What is transformer full-load current?
Full-load current is the current a winding carries when the transformer is delivering its full nameplate rating. Every transformer has two of them — one on the primary, one on the secondary — and they are almost never the same number. A 75 kVA three-phase unit stepping 480 V down to 208 V carries 90.21 A on the primary and 208.2 A on the secondary. The same 75 kVA passes through both windings. Less than half the voltage comes out the other side, so more than double the current has to flow to carry it.
That pair of numbers is what the rest of the job hangs on: conductor size, overcurrent protection, whether the feeder that's already in the wall can take a bigger unit, whether the transformer humming in the electrical room is running near its limit or loafing. It's also the number people get wrong most often, because the three-phase formula carries a √3 and the single-phase one doesn't. Forget it and your answer is 73% too high.
The Transformer Calculator answers that question and the reverse one — you have a load, you need a rating. Both directions are the same relationship between kVA, volts and amps read from opposite ends, so both live on one page, one toggle apart.
How to use the Transformer Calculator
- Pick the phase first. Three-phase is the default, since most units people size are commercial or industrial; single-phase is one click away. Every three-phase voltage on this page is line-to-line, the way transformer nameplates are rated.
- In Transformer specs mode, type the nameplate rating in kVA and both winding voltages. Use decimals like 37.5, not fractions. There's no Calculate button — the numbers move as you type.
- Tap the quick-volt chips under the fields to drop 120, 208, 240, 277, 480 or 600 into whichever voltage box you touched last. Those six cover most of North American distribution.
- Read the turns ratio and the type underneath the currents. Above 1 : 1 is a step-down, below is a step-up, exactly 1 : 1 is an isolation transformer — same voltage in and out, installed for safety and noise rather than for a voltage change.
- To go the other way, switch to Size from load and enter the load's voltage and current draw. You get the minimum rating, the rating to actually order once you allow headroom, and the next standard size that exists to buy.
- Copy any result, or copy the plugged-in formula line, straight into an email or your notes. Your entered values survive a mode or phase switch, so you can flip between the two questions without retyping.
Sizing pages in this corner of the web have a habit of putting the answer behind a form — name, company, email, and someone will be in touch about a quote. That's a lead-capture page wearing a calculator's clothes. This one does the arithmetic and gets out of the way.
The formulas behind the numbers
Two formulas do all the work. Which one you need depends only on the phase.
Single-phase: I = kVA × 1000 ÷ V
Three-phase: I = kVA × 1000 ÷ (√3 × V)
The kVA is the nameplate rating, multiplied by 1000 to get from kilovolt-amperes to volt-amperes. V is the voltage of the winding you're asking about — which is why the primary and secondary get different answers from the same rating. The √3, about 1.732, is there because the three line currents in a three-phase system sit 120° apart in time and don't simply add the way a single-phase circuit's do.
Take a 75 kVA three-phase transformer, 480 V primary, 208 V secondary. The Transformer Calculator shows its work on both windings:
I₁ = 75 kVA × 1000 ÷ (√3 × 480 V) = 90.21 A
I₂ = 75 kVA × 1000 ÷ (√3 × 208 V) = 208.2 A
The turns ratio comes out of the voltages alone: 480 ÷ 208 = 2.308, displayed as 2.308 : 1, and anything above 1 is a step-down. Notice the current ratio runs the other way — 208.2 ÷ 90.21 is also about 2.31. That's conservation of power staring back at you, and it's the reason the secondary conductors on a step-down unit are always the fat ones.
Sizing from a load runs the same relationship backwards: kVA = √3 × V × A ÷ 1000 for three-phase, without the √3 for single-phase. A 208 V three-phase load pulling 50 A needs a minimum of 18.01 kVA, which becomes 22.51 kVA once you divide by 0.8 for 20% headroom, which lands on a 30 kVA standard unit — the smallest three-phase size that actually exists above the number you need.
