Five calculators sit above this text. Each one is small, each one answers exactly one question, and none of them is useful on its own if you feed it the wrong number. That is the whole reason this page exists: the tools are easy, the order you use them in is what decides whether the answer is right.
We built these for our own sales desk first. A buyer writes in with an appliance, a destination and a quantity, and someone has to turn that into a transformer rating, a model number and a price before the end of the day. Doing it by hand invites the two mistakes that cost money: sizing on running current when the load is a compressor, and quoting a plug that cannot carry the current.
The five tools below are the same ones we use. They are free, they need no account, and there is nothing to download.
Which tool answers which question
Start from the sentence the customer actually wrote, not from the tool you happen to like. The table maps the common ones.
| What you are trying to find out | Tool | What you need to type in |
|---|---|---|
| Will my appliance work in this country at all? | Voltage by country | Country, plus the voltage on the appliance label |
| How big a transformer do I need? | Sizing calculator | Load in watts or VA, plus the type of load |
| My nameplate says amps, not watts | VA to watts | Amps and volts, or VA and power factor |
| What fault current will this transformer deliver? | Impedance calculator | kVA, secondary voltage, nameplate %Z, X/R ratio |
| Which plug and socket do I order with it? | Plug and socket types | Destination market and the appliance’s own plug |
Two of those are everyday questions and three are not. Voltage and sizing come up on almost every enquiry. The impedance calculator is for the panel builders and the maintenance engineers, the ones who have to know whether a 3 kVA transformer on a 400 V secondary will trip a 32 A breaker on a dead short. If that sentence means nothing to you, you probably do not need that tool, and that is fine.
The order that saves the most time
Run them in this sequence and you will not have to go backwards. Each step feeds the next, and the last one is the only step that involves a choice rather than a calculation.
Step one, country voltage. Look up the destination. You want three things out of it: nominal voltage, frequency, and the plug type. If the appliance label and the destination agree on voltage, you may not need a transformer at all, and the enquiry becomes a plug question. If they sit on opposite sides of the 110 to 220 divide, everything after this point is about sizing.
Step two, real load in VA. Appliance labels are inconsistent. Some give watts, some give amps, some give VA, and a few give a horsepower figure that has to be converted. Get everything onto one unit before you go near the sizing tool. Amps at a stated voltage multiply straight into VA. Watts divide by power factor to reach VA, and if the power factor is unknown, 0.8 is a reasonable guess for a motor-driven appliance.
Step three, sizing. Enter the VA figure and pick the load type. The calculator applies the margin: 1.25 for a resistive or simple load, 2 to 3 times for a motor or compressor because the starting current is what sets the size. This is the step where a wrong load type produces a transformer that runs fine for a month and then trips on the first hot afternoon.
Step four, impedance. Only when the transformer feeds a distribution board, a machine panel or anything with a breaker you care about. The nameplate %Z gives you the ohms, and the ohms give you the fault current the breaker has to interrupt.
Step five, plug and socket. The last decision, and the one most likely to be made badly at the end of a long quotation. Check the current rating against the transformer rating before you write it into the offer.
Done in that order, an enquiry that arrives in the morning gets a complete specification by the afternoon, and the specification has no gaps for the factory to fill in with guesswork.
One order worked end to end
A real pattern, with the numbers rounded for reading. A bakery in Lagos wants to run a US-bought commercial display fridge from the local 230 V supply. The label reads 120 V, 4.5 A, and the compressor has a locked rotor figure of five times running current.
Destination check. Nigeria is 230 V at 50 Hz with British Type G sockets. The fridge is 120 V at 60 Hz. Voltage is the wrong side of the divide, so a step down transformer is required. The frequency gap is real and worth a line in the quotation: a 60 Hz compressor running at 50 Hz turns 17 percent slower, which is usually acceptable for a fridge and is not acceptable for a pump on a timer.
Load in VA. 4.5 A at 120 V is 540 VA of running load. The power factor question does not arise here, because amps and volts already give volt-amps. This is why label amps are convenient when you have them.
Sizing. A fridge is a compressor load, so the running figure is not the one that sizes the unit. Five times 540 VA is 2700 VA at the instant of start. Round up, add a little headroom for a hot day and a mains sag, and the answer lands on a 3000 VA step up and step down unit with taps at 110, 120, 220 and 240 V. In our range that is the STU-3000.
Impedance. Skipped. One appliance on one transformer with no downstream distribution does not need a fault current study, and pretending otherwise would just slow the quote down.
Plug and socket. Input is British Type G at 13 A, which is 2990 VA at 230 V, so a 3000 VA transformer sits right at the ceiling. We quote a 16 A industrial inlet instead and explain that the 13 A plug would be running at its limit from day one. Output is North American Type B at 15 A for the fridge lead. Cord length 1.8 m, earth bonded to the enclosure.
Five steps, one afternoon, one specification with nothing left open. The size of that job is typical: most enquiries that reach us are between 500 VA and 5 kVA, and the arithmetic above covers that whole range.
