Power

Three-Phase Power

Convert between kW, kVA, kVAR, current and power factor for single- and three-phase systems. Enter the voltage and one value (power or current) and get every quantity, plus the recommended kVA to size a UPS or generator with margin. All in your browser.

redzilla.cl — pwr

kW —

kVA —

kVAR —

A —

Active power

—

Apparent power

—

kVA

Reactive power

—

kVAR

Line current

—

amperes

All quantities

Quantity	Value	Detail

UPS / generator sizing

Recommended kVA

—

with 20 % margin

Design margin

80 %

recommended max. load

Rule of thumb: kVA = kW / PF / 0.8. Keep the UPS or generator running at no more than 80 % to absorb inrush peaks and future expansion.

How it is calculated · kW, kVA, kVAR and PF

1. Three-phase (V = line-to-line): kVA = √3 · V · I / 1000 and I = kVA · 1000 / (√3 · V). Single-phase: without the √3 → kVA = V · I / 1000.

2. The power factor relates apparent to active power: kW = kVA · PF, where PF = cosφ is between 0 and 1.

3. The power triangle: kVA² = kW² + kVAR², so kVAR = √(kVA² − kW²) (reactive power).

4. Sizing: start from active power and leave margin → kVA = kW / PF / 0.8. The 0.8 reserves 20 % for inrush and growth.

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How it works

The calculator converts between all the electrical quantities of a single-phase or three-phase system from the voltage and one known value: power (kW or kVA) or line current, plus the power factor. For three-phase, with line-to-line V, it uses kVA = √3 · V · I / 1000 and I = kVA · 1000 / (√3 · V); for single-phase the same formulas without the √3. Powers are related by kW = kVA · PF and the power triangle kVA² = kW² + kVAR², which yields the reactive component: kVAR = √(kVA² − kW²).

It also sizes the UPS or generator with the practical rule kVA = kW / PF / 0.8: the 0.8 factor keeps the equipment loaded at 80 % of capacity at most, the reserve that good design practice (aligned with the NEC continuous-load criterion) sets aside for motor starting peaks and future growth. It includes presets for the common voltages: 220/230 V single-phase and 380/400/480 V three-phase.

Example: 10 kW motor on 380 V three-phase with 0.9 PF

Apparent power: kVA = 10 / 0.9 = 11.11 kVA.
Line current: I = 11,111 / (√3 × 380) = 16.9 A per phase.
Reactive: kVAR = √(11.11² − 10²) ≈ 4.84 kVAR.
Recommended UPS or generator: 10 / 0.9 / 0.8 ≈ 13.9 kVA (a commercial 15 kVA unit).

Frequently asked questions

What is the difference between kW and kVA?

kW is the active power, the part that actually does work and gets billed as energy; kVA is the apparent power, the voltage-times-current product that wiring and transformers must withstand. They are related by the power factor: kW = kVA × PF. At 0.8 PF, a 100 kVA unit only delivers 80 useful kW.

How many amps is 10 kW at 380 V three-phase?

With a 0.9 power factor, about 16.9 A per phase: I = 10,000 / (0.9 × √3 × 380). At PF 1 (resistive load) it drops to 15.2 A. Note that the current depends on the PF: the worse the power factor, the more amps for the same kW, which is why conductors are sized by kVA.

What UPS do I need for a 10 kW load?

With 0.9 PF and the 80 % design margin, the rule kW / PF / 0.8 gives 13.9 kVA: in practice a commercial 15 kVA UPS. Do not size it tight: starting peaks, battery degradation and future expansions eat that 20 % reserve quickly.

Why does √3 appear in three-phase formulas?

Because the voltage used is line-to-line (380/400/480 V), and in a balanced three-phase system that voltage is √3 ≈ 1.732 times the phase-to-neutral voltage. Multiplying √3 × V × I correctly adds up the contribution of the three phases; in single-phase, V × I is enough.

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We take great care to keep every tool accurate and review it thoroughly; even so, we can't guarantee it is free of errors or take responsibility for how the results are used. We recommend double-checking anything critical.

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