Amps to kVA Calculator

How to convert amps to kVA

To convert amps to kVA you need the current, the voltage and whether the supply is single phase or 3 phase. No power factor is needed.

1. Find the current. Use the rating of your supply or main breaker, a clamp-meter reading, or the equipment's data plate.
2. Pick the formula. Single phase: kVA = amps × volts ÷ 1000. 3 phase: kVA = 1.732 × amps × volts ÷ 1000.
3. Use the per-phase current for 3 phase. Don't add the three phases together.

Sizing a standby generator from your supply's amp rating

A quick way to estimate the generator size for a building is to convert its incoming supply rating from amps to kVA:

• A typical UK home has a single phase supply fused at 60-100A. At 230V, 100A is 23 kVA, though most homes use far less than their fuse rating. See our generators for homes.
• A small commercial unit with a 100A 3 phase supply has a capacity of about 69 kVA, so an 80 kVA generator would cover the full supply.
• A larger site with a 400A 3 phase supply is around 277 kVA. A 300 kVA generator covers it, with more headroom from a 350 kVA generator.

Sizing to the supply rating is the cautious approach. If you know your actual maximum demand, from half-hourly meter data or clamp-meter readings taken at peak times, convert that figure instead. It often allows a smaller, more economical standby generator.

Matching the generator to your changeover switch

Changeover switches and automatic transfer switch (ATS) panels are rated in amps, not kVA. Converting between amps and kVA lets you check that the generator and the switch suit each other: the switch must be rated for at least the generator's full-load current, and usually for the incoming supply too.

Common mistakes

• Adding the phases together. A 100A 3 phase supply means 100A on each phase, not 300A in total. The √3 in the formula already accounts for all three phases.
• Using the wrong voltage. UK 3 phase is 400V line-to-line. Using 230V in the 3 phase formula understates the kVA.
• Forgetting starting currents. Motors draw a high inrush current at start-up. If your load includes big motors, allow extra headroom or ask us to check.