Motors and transformers
Three-phase motor current from output power
- where
- line current (A)
- mechanical shaft output (W)
- line-to-line RMS voltage (V)
- power factor
- efficiency (decimal)
Explain this equation
Estimates the running line current of a three-phase motor from its rated shaft output. Use it as a check against the nameplate current.
P_out is the mechanical shaft output in watts. V_L is the line-to-line RMS voltage. PF is the power factor and η the efficiency, both as decimals. I_L is the line current in amperes.
Dividing by η turns shaft power into electrical input power, since the motor must draw more than it delivers. Dividing by √3 × V_L × PF then converts three-phase active power into line current; it is P = √3 × V_L × I_L × PF rearranged.
A 7.5 kW motor on 400 V with PF 0.8 and 90% efficiency draws about 15 A. A common rule of thumb is roughly 2 A per kW at 400 V for medium-sized motors.
Watch out: this is full-load running current, not starting current, which can be several times higher on direct-on-line starting. Enter efficiency as 0.9, not 90.
Motor shaft ratings do not directly describe electrical current. First allow for efficiency to obtain electrical input power, then allow for power factor and the three-phase relationship.
This combined equation is a useful check against nameplate current. Retain unrounded values through the calculation and round only the final current.
Enter your values to see the result.
Common mistake
Convert efficiency percentages to decimals and shaft kW to W.
Limits
Balanced sinusoidal three-phase operation at the stated load. This is running current, not starting current. Do not use it alone to choose overloads, cables or protective devices; drive input current also differs from motor current.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.