Breaker Size Calculator — 125% Rule & Wire Match
Who this is for: For electricians and serious DIYers adding circuits — water heaters, ranges, shop tools — who want the breaker-and-wire pair to agree before the permit inspection.
Enter watts, voltage and whether the load is continuous — get the breaker size, the 125% design current and the minimum copper at 75°C.
Quick answer: Breaker sizing: amps = watts ÷ volts; if continuous (3+ hours) multiply by 125% per NEC 210.20(A), then round up to the next standard size. A 2,400 W, 120 V water heater draws 20 A → sized at 25 A continuous → 25 A breaker → 10 AWG copper at 75°C. Enter your load below to get the breaker-wire pair.
| Current | I = W ÷ V (PF = 1 resistive) |
|---|---|
| Continuous rule | ×1.25 per NEC 210.20(A) (3 h+) |
| Standard sizes | 15 · 20 · 25 · 30 · 35 · 40 · 50 · 60 · 100 … |
| Wire pairing | Table 310.16 @ 75°C with 240.4(D) caps |
| Example | 2400 W @ 120 V continuous → 25 A → 10 AWG |
| Status | Figures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code. |
How does breaker sizing work?
Watts divided by volts gives load current. If the load runs three hours or more (NEC definition of continuous), 210.20(A) requires the overcurrent device rated at 125% of that current — a 2,400 W, 120 V water heater draws 20 A, so continuous sizing asks for 25 A. The result rounds up to the next standard size (15, 20, 25, 30, 35, 40, 45, 50, 60 …) and the calculator then finds the smallest 75°C copper conductor whose effective ampacity — Table 310.16 with the 240.4(D) caps — matches the breaker. For that 25 A breaker the answer is 10 AWG. Power factor defaults to 1 for resistive loads; motors are different animals (430.52 sizing, locked-rotor current) and deserve the motor branch-circuit path, not a generic calculator.
Common uses
- Adding a 240 V garage heater or compressor circuit
- Checking whether an existing 30 A circuit can take a new 5,000 W appliance
- Planning a tankless water heater replacement before the electrician visit
- Writing up a permit application's branch-circuit schedule
Motors, HVAC and nameplates
Air conditioners and heat pumps carry a nameplate with MCA (minimum circuit ampacity) and MOCP (maximum overcurrent protection) — those figures already embed the 125% and the compressor's unusual current profile. Wire to the MCA and break to the MOCP range; don't run watts ÷ volts on them. Same story for motors with Article 430 sizing. This calculator's home turf is resistive and general-purpose loads where the nameplate gives plain watts.