Three-Phase Power and Motor Current Calculator

Apparent, real and reactive power from line current, or the running current of a motor from its shaft power.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

For a balanced three-phase load the apparent power is S = √3 V I, with V the line-to-line voltage and I the line current. The real power is S times the power factor, and the reactive power is S times sin of the phase angle.

For a motor, the electrical input is the shaft power divided by the efficiency. The current is then the apparent power divided by √3 V. This is the running current only. Starting current is several times higher, and cable and protection sizing are for a licensed electrician or electrical engineer.

Formulas used

  • S = \sqrt{3}\,V\,I
  • P = S\cos\varphi
  • I = \frac{P_{shaft}}{\sqrt{3}\,V\,\eta\cos\varphi}

Worked example

With the default numbers in the calculator above:

  1. Electrical power in: 12.22 kW
  2. Apparent power: 14.38 kVA
  3. Line current: 20 A

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What voltage do I use?

The line-to-line voltage. In Australia that is 415 V for normal low-voltage three-phase supply (400 V nominal).

What power factor is typical?

About 0.8 to 0.9 for a loaded induction motor. It falls when the motor is lightly loaded.

Does this size the cable?

No. Cable size depends on the installation method, voltage drop, protection and the wiring rules.

Related calculators

Further reading: Any electrical power text; motor nameplate data.

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