How do you calculate the amperage of a three phase motor?

Divide the motor's rated output power in watts by the square root of 3 times the line-to-line voltage times the power factor times the efficiency. For a 7.5 kW motor on 400 V with a power factor of 0.85 and efficiency of 90%, the full load current is 7,500 / (1.732 x 400 x 0.85 x 0.90), which is about 14.2 A per line.

Why do power factor and efficiency matter for motor current?

The kW rating on a motor nameplate is mechanical output power at the shaft. The motor draws more electrical power than it delivers because of losses (efficiency), and it also draws magnetising current that does no useful work (power factor). Dividing by both converts shaft kW into the real electrical current in the supply cables, which is what matters for cable and switchgear sizing.

What voltage is three phase power in New Zealand?

Standard low-voltage three phase supply in New Zealand is 400 V between phases (line-to-line) and 230 V from any phase to neutral, at 50 Hz. Most three phase motors on NZ sites are wired for 400 V, so that is the default in this calculator, but you can enter any line-to-line voltage.

3 Phase Motor Amperage Calculator

This 3 phase motor amperage calculator works out the full load line current a three phase motor draws from its kW rating, the line-to-line voltage, the power factor and the efficiency. The kW figure on a motor nameplate is mechanical output at the shaft, not electrical input, so the motor always draws more from the supply than it delivers: efficiency accounts for the heat and friction losses inside the motor, and power factor accounts for the magnetising current that flows without doing useful work. The calculator divides the output power by the square root of 3 times the voltage times both of those factors to give the current in each supply conductor, along with the electrical input power in kW and the apparent power in kVA. Electricians, engineers and anyone speccing a pump, compressor, fan or machine tool use this to size cables, check switchboard capacity and sanity-check a nameplate before installation. The defaults reflect a common NZ setup: a 7.5 kW motor on the standard 400 V three phase supply with a power factor of 0.85 and efficiency of 90 percent. Typical induction motors run a power factor between 0.8 and 0.9 at full load and efficiency between 85 and 95 percent, with larger motors generally better on both counts.

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Motor rated power

Line-to-line voltage

Power factor

cos φ

Efficiency

14.2 A

full load current in each line conductor

Electrical input power8.33 kW

Apparent power9.80 kVA

Phase-to-neutral voltage230.9 V

A 7.5 kW motor on 400 V with power factor 0.85 and efficiency 90% draws about 14.2 A per line at full load.

Indicative only. Always use the full load current stamped on the motor nameplate for cable sizing and protection settings, and remember starting current can be 6 to 8 times the full load figure.

How it works

Full load current I = P / (√3 × V × pf × η), where P is the rated output power in watts, V is the line-to-line voltage, pf is the power factor and η is the efficiency as a decimal. The √3 factor (about 1.732) appears because in a balanced three phase system the power is shared across three conductors whose voltages are 120 degrees apart. The calculator also reports electrical input power, which is output divided by efficiency, and apparent power in kVA, which is output divided by both efficiency and power factor. Apparent power is what transformers and generators must be sized for, while input kW is what you pay for on the power bill.

Worked example

Take the defaults: a 7.5 kW motor on a 400 V supply with power factor 0.85 and efficiency 90%. The denominator is 1.732 × 400 × 0.85 × 0.90, which is about 530.0. Dividing 7,500 W by 530.0 gives a full load current of about 14.2 A in each line. The electrical input power is 7.5 / 0.90, about 8.33 kW, and the apparent power is 7.5 / (0.90 × 0.85), about 9.80 kVA. The phase-to-neutral voltage on a 400 V system is 400 / 1.732, about 230.9 V, which is why NZ single phase supplies are 230 V.

Related calculators

Three-Phase Power Calculator: kW, kVA and kVAR from volts and amps.
Amp to Wire Size Calculator: cable size for your current.
Watts to Amps Calculator: single phase current from watts.
Amps to Watts Calculator: power from current and voltage.
Volts Amps Watts Ohms Calculator: Ohm's law solver.

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