI (Ground Fault Circuit Interrupter) breaker monitors for current imbalance between hot and neutral conductors. It trips when it detects a ground fault of 4–6 mA, protecting people from shock. Required by NEC for bathrooms, kitchens, outdoors, garages, and other wet or damp locations.
What is an AFCI breaker?
An AFCI (Arc Fault Circuit Interrupter) breaker detects arcing faults that can cause electrical fires. Required by NEC for most living areas in new construction. Combination AFCI+GFCI breakers are available for locations that require both protections.
Can I use a 20A breaker on a 15A circuit?
No. The breaker must match the wire size. AWG 14 wire is rated 15A — installing a 20A breaker on AWG 14 wire means the wire can overheat before the breaker trips, creating a fire risk. Always match the breaker to the wire ampacity.
← Back to all calculators