Full-load current for common transformer sizes
Most of the time you're looking up one of a dozen ratings at one of three service voltages. Here's the whole grid, three-phase, line-to-line, as the calculator renders it:
| Rating | At 208 V | At 480 V | At 600 V |
|---|---|---|---|
| 15 kVA | 41.64 A | 18.04 A | 14.43 A |
| 30 kVA | 83.27 A | 36.08 A | 28.87 A |
| 45 kVA | 124.9 A | 54.13 A | 43.30 A |
| 75 kVA | 208.2 A | 90.21 A | 72.17 A |
| 112.5 kVA | 312.3 A | 135.3 A | 108.3 A |
| 150 kVA | 416.4 A | 180.4 A | 144.3 A |
| 225 kVA | 624.5 A | 270.6 A | 216.5 A |
| 300 kVA | 832.7 A | 360.8 A | 288.7 A |
Two things fall out of this table. First, current scales straight down with voltage: the 208 V column is exactly 480 ÷ 208 = 2.31 times the 480 V column, all the way down. That's why utilities push power at high voltage and step it down at the very end — the same kilowatts move on thinner copper.
Second, the 480 V column is almost exactly 1.2 amps per kVA, and the 208 V column is about 2.78 amps per kVA. Those two constants are worth memorising if you work on North American gear. A 45 kVA unit at 480 V draws roughly 45 × 1.2 = 54 A, and the Transformer Calculator says 54.13 A. Close enough to sanity-check a number in your head before you trust one on a screen.
Single-phase is simpler and the arithmetic is kinder: at 240 V, amps are just kVA × 4.17. A 25 kVA single-phase unit carries 104.2 A at 240 V and 208.3 A at 120 V.
Sizing a transformer from a load
Ask how to size a transformer and you get three answers, not one — and skipping the last two is how people end up ordering a unit nobody builds. The minimum is the raw arithmetic. The recommended rating divides that by 0.8, because nothing in an electrical system should sit at 100% of its nameplate forever and heat is what ends a transformer's life. The next standard rating is the smallest size a manufacturer actually builds above the recommendation. The Transformer Calculator returns all three at once, because the first two on their own send you shopping for a size that doesn't exist.
| Load | Minimum | Recommended (80%) | Next standard size |
|---|---|---|---|
| 208 V, 50 A, three-phase | 18.01 kVA | 22.51 kVA | 30 kVA |
| 208 V, 125 A, three-phase | 45.03 kVA | 56.29 kVA | 75 kVA |
| 480 V, 60 A, three-phase | 49.88 kVA | 62.35 kVA | 75 kVA |
| 400 V, 80 A, three-phase | 55.43 kVA | 69.29 kVA | 75 kVA |
| 240 V, 100 A, single-phase | 24.00 kVA | 30.00 kVA | 37.5 kVA |
| 230 V, 40 A, single-phase | 9.200 kVA | 11.50 kVA | 15 kVA |
Look at the 240 V single-phase row. The recommendation lands on exactly 30.00 kVA — and the answer is still 37.5 kVA, because the single-phase standard series has no 30 kVA size even though the three-phase series does. Rows three and four are the opposite surprise: two different loads, 5 kVA apart, both land on the same 75 kVA unit. Standard ratings are coarse, and the gaps get wider as they climb.
Where transformer calculations go wrong
The line-to-neutral mix-up is the big one. Transformer nameplates are rated line-to-line, so a 480 V three-phase transformer means 480 V between any two phase conductors. The line-to-neutral figure on that system is 277 V. Enter 277 where the tool wants 480 and a 75 kVA unit reports 156.3 A instead of 90.21 A — 73% high, and high in the direction that makes you buy copper you don't need. That's the same 73% as dropping the √3, for the same reason: 480 ÷ 277 is √3. If line-to-neutral is all you have, multiply by 1.732 first.
