Three-Phase Power Calculator
Calculate apparent power (kVA), real input power (kW), and reactive power (kVAr) for a balanced three-phase AC load using line-to-line voltage, line current, and power factor.
Enter electrical values
Enter power factor as a magnitude, such as 0.85—not 85. This input does not identify leading or lagging direction.
Results
Reactive power is an unsigned magnitude because this calculator does not receive leading/lagging information. Output power is shown only when efficiency is entered and is not a substitute for measured motor or driven-equipment performance.
Three-phase power formulas used
For a balanced, approximately sinusoidal three-phase system using RMS line-to-line voltage, RMS line current and power-factor magnitude:
If efficiency is entered, the calculator estimates useful output power as:
Worked example
For a balanced 400 V three-phase load drawing 50 A at 0.85 power-factor magnitude:
| Quantity | Calculation | Approximate result |
|---|---|---|
| Apparent power | 1.732 × 400 × 50 ÷ 1000 | 34.64 kVA |
| Real input power | 34.64 × 0.85 | 29.44 kW |
| Reactive power magnitude | √(34.64² − 29.44²) | 18.25 kVAr |
| Estimated output at 92% efficiency | 29.44 × 92 ÷ 100 | 27.09 kW |
How to use the result in maintenance
- Compare calculated input power with normal historical readings to investigate abnormal loading.
- Use measured voltage, current and power factor rather than nameplate assumptions when diagnosing an operating system.
- Compare phase currents separately before treating a three-phase load as balanced; significant imbalance needs separate investigation.
- For motors, remember that electrical input kW is not the same as mechanical shaft output kW because losses occur in the motor.
- For VFD-fed motors, harmonics and drive behavior can make simple hand calculations less representative than readings from a suitable true-power instrument.
Investigate a protection trip without repeated resetting
Record the exact protection or drive indication, operating state, load, phase readings and recent machine event before any authorized reset. Check the approved settings against OEM and design information; do not increase or bypass protection to keep equipment running. A trip may reflect an electrical supply, connection, motor, driven-load, cooling, parameter or process problem, so preserve the evidence before replacing parts.
Use a controlled troubleshooting sequence
- Define the symptom, operating state and affected function.
- Apply the approved electrical-safety process and verify the required safe state.
- Confirm the instrument and measurement method are suitable.
- Compare supply, command, power-path, motor and driven-load evidence logically.
- Correct only the confirmed cause under approved controls.
- Restore safeguards, perform the authorized functional test and document the result.
Common mistakes
- Using phase-to-neutral voltage when the formula expects line-to-line voltage.
- Entering power factor as 85 instead of 0.85.
- Assuming a motor's nameplate kW is its present electrical input power.
- Ignoring phase imbalance, voltage distortion or VFD harmonics.
- Using estimated efficiency as though it were a measured value.