Published June 28, 2026
One of the most common questions in electrical work is how to size an overcurrent protective device (OCPD) — whether that's a circuit breaker or a fuse. The answer isn't the same for every load type. The NEC has specific rules for general loads, motors, HVAC equipment, and welders, and getting it wrong can mean nuisance tripping, equipment damage, or a failed inspection.
For general non-motor loads, NEC 210.20(A) requires that the OCPD be rated at no less than 125% of the continuous load current, plus 100% of the non-continuous load. A continuous load is one that runs for 3 hours or more. So a 16A continuous load requires a minimum 20A breaker (16 × 1.25 = 20A). This is the rule most electricians use day-to-day for lighting, receptacle circuits, and equipment.
Motors are a special case because they draw a large inrush current when starting — often 6 to 8 times their full load amperage. If you sized the OCPD at 125% of FLA like a general load, it would trip on every start. The NEC addresses this in NEC 430.52, which allows significantly larger OCPDs for motor circuits:
These are maximums, not minimums. If a smaller size holds without nuisance tripping, it can be used. The OCPD must be rounded up to the next standard size per NEC 240.6(A) if the calculated value doesn't land on a standard size.
Air conditioning and refrigeration equipment falls under NEC 440.22. These loads have a compressor motor plus other components, so the NEC requires that you follow the equipment nameplate — specifically the Maximum Overcurrent Protection (MOP) rating listed by the manufacturer. The general rule is a maximum of 175% of the rated load current, but always verify the nameplate first since manufacturer ratings may differ.
Arc welders have a highly variable duty cycle — they draw very high current while welding and no current between passes. NEC 630.12 allows the OCPD to be sized at up to 200% of the rated primary current. This accounts for the intermittent nature of welding loads without requiring the OCPD to handle continuous full-load current.
Per NEC 240.6(A), overcurrent devices come in standard sizes. When your calculated value falls between standard sizes, you round up to the next available size. Common standard sizes include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, and 500 amperes.
OCPD sizing isn't one-size-fits-all. The type of load determines which NEC article applies, and each has different multipliers and limitations. When in doubt, pull up the relevant NEC section — and always verify with your local AHJ, since some jurisdictions have amendments that affect OCPD sizing requirements.
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