Circuit Breaker & Fuse Sizing Calculator
Select the correct overcurrent protection to keep your circuits and equipment safe.
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OCPD Sizing Rule
NEC Article 240 Overcurrent Protection
NEC 240.4 generally requires conductors to be protected at their ampacity. 240.4(B) allows the "next size up" usage for devices 800A or less if the conductor ampacity doesn't match a standard breaker size.
Frequently Asked Questions
Can I put a 15A wire on a 20A breaker?
No. The breaker is there to protect the wire. A 20A breaker allows too much current for 14 AWG wire, creating a fire hazard. (Exception: Module specific motor loads).
What is the "Next Size Up" rule?
If your calculated wire ampacity is 45A, but there is no 45A breaker, NEC 240.4(B) allows you to use the next standard size up (50A), provided the load is not more than 45A.
How do I size a breaker for continuous load?
The breaker (and wire) must be rated for 125% of the continuous load. Effectively, this means you can only load a standard breaker to 80% of its rating continuously.