The second one is kW where kVA belongs. A transformer is rated in apparent power, which is volts times amps before power factor enters the picture. That's why there's no power factor field here and why adding one would be wrong. If your load is given in kilowatts, divide by the power factor to get kVA before bringing the number over.
And the 0.8 loading factor is practice, not code. Continuous-duty loads, harmonic content from drives and electronics, ambient temperature and altitude all move the right number around. Sizing conductors and overcurrent protection from these currents brings in ampacity tables, derating and termination ratings — real code questions with real consequences. Use the full-load current here as the input to that conversation, then have it with your local code book and a licensed electrician.
Related electrical calculations
Once you have a full-load current, the next question is usually what wire carries it. The voltage drop calculator takes that current plus the AWG gauge and run length and checks it against the 3% guideline, for copper or aluminium, single- or three-phase.
If you're converting rather than sizing, the kVA to amps converter runs the same full-load current arithmetic on its own, and the kW to amps calculator handles the real-power side where power factor does matter. For smaller loads quoted in watts rather than kilowatts, the watts to amps calculator covers DC as well as AC.
And when the question drops below the distribution level entirely — a resistance, a voltage, a power dissipation — the Ohm's law calculator solves for whichever two of V, I, R and P you don't have.
Frequently asked questions
What size transformer do I need for a house?
Residential service is normally fed by the utility's own transformer, and typical single-family sizing runs 15 to 50 kVA depending on the service size and whether the unit is shared between houses. If you're sizing a transformer inside a property — for a shop, a barn, a detached garage on a different voltage — work from the actual load. A 240 V single-phase load pulling 100 A needs 24.00 kVA minimum, 30.00 kVA with headroom, and a 37.5 kVA standard unit. Your utility, not a web page, decides what feeds the meter.
Is a three-phase transformer's kVA rating per phase or for all three?
For all three, always. A 75 kVA three-phase transformer delivers 75 kVA total, not 225 kVA. That's exactly why the √3 belongs in the formula: it splits the total rating across three line currents correctly. If you want the per-phase figure for a bank of three single-phase units, divide by three — a 112.5 kVA three-phase bank is three 37.5 kVA single-phase transformers, which is where that odd-looking rating comes from in the first place.
Does this work for 230 V / 400 V systems outside North America?
Yes. The formulas don't care about the country or the frequency — 50 Hz and 60 Hz use identical kVA-volts-amps arithmetic. Type any voltage you like into the fields; the quick-pick chips just happen to cover the common North American values. A 400 V three-phase load drawing 80 A returns 55.43 kVA minimum and 69.29 kVA recommended. The one thing that does vary by region is the standard rating series, so treat the suggested next size as a North American shorthand and check it against a local manufacturer's list.
What happens if a transformer is overloaded — or badly oversized?
Overloading a transformer doesn't trip anything by itself. It runs hotter, the winding insulation ages faster, and a unit rated for decades quietly becomes a unit rated for years. That's the failure mode the 20% headroom exists to prevent. Oversizing has a cost too, just a gentler one: a transformer's no-load losses run whether anything is plugged in or not, so a vastly oversized unit burns electricity around the clock to do nothing. One standard size above your recommendation is the sweet spot; three sizes above is money leaking out of the electrical room.
How do I find the rating of a transformer that's already installed?
Read the nameplate — kVA, primary and secondary voltage, phase and frequency are all stamped on it, usually on the front face or inside the access cover. If the plate is missing or unreadable, you can work backwards: measure the primary and secondary voltages, enter them in the Transformer Calculator, and the turns ratio will tell you whether the unit matches what you assumed it was. The kVA rating itself can't be measured from outside, so that one needs the plate, the drawings, or the manufacturer's serial lookup.
Does the calculator send my numbers anywhere?
No. Everything runs in your browser — the arithmetic happens on your device and nothing you type leaves it. There's no account, no email gate, and no quote request standing between you and the answer. Using it costs nothing, and 10% of every dollar Microapp earns goes to charity, off the top, audited quarterly.