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Electrical Load Calculator

Connected watts to load current and a recommended breaker size, with the 80% continuous-load rule.

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Electrical load

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The 80% rule protects the breaker, not just the wire

A standard (non-continuous-duty rated) breaker is only meant to carry 100% of its rating for short loads. For anything running 3+ hours straight, NEC 210.19 and 210.20 call for sizing the circuit so the continuous load never exceeds 80% of the breaker's rating.

Load current is simply watts divided by voltage — a straightforward application of the power formula. The recommended breaker size accounts for one more thing: whether the load is continuous, meaning it runs at that level for 3 hours or more, like a water heater, some HVAC equipment, or shop tool running long jobs.

For continuous loads, the National Electrical Code effectively requires the breaker to be sized so the load only uses 80% of its rating, which is the same as saying the load's actual current, multiplied by 1.25, should not exceed the breaker size. This calculator applies that 1.25 factor automatically when you flag a load as continuous, then picks the next standard breaker size (15, 20, 30, 40, 50, or 60 A) that covers it.

A dedicated appliance breaker (like the folded-in "breaker size" case) works the same way: total that appliance's rated watts, check whether it runs continuously, and apply the same math.

Where these numbers come from

These are planning estimates only. Electrical work must meet the National Electrical Code and your local code, and should be installed or inspected by a licensed electrician. This is not a substitute for a permit or a licensed electrician's judgment.

Frequently asked questions

What counts as a continuous load?

NEC defines it as any load expected to run at its maximum current for 3 hours or more. Water heaters, some HVAC compressors, and certain lighting or shop equipment commonly qualify; a toaster or a drill does not.

Why does a continuous load need 25% more breaker capacity?

Standard breakers and the wiring feeding them are only rated for 100% continuous use in specific listed applications; otherwise, sustained near-100% loading generates more heat than the equipment is designed to shed, so code backs the actual load off to 80% of the breaker's rating instead.

Does this replace a proper panel load calculation?

No. This sizes one circuit for one load. A whole-panel service load calculation (required before adding major new circuits to an existing panel) is a separate, more involved NEC process that should be done by a licensed electrician.

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