Transformer Sizing Calculator: Size kVA From a Load List

The calculator on this page will tell you a VA rating in about four seconds. What it cannot tell you is whether the number on your load list is the number that matters, and that is where almost every sizing mistake starts. We build low voltage transformers in Yueqing for workshops across the Middle East, Africa, South and Southeast Asia and Latin America, and the enquiries that come back with a failed unit are rarely the ones where somebody typed the wrong figure into a web form. They are the ones where a pump was listed at its running watts, or a welder was listed as if it behaved like a lamp, or a 3000VA unit was ordered for a load that draws 2900VA continuously inside a sealed cabinet in a country where the shade temperature sits near 45 degrees.

So this page is organised the way we would talk through a sizing question on the phone. First the list, then the load behaviour, then the margin, then the model. The calculator is the fast part. The list is the part that decides whether the answer holds up in year three.

Why a sizing calculator is only as good as the load list behind it

Every sizing problem is really a data problem. A transformer has one job: carry the current the load pulls, without its winding getting hotter than its insulation class allows. The VA rating is a promise about that current at a stated voltage. If your list of loads is complete and each item is described honestly, the arithmetic is trivial and any calculator will get it right. If the list is short by one machine, or one machine is described as something it is not, the arithmetic is still correct and the answer is still wrong.

The three things that go missing most often are the load that runs only in summer, the load that nobody thinks of as a load, and the load that starts with a bang. An air conditioner in Karachi is not a seasonal extra, it is the largest single item on the list for four months a year. A workshop’s lighting, fans and small chargers add up to a few hundred watts that nobody counts, and then a 2000VA unit that was sized to the exact total runs warm all day. A 3 HP water pump draws about 2200 W running and something close to 15 kW for the half second it takes to accelerate, and that half second is what sizes the breaker and often the transformer as well.

There is a fourth problem worth naming. People size to the number they want to see. A buyer with a 2100VA load wants to hear 2000VA, because the 3000VA frame costs more and weighs more and takes a longer lead time on a custom voltage. We understand the pressure, and we will still say the same thing: on a 2100VA continuous load, a 2000VA unit is running at 105 percent from the day it is installed. It will work, for a while, and then it will not, and the failure will happen at the worst possible time because it will happen under the heaviest load of the year.

The four inputs that change the answer

The tool asks for four things, and each one moves the result for a different reason.

Total load in watts. This is the sum of every appliance that can run at the same time, not the sum of everything that exists on site. Two machines on a changeover switch are one load. Ten sewing machines in a room where eleven operators work are ten loads, because the eleventh machine is the spare.

Load type. Resistive loads such as heaters, lamps and kettles pull current in step with the voltage. Motor loads pull a large current at the moment of starting and a smaller one afterwards. Electronics and LED drivers pull current in short spikes near the top of the waveform, and that shape matters to the transformer even when the wattage looks modest.

Phase. Single phase or three phase. This is not a preference. It is set by what your supply offers and by the size of the load. A 15 kVA single phase load on a 400V three phase supply is a bad idea for reasons covered further down.

Supply voltage. The voltage you actually have at the wall, not the voltage printed in a brochure about your country. A site in Lagos measured at 228V and a site in Lagos measured at 205V need different taps even though they are in the same city.

Everything else the calculator does is arithmetic on top of these four. If one of them is wrong, no amount of careful calculation downstream will recover it.

Building the load list, appliance by appliance

Walk the site with a notebook, or open the panel and read the plates. Do not work from memory and do not work from the purchase invoices, because invoices list what was bought and the plate lists what it draws. Where a plate gives amps only, multiply by the nominal voltage to get VA. Where a plate gives watts and a power factor, divide watts by power factor to get VA. Where a plate gives neither, the table below is a reasonable starting point for common equipment.

Add the column up. Then add the things that are not appliances: the door motor, the workshop lighting, the battery chargers left on overnight, the phone chargers at the counter. These are small individually and they are not small in total. On one site in Manila the counted appliances came to 7.1 kVA and the measured running current worked out to 8.4 kVA once lighting, fans and standby chargers were included. That gap decided the model.

Load type and start-up current: where most undersizing happens

A transformer can carry a short overload without complaint because its thermal time constant is measured in tens of minutes. A motor’s start-up inrush lasts a fraction of a second, so the transformer usually survives it. What does not survive is the breaker, and what also does not survive is the voltage dip that the surge creates on everything else sharing the transformer. That dip is why the lights blink when the compressor kicks in, and it is why a factory with a marginal transformer gets mysterious control-board resets.

