By David Robinson
Updated Mar 24, 2022
Every electrical appliance transforms stored electrical energy into another form—movement, light, or heat. An electric motor converts electricity into motion, with a small portion lost as heat and light. Knowing a motor’s current draw is essential for selecting proper wire sizes, circuit breakers, and estimating operating costs.
Locate the small technical‑data plate on the motor’s casing. Most motors display power rating, voltage, and wattage. Often two wattage figures are listed: running wattage and starting (inrush) wattage. Use the higher value for safety calculations.
Current (amps) can be approximated by dividing wattage by voltage. For example, a 500‑W motor on 50 V draws 10 A, while the same motor on 20 V draws 25 A. This theoretical figure provides a quick “average” amp value.
With the motor off, disconnect one power lead. Connect a DC‑current rated multimeter (or ammeter) in series between the power source and the motor. Set the meter to the highest range, then power the motor. Observe the reading as it rises during startup and stabilizes at running current.
Use the wattage‑derived ampere figure for general sizing or budgeting. For precise load calculations—especially under varying conditions—rely on the multimeter reading.
Motor current increases with load. If voltage doubles, current roughly halves.
Always start the multimeter on the highest range (e.g., 0‑100 A) and reduce it once the reading falls within a lower band. Even low voltages can cause painful shocks. Verify the power is off before connecting or disconnecting any leads.
For visual reference, the image below is courtesy of Nordroden/iStock/GettyImages.
