Field calculator
Single-phase amps calculator (watts to amps)
Converting power to current is one of the most common electrical field calculations, and for a single-phase load it is straightforward: amps equal watts divided by the voltage times the power factor. Enter the power in watts, the line voltage (commonly 120, 208, or 240 for single-phase), and the power factor. Power factor is 1.0 for purely resistive loads like electric heat and incandescent lighting, and lower (often 0.8 to 0.95) for motors and electronic loads that draw reactive current. This is the single-phase relationship; a three-phase load divides further by the square root of three. Use the actual running watts rather than a nameplate maximum, and the real voltage at the load. The result is the load current only: size the conductor and the overcurrent device per the NEC, which adds the 125 percent rule for continuous loads and any derating for temperature and conductor bundling. For a quick reverse check, watts equal amps times volts times power factor.
Result
Single-phase current: amps = watts / (volts × power factor). Enter the power in watts, the line voltage (commonly 120, 208, or 240), and the power factor. Power factor is 1.0 for resistive loads like heaters and incandescent lighting and lower for motors and electronic loads. This is the single-phase relationship; a three-phase load divides further by the square root of 3. Use the actual running watts (not a nameplate maximum) and the real voltage, and remember this is the load current only: size the conductor and breaker per the NEC, including the 125 percent rule for continuous loads and any temperature or bundling derating.
Worked example
A 3,000 W resistive heater runs on 240 V single-phase. What current does it draw?
- Power3,000 W
- Voltage240 V
- Power factor1.0
- Amps = watts ÷ (volts × PF) = 3,000 ÷ (240 × 1.0) = 12.5 A.
12.5 A. For a continuous load, size the circuit at 125% (about 15.6 A), so a 20 A circuit fits.
anvilfield.com/calculators/single-phase-watts-to-amps-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Watts to amps FAQ
What is an electrical load calculation?
An electrical load calculation totals a building's connected load, applies the NEC Article 220 demand factors, and returns the minimum service or feeder size in amps. It accounts for the fact that no building runs every load at once, so the diversified demand sizes the gear, not the sum of all nameplates.
What is a demand factor in a load calculation?
A demand factor is the fraction of a connected load you may count toward the service, because that load will not run at full output with everything else. The Article 220 tables set them by category from metered data. The dwelling general-lighting demand, for example, takes the first 3000 VA at 100 percent and the rest at 35 percent.
What is the optional method for a dwelling load calculation?
The optional method, NEC 220.82, lumps general lighting, small-appliance, laundry, and appliance nameplates together, then takes 100 percent of the first 10,000 VA and 40 percent of the remainder. It is allowed for a single 120/240 V service rated 100 A or more and usually yields a smaller service than the standard method.
How do you add an EV charger to an existing service?
Use NEC 220.87 to find the existing load from the metered annual peak, or a 30-day recording, taken at 125 percent. Subtract that from the service rating for the spare capacity. The EV charger, a continuous load under Article 625, enters at 125 percent of its rating and has to fit inside the headroom.
Do you count both heating and air conditioning in a load calculation?
No. You count only the larger of the two, because heating and air conditioning are noncoincident under NEC 220.60 and cannot run at the same time. Compare the heat load against the AC load and carry the larger into the total. Counting both is a common error that oversizes the service.