Formula
Single phase: S (kVA) = I (A) × V (V) ÷ 1000
Three phase: S (kVA) = √3 × I (A) × VL-L ÷ 1000
Worked example
A three-phase load drawing 50 A at 400 V: S = 1.732 × 50 × 400 ÷ 1000 = 34.64 kVA. Choose the next standard generator size up (e.g. 40 kVA) for headroom.
Reference table
Quick reference
| Amps | kVA @ 230 V (1φ) | kVA @ 400 V (3φ) |
|---|---|---|
| 5 | 1.15 | 3.46 |
| 10 | 2.3 | 6.93 |
| 20 | 4.6 | 13.86 |
| 50 | 11.5 | 34.64 |
| 100 | 23 | 69.28 |
| 200 | 46 | 138.56 |
Where this shows up in the real world
Measured a feeder at 180 amps and need to know what generator keeps it alive through an outage? That's this conversion. It's the standby-generator and UPS sizing direction: clamp the real current, convert to kVA, then buy the next standard size up. Rental houses in the US quote generators in kVA or kW at 0.8 PF, so speaking kVA gets you the right machine the first time.
Common mistakes to avoid
Don't size at 100% of the calculated kVA — motor inrush on air conditioners, well pumps and refrigerators briefly demands several times running current. Standard practice is loading a generator to 70–80% of its rating. And measure current on the hottest, busiest day you can, not an idle Tuesday morning, or your 'measured load' will be fiction.
Frequently asked questions
Why size generators in kVA, not kW?
A generator's windings are limited by current, which depends on apparent power (kVA) regardless of the load's power factor. The kW it can deliver is kVA × PF.
Should I add a safety margin?
Yes — common practice is to size a generator or transformer at 70–80% of its kVA rating for continuous load, leaving headroom for motor starting surges.
What voltage do I enter?
The operating voltage of the circuit: line-to-neutral for single phase, line-to-line for three phase.