Watts, Amps and Volts Calculator

Pick the supply, then enter the two numbers you have. You get the rest, plus the apparent power, and, if you add the breaker rating, how much of the circuit the load uses.

Supply

What you already know

Power

Real power in watts, off the nameplate. For kilowatts, multiply by 1,000.

Voltage

Single-phase: across the two conductors. Three-phase: line to line.

Power factor

Exactly 1 for a heater, a kettle or an incandescent lamp. Motors, compressors and LED drivers run lower, often 0.8 to 0.95: read it off the nameplate. Left at 1 on one of those, the amps shown are too low.

Result

Fill in a few more boxes and the answer will show here.

Opens your print dialog. Choose Save as PDF. Your figures stay on this device.

Loading the calculator.

This is a calculator, not an electrical design. Use the nameplate ratings where they exist, and size conductors and breakers under the code and the inspection authority where you work.

Watts, amps and volts are one equation, until the load is a motor

On a DC circuit the relationship is as simple as it gets: watts are volts times amps, and Ohm’s law gives the third from any two. A 1,500 W heater on 120 V draws 12.5 A, and its element measures 9.6 ohms. On a single-phase AC circuit the same holds for anything resistive, which is why heaters, kettles and incandescent lamps are easy to size.

Motors, compressors and LED drivers are not purely resistive. Part of the current they draw does no work, and the power factor is the share that does. At a power factor of 0.8 the same 1,500 W draws 25% more current than the heater, and it is the current that heats the conductor and trips the breaker. So the nameplate amps beat a calculation from watts every time; use this when the nameplate gives watts only.

Three-phase adds one more factor, the square root of 3, because the voltage you measure is between two lines. A 10 kW load at 400 V and a power factor of 1 draws about 14.4 A per line, not 25.

The usual mistake is putting the calculated amps straight against the breaker. A load that runs for hours is held below the full rating in Canada and the US, and everything else on the circuit counts too. Write the nameplate down while you are standing at it. In Zeus the photo and the figures stay on that address, so the next call starts from them.

Built for Electrical work: see the Electrical job app.

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