Calculator inputs
Use nameplate values, your expected operating load, and loss data when available. If loss data is blank or zero, efficiency input drives the loss estimate.
Example data table
Use these sample cases to check your page setup, compare winding choices, or test the export buttons before publishing.
| Case | kVA | HV / LV | Connection | PF | Load % | Primary A | Secondary A | Efficiency % |
|---|---|---|---|---|---|---|---|---|
| Distribution unit | 250 | 6600 / 415 V | Δ / Y | 0.85 | 70 | 15.31 | 243.46 | 97.98 |
| Plant feeder | 500 | 11000 / 415 V | Y / Δ | 0.90 | 80 | 20.99 | 542.91 | 98.31 |
| Substation step-down | 1000 | 33000 / 11000 V | Y / Y | 0.95 | 60 | 10.50 | 31.97 | 98.62 |
Formula used
1) Three phase apparent power and line current
S = √3 × VL × IL. Rearranging gives line current as IL = S / (√3 × VL). The calculator applies this to both windings.
2) Phase voltage and phase current by connection
For star, Vphase = Vline / √3 and Iphase = Iline. For delta, Vphase = Vline and Iphase = Iline / √3.
3) Turns ratio and line voltage ratio
Turns ratio = primary phase voltage / secondary phase voltage. Line voltage ratio = primary line voltage / tap-adjusted secondary line voltage.
4) Output power, input power, and efficiency
Output power = rated kVA × load fraction × power factor. If core and copper losses are entered, input power = output power + losses. Otherwise, input power comes from the efficiency fallback value.
5) Copper loss at load and regulation estimate
Copper loss at load = full-load copper loss × load fraction². Regulation uses an approximate lagging-load model: %Reg = load fraction × (R% × pf + X% × sinφ), where X% = √(Z%² − R%²).
How to use this calculator
- Enter the transformer rated kVA and both line voltages from the nameplate.
- Select star or delta for the primary and secondary winding connections.
- Type the operating frequency, expected power factor, and loading percentage.
- Add tap position if the secondary is intentionally raised or lowered.
- Enter impedance and resistance percentages if you want a regulation estimate.
- For better loss accuracy, provide core loss and full-load copper loss values.
- Use efficiency fallback when detailed loss values are unavailable.
- Press the calculate button and review the result panel above the form.
- Download CSV for spreadsheets or PDF for reports and documentation.
Frequently asked questions
What does this calculator estimate?
It estimates turns ratio, line and phase currents, voltage ratios, output power, input power, efficiency, losses, tap-adjusted voltage, and approximate regulation for three phase transformers.
Why do star and delta selections matter?
They change the relationship between line values and phase values. Because transformer turns ratio is based on phase voltage, winding connection directly affects the reported ratio.
Does the regulation result match laboratory tests exactly?
No. The regulation figure is an engineering estimate using percent resistance, percent impedance, power factor, and a lagging-load assumption. Field tests may differ.
When should I enter core and copper losses?
Enter them whenever you know no-load and full-load loss data. The calculator then builds efficiency from actual loss components instead of a single efficiency assumption.
Why is copper loss reduced at partial load?
Copper loss follows current squared. When load falls, winding current falls too, so copper loss drops roughly with the square of the load fraction.
Can I use this for step-up transformers?
Yes. Enter the actual primary and secondary sides exactly as energized in your application. The calculator works for step-up and step-down arrangements.
What if I only know nameplate efficiency?
Leave detailed loss fields at zero and enter the efficiency fallback value. The calculator will estimate input power and losses from that efficiency figure.
Can I export the results for reports?
Yes. Use the CSV button for spreadsheet-ready output. Use the PDF button or print option when you need a clean report-style export.