kVA Calculator — Apparent, Real & Reactive
Transformers, generators, and UPS units are rated in kVA because their limit is current, not work. Your load is usually known in kW. Power factor is what separates them, and the third leg — kVAR — is the reactive current that inflates your conductors and your demand charge. This calculator solves all three from whichever one you have.
Solve the power triangle
Converted to apparent power
Reactive power is circulating without doing work.
Real power
70.67 kW
Line current
100 A
See the breakdown
Transformer primary and secondary conductors and overcurrent protection are sized per NEC 450.3, generally at 125% of the respective full-load current.
The formula, explained in plain English
Three quantities forming a right triangle. Power factor is the cosine of its angle.
Equipment is rated in kVA for a reason
A winding heats from the current through it, in phase or not. Rating in kVA means the manufacturer never has to assume your power factor — and means you must compare kVA to kVA.
Power factor is a discount on capacity
At PF 0.8, a 100 kVA transformer delivers 80 kW. Raise the load's power factor and the same transformer delivers more useful work with no hardware change.
Worked examples
A feeder measurement, a transformer sizing, and a generator check.
100 A measured at 480 V three-phase, PF 0.85
A clamp-meter reading on an existing feeder. The defaults above.
kW = 83.14 × 0.85 = 70.67 kW
kVAR = √(83.14² − 70.67²) = 43.80 kVAR
next standard transformer: 112.5 kVA
Result: 83 kVA of load doing 71 kW of work. The 43.80 kVAR is what a capacitor bank would target — and what a kVA demand charge would bill.
Sizing a transformer for a 60 kW load
208 V three-phase secondary, PF 0.9.
secondary current = 66,670 ÷ (1.732 × 208) = 185.0 A
next standard size: 75 kVA
Result: sizing on the 60 kW figure would have suggested a 60 kVA unit and undersized it. Always convert to kVA first — the 0.9 power factor adds 6.67 kVA of capacity the transformer must actually carry.
Single-phase: 200 A at 240 V
A residential service at unity power factor.
at PF 1.0: kW = 48 kW · kVAR = 0
Result: a fully loaded 200 A residential service is 48 kVA. Useful for generator sizing — and note that a "48 kW" generator published at 0.8 power factor is a 60 kVA machine, so it would cover this comfortably.
Line current by kVA and voltage
Full-load line current for standard transformer sizes. Power factor does not appear because apparent power already accounts for it — this is the current the conductors carry.
| System | 15 kVA | 45 kVA | 75 kVA | 150 kVA | 300 kVA |
|---|---|---|---|---|---|
| 208 V 3φ | 41.6 A | 124.9 A | 208.2 A | 416.4 A | 832.7 A |
| 240 V 1φ | 62.5 A | 187.5 A | 312.5 A | 625.0 A | 1,250.0 A |
| 240 V 3φ | 36.1 A | 108.3 A | 180.4 A | 360.8 A | 721.7 A |
| 480 V 3φ | 18.0 A | 54.1 A | 90.2 A | 180.4 A | 360.8 A |
| 600 V 3φ | 14.4 A | 43.3 A | 72.2 A | 144.3 A | 288.7 A |
Sources & standards: √3 = 1.7321. NEC (NFPA 70) 2023 — Article 450 transformers, 450.3 overcurrent protection at 125% of full-load current, Table 310.16 conductor ampacities, 220.61 neutral load. Standard dry-type transformer kVA ratings per industry convention. Generator kVA ratings are conventionally published at 0.8 power factor. Local amendments override the model code.
Frequently asked questions
Common questions about kVA, kW, kVAR, and transformer sizing.
How do I calculate kVA?
Multiply voltage by current and divide by 1,000 — with the √3 factor for three-phase. Single-phase: kVA = V × I ÷ 1,000. Three-phase: kVA = √3 × V × I ÷ 1,000. So 100 A at 480 V three-phase is 1.732 × 480 × 100 ÷ 1,000 = 83.14 kVA.
What's the difference between kVA and kW?
kVA is apparent power — everything the conductor and transformer must carry. kW is real power — the part doing useful work, and what the energy meter records. They are related by power factor: kW = kVA × PF. At unity power factor they are identical; at 0.8 power factor a 100 kVA load delivers only 80 kW.
Why are transformers and generators rated in kVA?
Because their limits are thermal and current-driven, not work-driven. A transformer winding heats according to the current flowing through it regardless of whether that current is in phase with the voltage. Rating in kVA is therefore honest — the manufacturer doesn't have to guess your power factor. It also means comparing your calculated kVA against the nameplate is the only valid check.
What is kVAR and do I care?
Reactive power — the component that circulates without doing work, from motor and transformer magnetising current. kVAR = √(kVA² − kW²). On a residential job you can ignore it. On a commercial or industrial job it matters twice: it inflates the current your conductors carry, and utilities frequently bill a kVA demand charge or a power-factor penalty because of it.
How do I size a transformer from a load?
Calculate the load in kVA — not kW — then choose the next standard rating above it, allowing headroom for growth. Standard dry-type sizes run 3, 6, 9, 15, 22.5, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 and 1,000 kVA. Primary and secondary conductors and overcurrent protection are then sized per NEC 450.3, generally at 125% of the respective full-load current.
Does a higher power factor reduce my kVA?
Yes, and that is exactly what power-factor correction buys. For a fixed real load, raising power factor from 0.75 to 0.95 drops apparent power from 133 kVA to 105 kVA — a 21% reduction in current, which frees conductor and transformer capacity and cuts any kVA demand charge. The Power Factor Correction Calculator sizes the capacitor bank.
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