Sum device wattages into total load, see the amp draw at your voltage, and get a standard breaker recommendation.
One device wattage per line (or comma-separated).
Supply Voltage — Value for supply voltage. Provide accurate numbers for a reliable Total Load. Example: 120.
Total Load
25.00 A
Total Load — Calculated outcome A. Derived from your inputs via the displayed formula; use to plan.
Total Wattage
3,000 W
Total Wattage — Calculated outcome W. Derived from your inputs via the displayed formula; use to plan.
Derated Load (125%)
31.25 A
Derated Load (125%) — Calculated outcome A. Derived from your inputs via the displayed formula; use to plan.
Recommended Breaker
25 A
Recommended Breaker — Calculated outcome. Derived from your inputs via the displayed formula; use to plan.
Breaker for Continuous Load
35 A
Breaker for Continuous Load — Calculated outcome. Derived from your inputs via the displayed formula; use to plan.
What this means
Wattage is additive: a 1,200 W space heater and an 800 W toaster on one 120 V circuit draw 2,000 W ÷ 120 V ≈ 16.7 A. Standard breaker sizes are 15, 20, 25, 30, and up, and the tool selects the smallest size that clears the load. For loads expected to run 3+ hours continuously, the NEC sizes the circuit at 125% of th...
Every device draws current proportional to its wattage divided by the supply voltage. Sum your devices to find the circuit load in amps, then pick a breaker that comfortably clears it — using the 125% factor the NEC requires for continuous loads.
Formula
load (A) = Σ wattage ÷ voltage; breaker ≥ max(load, 1.25 × load)
Worked examples
FAQ
What does derating to 125% mean?
The NEC sizes circuits that carry continuous (3+ hour) loads at 125% of the nameplate to avoid nuisance trips and overheating. The derated figures reflect that rule.
Can I just use a bigger breaker?
No — the breaker protects the wiring. The ampacity of the wire and the rating of every device on the circuit must also be sized correctly; a breaker alone does not make a circuit safe.