By J.R. Kambak – Updated Mar 24, 2022

Three‑phase generators and motors deliver power evenly across three conductors, making accurate current calculations essential for sizing equipment, protecting circuits, and ensuring efficient operation. Below is a concise, step‑by‑step method for converting three‑phase power (in watts or kW) to the corresponding line current (amps) using voltage and power factor values found on the motor’s nameplate.

Key Power Definitions

• Active (real) power – the useful work performed, measured in watts (W).
• Reactive power – stored and released by inductive loads such as motors and transformers, measured in volt‑amperes reactive (VAR).
• Apparent power – the total power drawn from the supply, equal to the vector sum of active and reactive power, measured in volt‑amperes (VA).

The relationship between active and apparent power is:
1 kVA = 1 kW ÷ power factor

Step 1 – Use Standard Units

Convert the motor’s power rating to watts if it’s given in kilowatts: 1 kW = 1,000 W.

Step 2 – Measure the Line‑to‑Line Voltage

Use a calibrated digital multimeter to read the voltage between any two of the three phases. Record this value in volts (V).

Step 3 – Determine the Power Factor

Check the nameplate for the power factor (p.f.). A purely resistive load has a power factor of 1.0; most motors operate at 0.8–0.9.

Step 4 – Apply the Three‑Phase Current Formula

The general Ohm’s Law expression for three‑phase systems is:
P = √3 · V · I · pf
Rearranged to solve for current (I):
I = P ÷ (1.732 · V · pf)

Step 5 – Calculate the Current

Insert the values into the formula. For example, a 114 kW generator operating at 440 V with a power factor of 0.8:

I = 114,000 W ÷ (1.732 · 440 V · 0.8)
≈ 187 amps

Tools and Materials Needed

• Scientific calculator or spreadsheet
• Pen or pencil and paper
• Digital voltmeter
• Motor nameplate or datasheet (for power factor)