Calculator inputs
Formula used
Single-phase apparent power: kVA = (V × I) / 1000
Three-phase apparent power: kVA = (√3 × V × I) / 1000
Real power estimate: kW = kVA × Power Factor
Efficiency adjustment: Adjusted kVA = Base kVA / Efficiency
Load-factor adjustment: Adjusted kVA = Prior kVA / Planned Load Factor
Derating adjustment: Adjusted kVA = Prior kVA / (1 − Derating)
Final design size: Required kVA = Adjusted kVA × (1 + Margin) × (1 + Growth)
Phase current from rating: I = (kVA × 1000) / V for single-phase, or I = (kVA × 1000) / (√3 × V) for three-phase.
This method is intended for planning and preliminary selection. Final transformer choice should also consider inrush, harmonics, impedance, cooling class, ambient temperature, insulation class, and applicable electrical codes.
How to use this calculator
- Select whether you are calculating from current or from an existing kVA value.
- Choose single-phase or three-phase operation.
- Enter the operating voltage and either the operating current or known kVA.
- Provide primary and secondary voltages to estimate full-load currents on both sides.
- Set power factor, efficiency, planned load factor, derating, design margin, and future growth.
- Press Calculate transformer size to show the result above the form.
- Review the recommended standard size, current estimates, utilization, and graph.
- Use the CSV or PDF buttons to export the result summary.
Example data table
| Scenario | Phase | Voltage (V) | Current (A) | Base Load (kVA) | Suggested Standard Size |
|---|---|---|---|---|---|
| Small lighting panel | Single-phase | 240 | 80 | 19.20 | 30 kVA |
| Workshop feeder | Three-phase | 415 | 120 | 86.27 | 150 kVA |
| Motor control group | Three-phase | 400 | 210 | 145.49 | 225 kVA |
| Commercial floor load | Three-phase | 480 | 260 | 216.13 | 300 kVA |
| Mixed expansion load | Three-phase | 415 | 420 | 301.95 | 500 kVA |
Frequently asked questions
1. What does transformer kVA mean?
Transformer kVA is the apparent power capacity a transformer can safely deliver. It reflects voltage and current capability together, regardless of load power factor, and is the standard rating used for transformer selection.
2. Why does the calculator ask for power factor?
Power factor helps estimate real power in kilowatts. Transformer sizing is still based on kVA, but kW is useful for understanding how much real working power the load represents.
3. Why is load factor included?
Load factor lets you size the transformer so normal operation stays below a chosen percentage of nameplate capacity. That supports cooler operation, reliability, and reserve capacity for unexpected peaks.
4. What is derating in this calculator?
Derating reduces usable capacity to account for heat, altitude, harmonics, ventilation limits, or other real-world conditions. A higher derating percentage increases the recommended transformer size.
5. Should I size to current or kVA?
Use current when you know measured operating amps and voltage. Use known kVA when your connected equipment or design documents already state apparent load directly.
6. Why does the recommendation jump to a standard rating?
Transformers are manufactured in standard sizes. After calculating the required design kVA, the tool rounds upward to the next common rating so the selection is practical for procurement.
7. Are primary and secondary current values actual operating currents?
They are full-load current estimates for the recommended transformer rating at the entered voltages. Actual operating current depends on the connected load and how much of the rating is being used.
8. Can I use this for final procurement decisions?
You can use it for planning and comparison, but final selection should be confirmed against manufacturer data, impedance, temperature rise, protection coordination, and local electrical requirements.