Amps to kVA Calculator

This is the conversion you reach for when the current is the thing you already know — clamped off an existing feeder, taken from a load schedule, or arrived at as the capacity a new installation has to have — and the equipment you are about to choose is sold by a different measure. Transformers, standby sets and UPS units are specified in kilovolt-amperes, so the current has to be translated before a catalogue is any use.

Volt-amperes are simply volts multiplied by amps. Divide by a thousand for the kilo, and add the three-phase factor where it applies:

Single phase: kVA = (I × V) ÷ 1,000
Three phase: kVA = (I × V × 1.732) ÷ 1,000

The 1.732 is where this conversion goes wrong most often, and it goes wrong quietly. Leave it out and the answer is 42% below where it should be — small enough to look like a reasonable number and large enough to buy a machine that cannot do the job. There is no partial credit here: either the supply has three line conductors and the factor applies, or it has one and it does not.

What the arithmetic produces is a minimum, and treating it as an answer is the second trap. Standard ratings exist in a fixed ladder — 15, 25, 37.5, 50, 75, 112.5, 150, 225 and upward — and nothing is manufactured between the rungs. A figure of 166.27 kVA is not something you can order; it is a threshold that tells you the 150 kVA unit is out and the 225 kVA unit is in.

Two things this conversion deliberately does not do. It applies no power factor, because volts times amps is apparent power by definition and the result is already in the units the nameplate uses. And it adds no design margin: the figure describes the load as entered, with nothing set aside for the circuit somebody adds in four years' time.

A 200 A three-phase feeder at 480 V, on a small industrial unit that is getting its own transformer rather than continuing to be fed from the main switchboard.

The conversion. 200 × 480 = 96,000, and because this is a three-phase supply that figure is multiplied by 1.732 to give 166,272 VA, or166.27 kVA. Worth pausing on the intermediate number: 96 kVA is what the naive sum produces, and it is the answer a great many people write down. It is not conservative or approximate — it is 42% short.

Finding the rung. Looking up the standard ladder from 166.27, the next rating is 225 kVA. The one below it is 150 kVA, which is 10% under what the feeder can pass and therefore not a candidate. So the answer is a 225 kVA transformer carrying 58.73 kVA of spare capacity, or 35% above the calculated figure.

Is that spare capacity a bonus? Not quite, and this is where a converter stops being enough. Thirty-five percent sounds like generous room for growth, but it is an accident of where the rungs happen to fall rather than a decision anybody made. Had the feeder been 250 A, the calculation would give 207.84 kVA — still landing on the same 225 kVA unit, but with only 8% to spare and no room at all for the next machine on the floor. Two similar-looking jobs, the same transformer, and completely different amounts of future.

That is the question the sizing calculator answers and this one does not: it applies a headroom percentage before rounding, so growth is something you specify rather than something the ladder hands you at random. Use this page to find out where you stand; use that one to decide what to buy.

This table starts from the ratings services and feeders are actually built at, which is the number most readers arrive with — a 400 A switchboard, an 800 A main, a breaker frame size off a drawing. Look up the row, read across to the supply you are working on, and the last column names the transformer rung that figure rounds up to at 480 V.

Use it instead of the converter when you are scoping rather than calculating: checking roughly what a service would need if it were transformed, or sanity-testing a proposal someone has already put in front of you. The converter is the better tool the moment your current is a real measured or calculated figure rather than a standard rating, because those almost never fall on one of these rows.

The two three-phase columns are worth comparing directly. The same current at 480 V represents 2.3 times the apparent power it does at 208 V, which is the entire argument for distributing at the higher voltage: more power down the same size of conductor. The 240 V single-phase column is there for context and is the one to use for a residential service or a small shop.

The rounding column is a starting point and not a specification. It applies no headroom, and it reads from a combined ladder that includes rungs which only exist on one phase's series in the real market.