By Michael Logan – Updated March 24 , 2022
The National Electrical Code mandates that every motor’s nameplate lists its operating voltage and full‑load current, regardless of phase or voltage type. When a three‑phase motor runs at full load and its rated speed, the power it consumes is expressed in watts or kilowatts—both standard units of electrical power. You can determine this power directly from the voltage and current values found on the nameplate.
Locate the voltage rating on the motor’s nameplate. Some motors display multiple voltages (e.g., 230/460 V). Use the first voltage listed for the calculation. For example, if the rating reads 230/460 V, select 230 V.
On the same nameplate, locate the full‑load current. When multiple voltages are listed, the nameplate will provide a corresponding set of currents. Again, use the first current value. For instance, if the rating reads 20/10 A, select 20 A.
Multiply the chosen voltage by the chosen current to obtain power in watts, then divide by 1,000 to convert to kilowatts. Example: 230 V × 20 A = 4,600 W; 4,600 W ÷ 1,000 = 4.6 kW.
Match the appropriate voltage with its corresponding full‑load current. Either voltage/current pair will yield the correct power rating. In the example above, the motor’s rating is 4.6 kW.
