Amps Draw Calculator

Three quantities define every electrical circuit: voltage (V), current (A), and power (W). They are linked by a simple formula:

Amps = Watts / Volts

The calculator opens at 1,800W and 120V: 15A x 120V. 15A is one of the standard fuse ratings Mersen lists, and the default describes no particular device. The same 1,800W at 240V draws 7.5A. Higher voltage means less current for the same power.

This formula assumes a power factor of 1.0, which is true for purely resistive loads: heaters, incandescent lights, toasters, and hair dryers. These devices convert all their electrical energy directly to heat or light.

Motors, fluorescent ballasts, and power supplies can have a power factor below 1.0. They draw more current than the simple formula predicts because some of that current oscillates back and forth without doing useful work. For a 1,000W motor with a 0.80 power factor at 120V, the actual current is 1,000 / (120 x 0.80) = 10.4A instead of the 8.3A you would expect from watts and volts alone.

For three-phase systems, the formula becomes: Amps = Watts / (Volts x 1.732 x Power Factor). The 1.732 factor accounts for the three-phase voltage relationship. Three-phase loads draw less current per conductor than single-phase loads of the same power, one reason commercial and industrial buildings use three-phase power.

The amps column is this calculator's result for the row's watts and volts at a power factor of 1. Where a maker prints its own amps, the maker's figure governs. For any other device, read the watts or amps on its nameplate or its maker's sheet and enter them above; for a motor load, enter its power factor too.

Devices over 1,800W at 120V need a dedicated circuit. If you plan to run any of these from a battery-based inverter, our battery size for inverter calculator shows how much capacity you need. Conair prints the watts of its dryer; the 120V its preset enters is the outlet voltage, your own input. At 120V the Conair dryer draws 15.63A and the Panasonic microwave 17.3A, so either one alone is more than a 15A circuit is rated for.

A standard US bedroom circuit is 15A at 120V. NEC says you should load it to no more than 80% for continuous use: that is 12A or 1,440W.

Say the loads you plug in add up to 380W. At 120V that is 3.17A, or 26.4% of the 12A continuous limit (21.1% of the 15A rating), with room for another load.

Now try a heavier load. A 1,500W load at 120V draws 12.5A: 104.2% of the 12A continuous limit by itself (83.3% of the 15A rating). Run for hours, it is already over the continuous limit, and any other load on the same circuit adds to that. This is why kitchens and bathrooms have 20A circuits - high-draw appliances need the extra headroom. On long wire runs, the current you calculate here also determines the voltage drop in the circuit.

If you are adding up multiple loads to size a backup power source, our generator sizing calculator factors in running watts and motor starting surges to recommend the right unit.