kVA to amps calculator

To convert kVA to amps, multiply the kVA by 1,000 and divide by the voltage. On a 230 V single-phase supply that's kVA × 1000 ÷ 230, about 4.3 A per kVA. On a 400 V three-phase supply it's kVA × 1000 ÷ (1.732 × 400), about 1.4 A per kVA, per phase. No power factor needed for this one.

Every generator on a hire fleet, every transformer nameplate and every supply agreement talks in kVA, but your breakers, cable tables and clamp meter all talk in amps. Getting from one to the other is a ten second job, and the reason the power factor stays out of it is that kVA is the apparent power the supply has to push out regardless of what the load does with it.

Single-phase: I = kVA × 1000 ÷ V

Three-phase: I = kVA × 1000 ÷ (1.732 × V)

The 1.732 is the square root of 3, and V is the line voltage, 400 V on a standard UK three-phase supply. So a 10 kVA load on three-phase draws 10,000 ÷ (1.732 × 400) = 14.4 A per phase, while the same 10 kVA on a 230 V single-phase supply pulls a much beefier 43.5 A down one conductor. That gap is the whole argument for putting big loads on three-phase.

Step by step

1. Read the kVA rating off the nameplate, the hire docket or the supply agreement.
2. Pick the supply: 230 V single-phase or 400 V three-phase. For three-phase, always use the 400 V line voltage, not 230 V.
3. Multiply the kVA by 1,000, then divide by V for single-phase or by 1.732 × V for three-phase.
4. Remember three-phase answers are per phase, then size the cable and protective device from that current, not from the kVA.

kVA vs kW, what's the difference?

Short version: kW = kVA × power factor. kVA is the apparent power, the total the supply must deliver. kW is the real power, the bit that actually does the work. With purely resistive loads like heaters the power factor is 1 and the two are identical. Add motors, drives and anything inductive and the kW drops below the kVA.

This is why generators and transformers are rated in kVA. The manufacturer has no idea what you are going to plug in, so they cannot know your power factor in advance. What they do know is how much current the windings can carry, and that caps the apparent power. The kVA rating is the honest one, and the real kW you get out is down to your load. If you are working the other way, from a kW load to a current, use the kW to amps calculator instead, that one has the power factor built in. For small single-phase kit rated in watts, the watts to amps calculator is quicker still.

kVA to amps at 400 V three-phase

The sizes you will meet on generators, transformers and commercial supplies. Remember these currents are per phase.

kVA to amps at 230 V single-phase

Smaller gennies and single-phase site supplies. Notice how quickly the current climbs, 10 kVA on single-phase is already past a 40 A breaker.

Sizing a generator from a supply current

Working backwards is just as easy, flip the formula: kVA = 1.732 × V × I ÷ 1000. Say you need to back up a 63 A three-phase supply: 1.732 × 400 × 63 ÷ 1000 = 43.6 kVA, so you would round up to the next standard set size and keep some headroom on top. Once you have the current, size the run to it with the cable size calculator.

Checking whether a supply can take the load

This conversion earns its keep the day a client asks "can our supply handle it?". On commercial sites with a half-hourly meter, the number that matters is the Maximum Import Capacity (MIC), the kVA figure agreed with the network operator. Exceed it and excess capacity charges land on the bill, so convert the site's existing demand and the proposed load into kVA before you promise anything. If the sums don't fit, the conversation moves from "new circuit" to "new connection agreement", and it's far better to have that conversation at quote stage than after the kit's been ordered.

• A typical UK domestic single-phase supply with a 100 A cut-out is about 23 kVA. Handy for a quick sanity check on whether a load stands any chance on a domestic board.
• Three-phase currents are per line, not totals. A 14.4 A result at 10 kVA means 14.4 A flowing in each of the three phases.
• Sizing a genny for motor loads? Allow for starting currents of 3 to 6 times the running current. A set that idles happily at running load can stall the moment a big motor kicks in.