Should I use line or phase voltage?

Line-to-line voltage with line current, which is what the 1.732 constant assumes and what you measure at the panel.

Why does my drive-fed motor not match this?

A variable frequency drive draws non-sinusoidal current. Harmonic content raises the apparent power without raising the real power, so displacement power factor and true power factor differ and this formula understates the loading.

Electrical

Three-Phase Power Calculator

Apparent, real, and reactive power from line voltage, line current, and power factor, or the current a given load will draw.

Read before using this calculator

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Calculated reference results

Apparent power: 54.04kVA
Real power: 45.93kW
Reactive power: 28.47kVAR
Real power: 45,934W
Approximate mechanical output: 57.26hp

The arithmetic

Constant = 1.732 (three phase)
kVA = 1.732 x 480 V x 65.0 A / 1000 = 54.04 kVA
kW = 54.04 kVA x 0.85 power factor = 45.93 kW
kVAR = sqrt(54.04 squared - 45.93 squared) = 28.47 kVAR

What this does not account for

Assumes a balanced three-phase load. An unbalanced load needs to be evaluated per phase.
Power factor varies with load. A motor at 30% load has a much worse power factor than the nameplate figure.
The mechanical output figure is an estimate from electrical input and an assumed efficiency. It is not a substitute for the motor nameplate.

How this is calculated

For a balanced three-phase load, apparent power in kVA equals 1.732 multiplied by line voltage, multiplied by line current, divided by 1000. Real power in kW is that figure multiplied by the power factor. Reactive power in kVAR is the square root of kVA squared minus kW squared.

Formulas

kVA = (1.732 x V_line x I_line) / 1000

kW = kVA x power factor

kVAR = sqrt(kVA squared - kW squared)

Assumptions and limitations built into this calculator

Balanced three-phase load with sinusoidal waveforms.
Line-to-line voltage and line current.
Ignores harmonic distortion, which inflates apparent power on drive-heavy systems.

Frequently asked questions

Should I use line or phase voltage?: Line-to-line voltage with line current, which is what the 1.732 constant assumes and what you measure at the panel.
Why does my drive-fed motor not match this?: A variable frequency drive draws non-sinusoidal current. Harmonic content raises the apparent power without raising the real power, so displacement power factor and true power factor differ and this formula understates the loading.

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Content on CurryControls.com is general technical reference information provided for educational and reference purposes only. It is not engineering advice for a specific installation. All information must be independently verified against current codes, standards, manufacturer documentation, project requirements, and qualified professional judgment before it is relied upon or applied.