Breaker Size Calculator — NEC 240.6 Standard Ratings
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Turn a calculated load into a breaker rating the way the code book does: add 125% of the continuous load, round up to a standard rating, then check that the conductor can take it. OhmBase shows the minimum rating, the standard size, and whether the wire allows it.
Open the free calculator →How the math works
The device rating must be at least the noncontinuous load plus 125% of the continuous load (NEC 215.3 for feeders, 210.20(A) for branch circuits), then rounded up to a standard rating from 240.6(A). The conductor ampacity (after derating) normally sets the ceiling; when that ampacity is not itself a standard rating and the device is 800 A or less, 240.4(B) lets you use the next standard size up — except on multi-outlet receptacle branch circuits. 14, 12 and 10 AWG copper are capped at 15, 20 and 30 A by 240.4(D) regardless.
24 A continuous lighting load · 10 A noncontinuous · conductor ampacity 50 A
- Minimum rating = 10 + 1.25 × 24 = 40 A
- Standard size = 40 A (240.6(A))
- Conductor allows a 50 A device, so a 40 A breaker is fine.
FAQ
- What counts as a continuous load?
- A load expected to run at its maximum current for three hours or more — most lighting, EV charging, and many HVAC loads. It gets the 125% factor; everything else counts at 100%.
- My conductor is good for 55 A. Can I use a 60 A breaker?
- Usually yes under 240.4(B): 55 A is not a standard rating, the next size up is 60 A, and that is under 800 A — provided the circuit is not a multi-outlet receptacle branch circuit. The calculator says so when the rule applies.
- Does this size motor breakers?
- No — motor branch circuits follow Article 430 (up to 250% of FLA for an inverse-time breaker). Use the Motor wiring calculator for those.