The standard figures below are what we use when a customer cannot give a locked rotor current. They are multipliers on running current, applied for the first second or so.

If the load list is mostly motors, the honest way to size is not to add margin at all, but to add a soft starter or a variable speed drive and then size to running load. A 5.5 kW pump with a soft starter behaves like a 5.5 kW load, and the transformer you need is maybe 40 percent smaller than the one you would need with direct on line starting. That is a cheaper answer than the bigger transformer, and it also fixes the voltage dip that the rest of the plant is complaining about.

How much margin to add, and what margin actually costs

Margin is not wasted money, but it is not free either. A transformer running at 50 percent load has lower winding temperature and longer insulation life, and it also has lower efficiency than the same unit at 80 percent load, because the fixed iron losses are spread over less output. There is a sweet spot around 70 to 85 percent loading for continuous duty, and a different answer for intermittent duty.

The practical rule we give customers is 25 percent margin on a resistive load, 50 percent on a mixed workshop load, and 100 percent on a list dominated by direct on line motors. Then adjust once for the site: if the transformer will live in a closed metal cabinet, take one step up, because a cabinet in the sun adds ten to fifteen degrees to the ambient and every ten degrees above the rated ambient roughly halves insulation life. If the load runs less than an hour a day, you can take one step down, because thermal cycling at low duty does far less damage than the same overload held permanently.

On price, the step from 2000VA to 3000VA in our ST series is small compared with the cost of replacing a failed unit and losing a day of production. The step from 5000VA to 10000VA is not small, and at that point it is worth doing the arithmetic properly rather than defaulting to the bigger frame.

A worked example: eleven machines in one workshop

Here is a real pattern from a furniture workshop in Johor. Eleven machines, all single phase 220V, and the owner wanted one transformer to feed the whole room. The list came in as nameplate horsepower, which is where we started.

Running total is 8820 VA. The naive answer is a 10000VA single phase unit, and that is what a catalogue would suggest. But look at the start column. The compressor alone asks for 11400 VA, and if the table saw and the extractor happen to be starting at the same moment, the instantaneous demand passes 20000 VA. The transformer can ride through that for a fraction of a second, but the 40 A breaker cannot, and it did not: the workshop was tripping the breaker on the transformer primary every morning when the compressor restarted against line pressure.

We solved it in two moves. First, a pressure switch and unloader valve on the compressor, so it never restarts against a loaded head. Second, the transformer went to 15000VA rather than 10000VA, which brought the loading down to 59 percent and gave the compressor surge somewhere to go. The cost difference was less than one month of lost production. A 10000VA unit would have worked on paper and failed on the floor.

This is the single most useful thing to understand about sizing: the total VA tells you the thermal requirement, and the largest single start-up tells you the dynamic requirement. You need to satisfy both.

Matching the result to a model in the range

Once you have a VA figure with margin, the model is a lookup. Our standard frames step through the sizes below. Custom voltages and custom socket configurations are built to order, and the minimum order for a custom voltage is 10 pieces, which is worth knowing before you ask for a one-off 127V unit.

Two notes on this table. The ST series is a fixed step down or a fixed step up; the STU and DT frames are dual direction, which means the same box can step 220V down to 110V or 110V up to 220V depending on how you wire it. The DT series is also available from one piece, which makes it the practical choice for a single site test or a one-off replacement. For everything else the standard order quantity is 10 pieces, and lead time runs 15 to 25 days depending on the frame and the winding configuration.

When single phase stops being an option

Single phase transformers get expensive and awkward above roughly 10 kVA. The reason is current: at 220V, 10 kVA is 45 A, and at 20 kVA it is 91 A, which means thick windings, heavy terminals and a lot of copper. A three phase unit at the same rating splits that current three ways across three limbs, so the frame is smaller, the windings are lighter and the cost per kVA drops.

There is a second reason that matters more in practice. A large single phase load on a three phase supply loads one phase heavily and the other two lightly. That unbalance shows up as voltage variation between phases, and motors on the lightly loaded phases run at the wrong voltage. If your supply is 400V three phase and your load is above 10 kVA, put in a three phase transformer and distribute the single phase loads across the three phases downstream.

We cover the arithmetic for three phase in more detail on the page about single phase versus three phase sizing, and the current calculation itself is on the amps to VA page.

What the calculator cannot know

No web tool knows the following, and each of them has changed an answer we have given.

