Transformer Sizing & Ratio
Full-load current on both sides of a transformer, the turns ratio including the delta/star √3 caveat, and the available short-circuit current from the nameplate impedance — plus sizing a transformer from a kW load and rounding up to the next standard kVA. IEC and ANSI size families, fully offline.
Open Transformer Sizing & Ratio →What is the transformer sizing & ratio calculator?
A free transformer calculator that runs entirely in your browser. Enter a kVA rating and the two voltages and it gives the full-load line current on each side, or work backwards from a measured voltage and current to see the apparent power actually flowing and how loaded the unit is. A second tab handles the turns ratio, including the part most calculators skip: on a delta-star pairing the per-phase turns ratio is not the line-voltage ratio, because a winding sees its own phase voltage. A third turns the nameplate impedance into the available short-circuit current at the secondary terminals, with the transformer's ohmic impedance and the worst case allowed by the impedance tolerance of the standard it was built to — and sizes a transformer from a kW load, rounding up to the next standard kVA in either the IEC or the ANSI family. Nothing is uploaded.
How to use Transformer Sizing & Ratio
- Pick the supply and size family — Choose single- or three-phase, then the standard size family — IEC preferred numbers, the commonly stocked IEC subset, or the ANSI/IEEE ratings. Three-phase voltages are line-to-line throughout.
- Enter the nameplate — On the first tab, type the kVA rating and both voltages. The full-load current on each side appears immediately, along with the voltage ratio and, on three-phase, the line-to-neutral pair so you can spot a transposed voltage.
- Check the ratio — On the Turns ratio tab, enter the two voltages or a known ratio, plus the primary turns and current if you have them. On three-phase, pick the winding connection to see how a delta-star pairing shifts the per-phase turns ratio by √3.
- Work out the fault current — On the third tab, enter the kVA, the secondary voltage and the nameplate %Z. The available fault current, the multiplier, the through-fault level and the transformer's ohmic impedance all update as you type.
Frequently asked questions
Why is the nameplate impedance never filled in for me?
Because a guessed impedance quietly becomes a wrong fault current, and fault current decides interrupting ratings. As a rough sense-check, European distribution transformers are commonly specified at 4% up to 630 kVA and 6% from 630 to 2500 kVA (EN 50588-1), and small liquid-filled single-phase units sit lower still — but the only figure that matters is the one printed on the transformer in front of you. Many online calculators auto-fill this; that is a deliberate difference here.
Why does the turns ratio differ from the voltage ratio on a delta-star transformer?
A winding sees its own phase voltage. In a delta winding that is the full line-to-line voltage; in a star winding it is the line voltage divided by √3. So a delta-primary, star-secondary transformer needs a per-phase turns ratio √3 times the line-voltage ratio, and a star-delta one needs it √3 times smaller. The tool shows both.
What does the fault current figure include?
It is the symmetrical RMS current at the secondary terminals with an infinite source — no source impedance, no conductor impedance and no motor contribution. That makes it a conservative upper bound at the terminals. Running motors add to it for the first few cycles, and it is not the asymmetrical first-cycle value, which needs an X/R ratio most nameplates do not carry. The worst-case row applies the tolerance of the standard the unit was built to: ±7.5% under IEEE C57.12.00, and ±10% under IEC 60076-1 below 10% impedance.
Why round the sizing up to a standard rating?
Because you can only buy the sizes that are made. The IEC list is the ISO 3 preferred-number series, generated here from its own definition rather than typed in; the ANSI list is the IEEE preferred continuous ratings. If the requirement exceeds the largest listed size the tool says so instead of quietly returning the biggest one.
Tips
- There is no metric/imperial switch here because volts, amps, kVA and ohms are the same everywhere — the toggle picks a standard size family instead.
- On three-phase, glance at the line-to-neutral row: seeing the familiar 400/231 or 480/277 pair is a quick check that you typed a line-to-line voltage.
- The through-fault level in kVA is a handy sanity check — it is simply the rating divided by the per-unit impedance.