How this calculation works
A continuous load is one expected to run at its maximum for three hours or more. Office lighting, most commercial HVAC, EV charging, sign circuits. Not a receptacle circuit, not a residential kitchen.
Two rules say the same thing from opposite directions. 210.20(A) sizes the overcurrent device at 125% of the continuous load. 210.19(A) sizes the conductor to the same figure. They have to be applied together — a 25 amp breaker on a conductor sized for 20 does not protect anything.
Standard sizes, and the round-up rule
The standard sizes in 240.6(A) start 15, 20, 25, 30, 35, 40, 45, 50, then jump in wider steps. 240.4(B) generally lets you round up to the next standard size when the conductor ampacity does not land on one, as long as the circuit does not supply receptacle outlets and the device is 800 amps or less. Above 800 amps you round down.
Where this calculator stops
Motor circuits do not work this way. A motor branch circuit sizes its conductors at 125% of the table full-load current, but its short-circuit protection can go as high as 250% of FLC for an inverse-time breaker, because starting inrush would trip anything sized to the running load. Motor overload protection is a third, separate device. Use the motor calculator for those.
Same for air conditioning and refrigeration equipment, which carries its own minimum circuit ampacity and maximum overcurrent protection on the nameplate. Those nameplate numbers are the answer — do not recalculate them.
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Frequently asked questions
What counts as a continuous load?
Any load expected to operate at its maximum current for three hours or more. Store and office lighting, most commercial HVAC, EV supply equipment and sign circuits are continuous. General-purpose receptacle circuits and residential kitchen circuits are not, because nothing runs flat out for three hours.
Why 125% specifically?
It is the inverse of 80%. The rule limits a continuous load to 80% of the device rating, and 1 divided by 0.8 is 1.25. Overcurrent devices are calibrated and tested in open air, but installed in a panel surrounded by other loaded breakers they run hotter, and the 80% margin covers that.
Can I always round up to the next standard breaker?
Usually, under 240.4(B), but not always. The round-up is permitted when the device is 800 amps or less, the conductor is not part of a multi-outlet branch circuit supplying receptacles, and the ampacity does not correspond to a standard size. Above 800 amps you go to the next size down.
Does the conductor get the 125% too?
Yes. 210.19(A) for branch circuits and 215.2(A) for feeders both require the conductor to be sized at 125% of the continuous load. Sizing the breaker up without sizing the conductor up is a violation and a fire risk.
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