Ambient temperature. A transformer rated 40 degrees Celsius ambient loses life fast at 50. Motors and transformers both derate, and a unit in a roof space in Dubai is not working in the same conditions as the same unit in a workshop in Nairobi.

Altitude. Above about 1000 m the thinner air cools less effectively and the insulation has less dielectric margin. For most of our customers this is irrelevant. For a site in Bogota or Addis Ababa it is not.

Duty cycle. A welder at 20 percent duty and a welder at 60 percent duty can share the same nameplate and need different transformers. If the load is intermittent, work out the RMS over a ten minute window rather than taking the peak.

Harmonics. Six pulse drives, cheap LED drivers and inverter air conditioners all inject harmonic current. Harmonic current heats a transformer winding more than the same RMS at fundamental frequency, because of skin and proximity effects. If more than about 30 percent of the load is electronic, add 10 to 20 percent to the VA figure.

Cable losses. A transformer 30 m from the load with an undersized cable delivers less voltage at the machine than at its own terminals. That is a cable problem, but it is often solved by buying a bigger transformer, which is the expensive way round. We wrote about this separately under voltage drop and cable sizing.

Five sizing mistakes we see in enquiries

Sizing to watts for a motor load. A 2.2 kW motor at 220V draws 10 A running, which is 2200 VA at unity power factor, but its real running VA at 0.82 power factor is 2680, and its start is over 13000. If the list says 2.2 kW and the buyer converts at 1.0, every downstream number is low.

Adding margin twice. Some customers add 30 percent for safety, then the local agent adds another 30 percent, then the installer adds a third. A 5 kVA load becomes a 11 kVA order. Margin should be applied once, by one person, with the site conditions in front of them.

Forgetting the second shift. A transformer sized for one shift at 80 percent loading runs at 80 percent for eight hours. When the plant adds a night shift it runs at 80 percent for twenty hours, and the thermal picture changes completely. Duty is not just watts, it is watts times hours.

Ignoring the existing transformer’s taps. If a site already has a transformer with a 220V and a 240V tap and the actual supply is 208V, the right answer might be a different tap, not a new unit. We ask for a photo of the plate for exactly this reason.

Buying the frame that ships fastest. Lead time pressure is real, and a 3000VA unit in stock will beat a 5000VA unit built to order by three weeks. But a transformer that is 40 percent undersized will not last three weeks past its warranty if the load is continuous. If the schedule is tight, tell us the dead line and we will tell you honestly whether the right frame can make it.

Checking the answer against the nameplate you already have

If you are replacing a unit rather than sizing a new installation, the old nameplate is the most useful document on site. Photograph it. The VA rating tells you what the previous installer thought the load was. The voltage taps tell you what supply was expected. The temperature rise or insulation class tells you what duty was planned. If the old unit failed, the way it failed is diagnostic: a burnt primary winding usually means overvoltage or a shorted turn, a burnt secondary usually means sustained overload, and a blackened terminal block usually means a loose connection rather than a sizing problem.

Reading plates is its own small skill, and we wrote a separate guide to reading a transformer nameplate that walks through each field in order. If you want to sanity check the short circuit behaviour of the unit you are considering, the impedance calculator turns the percentage impedance into fault current, which is what tells you whether your breaker will actually clear a fault.

What to send us for a same-day answer

We answer sizing questions within 24 hours on working days, and the answers are better when the message contains four things: the load list with nameplate ratings, the supply voltage measured at the site, the phase configuration, and a photo of the space where the transformer will sit. Send those and you will usually get a model recommendation plus a note on anything that looks marginal.

What you will not get from us is a bigger transformer than you need. We would rather sell you a 3000VA unit that fits than a 5000VA unit that sits at 40 percent loading for ten years, because the second one comes back as a support problem and a bad reference. If a different model in the range fits better, we will say so, including when the better model is the cheaper one.

The other tools on this site fill in the parts this page leaves out. If you are still working out what the load actually is in VA, the VA to watts converter handles the power factor conversion. If you are not sure what voltage you are dealing with in the destination country, the voltage by country reference covers eighty markets. And if you want the long version of the selection logic, the guide on what size low voltage transformer you need goes through it from the other direction.

Finally, a note on start-up. If your list contains a compressor, a pump or anything else that starts under load, read the page on inrush and surge current before you finalise the order. It is the difference between a transformer that works and a breaker that trips every morning.

Frequently asked questions