What the numbers behind the tools are
None of the calculators are black boxes. The equations are the standard ones, and having them in front of you is the quickest way to spot a data entry mistake.
| Quantity | Equation | Where it is used |
|---|---|---|
| Apparent power | VA = V × A | Sizing, VA to watts |
| Real power | W = VA × PF | VA to watts |
| Transformer ratio | V1 / V2 = N1 / N2 | Sizing, tap selection |
| Full load current, single phase | I = kVA × 1000 / V | Sizing, impedance |
| Full load current, three phase | I = kVA × 1000 / (1.732 × V) | Sizing, impedance |
| Base impedance | Zb = V2 / S | Impedance |
| Impedance in ohms | |Z| = Zb × %Z / 100 | Impedance |
| Resistance and reactance | R = |Z| / √(1 + (X/R)2) | Impedance |
| Short circuit current | Ik = Ifl × 100 / %Z | Impedance |
| Peak factor, IEC 60909 | κ = 1.02 + 0.98 e−3R/X | Impedance |
| Peak asymmetrical current | Ip = κ × √2 × Ik | Impedance |
The two that surprise people are the last two. A short circuit on the secondary of a transformer is not limited by the load, it is limited by the impedance, so the current can be fifteen or twenty times the rated current. And the first peak of that current is higher than the steady fault current, because the fault starts on a DC offset that decays over a few cycles. That peak is what a breaker’s breaking capacity has to survive, which is why the κ factor exists.
Three numbers that cause almost every wrong order
When a transformer turns up undersized, the cause is nearly always one of these three, and all three look like correct data until you check where they came from.
Running current used as starting current. A motor nameplate gives the full load current, which is what the machine draws once it is spinning. The first half second is a different story: an induction motor pulls five to eight times its rated current until the rotor comes up to speed, and a hermetic compressor in a fridge or an air conditioner is worse because there is gas pressure on top of the inertia. Size on the running figure and the transformer will hold for weeks and then trip the first time the compressor restarts against a warm head. This single mistake accounts for most of the returns we see.
Watts treated as volt-amps. A 1000 W heater is 1000 VA, because a resistive element has a power factor of 1. A 1000 W motor is closer to 1250 VA, because the current and the voltage do not peak together and the extra current does useful work for nobody. The transformer has to carry the current, not the watts, so a 1 kVA unit behind a 1000 W motor is already loaded past its rating before any margin is applied. When only watts are known and the load has a motor in it, divide by 0.8 before sizing.
Nominal voltage read as measured voltage. The country table says 220 V, so the customer orders a 220 V to 110 V unit, and then the site turns out to measure 240 V in the afternoon. The output follows the input in proportion, so a 220 V to 110 V transformer fed with 240 V delivers 120 V, and the 110 V appliance sees ten percent over its rating all day. This is why the tap arrangement matters more than the headline ratio on export orders, and why our standard step down units carry taps at 110, 120, 220 and 240 V rather than a single pair.
None of the three requires more arithmetic. All three require knowing which number to distrust, which is the part a calculator cannot do for you.
Where the data comes from, and where it stops being reliable
Every calculator is only as good as the numbers behind it, so here is where ours come from.
Voltage and frequency. Nominal values follow IEC 60038 and are cross-checked against what the local utility publishes. Where a market runs two voltages, both are listed rather than averaged. Nominal means what the utility is required to supply, not what your meter will read; a 220 V nominal supply that measures 205 V in the evening is inside the allowed tolerance band and is not a fault.
Plug and socket types. Taken from the national standards themselves, not from travel websites. Where a country has a legacy type still in service alongside the current one, both appear.
Power factor and efficiency defaults. From our own test records across the units we build. The efficiency table runs from about 82 to 86 percent at 100 VA up to 96 to 98 percent at 20 kVA, which is why a small transformer wastes proportionally more than a large one.
Where it stops. Three places. The impedance tool does an IEC 60909 style first approximation and is not a substitute for a protection coordination study by a licensed engineer. The sizing tool does not model harmonic loads, variable frequency drives or regenerative loads; those need a conversation. And no calculator knows what your site voltage actually does at seven in the evening, which is why we ask for three meter readings before finalising a tap range.
If a number matters to a safety case, get it checked by the engineer who signs the installation. Use these tools to get into the right order of magnitude quickly, which is what they are good at.
Frequently asked questions
Are the calculators free, and do I need an account?
All five are free and none of them require an account, an email address or a download. They run in the browser on the page. We do not gate the result behind a form, because the people who find these tools useful are usually the ones who come back with a real enquiry afterwards.
Which calculator should I use first?
Start with voltage by country. It tells you whether a transformer is needed at all, and roughly half of the enquiries we receive turn out to need only a plug change rather than a transformer. If the appliance voltage and the destination voltage sit on opposite sides of the 110 to 220 divide, move to the sizing calculator next and leave the impedance tool alone unless the transformer feeds a distribution board.
Can the sizing calculator replace an electrical engineer?
No. It applies standard margins to the load you enter, which handles the ordinary cases correctly and quickly. It cannot see your wiring, your breaker curve, your harmonics or the state of the neutral. For a single appliance on a single transformer it is enough. For anything feeding a panel, a three phase installation or a medical location, treat the result as a starting number and have it reviewed.
Where does the country voltage data come from?
Nominal values follow IEC 60038, cross-checked against what each utility publishes, and we update a market when a customer sends us meter readings that disagree. Where a country runs two voltages, both are listed. The data is nominal by design: it tells you what the supply is supposed to be, not what a particular building measures on a particular evening.
Do you build to a specification the tools produce?
Yes, and that is the point of publishing them. Send the resulting figures with your destination market and quantity and we quote from them. Stock configurations start at 1 piece, a non-standard socket or cord length at 50, and lead time is 15 to 25 days FOB Ningbo or Shanghai.