Identify what the power means
This is a teaching scenario, not a record of an actual installation. Assume a balanced three-phase motor at full load: shaft output 15 kW, line-to-line voltage 400 V, power factor 0.85 and efficiency 0.90.
Motor nameplate kW normally means mechanical shaft output. Electrical input is higher: P₁ = P₂ / η. If you already measured electrical active input power, do not divide it by motor efficiency again.
Work through the 15 kW example
Input power is 15 / 0.90 = 16.67 kW. For a balanced three-phase system, I = P₁ × 1000 / (√3 × U × power factor). Substitution gives I = 16666.67 / (1.73205 × 400 × 0.85) ≈ 28.30 A.
Using 15 kW directly as electrical input gives 25.47 A. That result is valid for 15 kW electrical input; it does not describe this motor with 15 kW shaft output.
See how assumptions change the result
At the same shaft output, voltage and power factor, 85% efficiency gives about 29.97 A and 95% efficiency gives about 26.81 A. These are teaching comparisons. Use the actual nameplate data for selection.
The calculation represents full load. Efficiency and power factor can change at partial load. A drive’s supply input current and motor output current should not be treated as the same quantity.
Current does not finish equipment selection
A current of 28.30 A does not determine the breaker, contactor or cable by itself. Starting method and time, protection characteristics, prospective fault current and manufacturer coordination data must be assessed together.
Cable checks include ampacity, installation method, ambient temperature, grouping, length and voltage drop. This example does not prescribe one cable size or fuse rating for every 15 kW motor.
Check the calculated current
Compare the result with the motor’s rated current for the correct connection voltage. If the nameplate gives several voltage/current pairs, use the matching pair. Investigate power type, efficiency, power factor and load when results differ.
Measurements require competent personnel and appropriate procedures. Do not infer exact three-phase power from one phase current on an unbalanced or distorted supply.
Sources and further reading
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