Utility
Three Phase Power Calculator
Convert between kW, kVA, amps and horsepower on three-phase, single-phase or DC. Motor horsepower is asked for by standard — metric or mechanical — and by efficiency, because a nameplate states shaft output rather than what the supply delivers.
The supply
- Line voltage — the figure between two phases, which is what a meter reads and a nameplate states. 415 V and 400 V are the usual three-phase values; 230 V and 240 V the single-phase ones.
- cos φ. Around 0.85 for a loaded induction motor, lower when lightly loaded, close to 1 for heaters and incandescent lighting.
What you know
The motor
Two different units with the same name. Metric horsepower is 75 kgf·m/s and is what nameplates state across India, the Gulf and Europe; mechanical horsepower is 550 ft·lbf/s and is the American one. They are 1.4% apart — small enough to look like rounding, large enough to be the wrong answer.
Horsepower on a nameplate is shaft output, not what the motor draws. A 10 HP machine takes its 7.36 kW divided by its efficiency, so leaving this out understates the current by more than a tenth. Ordinary induction motors run 85–92%; check the nameplate or the IE class.
The load
Line current
13.68 A
- Real power
- 8.358 kW
- Apparent power
- 9.833 kVA
- Reactive power
- 5.18 kVAr
- Shaft power
- 10 HP
Star and delta
Line and phase quantities are not the same thing, and which is which depends on how the winding is connected. This is where 415 V between lines and 240 V to neutral come from — one supply, two numbers.
- Line voltage
- 415 V
- Phase voltage
- 239.6 V
- Line current
- 13.68 A
- Phase current
- 13.68 A
In star the line voltage is √3 times the phase voltage and the currents are equal; in delta the voltages are equal and the line current is √3 times the phase current. Starting a motor in star drops both the phase voltage and the phase current by √3, so the line current falls to a 3th of the delta value — not to √3 of it. That factor is the whole reason a star-delta starter exists.
How to use it
- Choose the supply type and enter the line voltage, plus the power factor for AC.
- Pick what you already know — horsepower, kW, kVA or amps — and enter it.
- For horsepower, set the standard the nameplate uses and the motor efficiency.
- Read the line current, and the real, apparent and reactive power alongside it.
- Use the star and delta panel to move between line and phase quantities.
How it works
Three-phase power is √3 × line voltage × line current × power factor. Single-phase and DC drop the √3.
Apparent power is real power over power factor; reactive power is what closes the triangle, √(kVA² − kW²).
Horsepower converts at 735.49875 W for metric and 745.69987 W for mechanical, and is divided by efficiency to get the electrical power the supply must deliver.
In star the line voltage is √3 times the phase voltage; in delta the line current is √3 times the phase current.
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Common questions
Why does horsepower need a standard?
Because there are two units with that name. Metric horsepower — PS — is 735.5 W and is what motor nameplates say across India, the Gulf and Europe. Mechanical horsepower is 745.7 W and is the American figure. The gap is 1.4%, which looks like rounding on one motor and stops looking like rounding when it is a bank of them.
Why does the tool ask for efficiency?
Because horsepower on a nameplate is shaft output — mechanical power delivered to the load — not electrical power drawn from the supply. The difference is the motor's losses. A 10 HP motor at 88% efficiency draws about 8.36 kW rather than 7.36, and the current with it: 13.7 A at 415 V rather than 12. Sizing a cable or a contactor on the lower number is the mistake this avoids.
kW or kVA — which do I size for?
Supplies, transformers, generators and cables are sized in kVA, because they carry the current whether or not it does useful work. Energy bills are usually in kW, sometimes with a penalty for poor power factor. The two are equal only at unity power factor, and the reactive part is the difference.
Do I enter line voltage or phase voltage?
Line — between two phases. That is what a meter reads, what a nameplate states and what the √3 in the formula expects. If you have a phase voltage, the star and delta panel converts it.
Why does a star-delta starter cut current to a third?
Because star drops the voltage across each winding by √3, and the current with it by another √3. Both together make three. The common belief that it falls by √3 misses one of them, and it is the reason the starter is worth the extra contactor.