Formula used
Single-phase apparent power
kVA = (V × I) ÷ 1000
Three-phase apparent power
kVA = (√3 × V × I) ÷ 1000
Load entered in kilowatts
kVA = kW ÷ power factor
Horsepower conversion
kW = (HP × 0.746) ÷ efficiency
Final required kVA = max(diversified running kVA, running kVA + largest motor starting increment) ÷ effective derating × safety × future growth × spare allowance
How to use this calculator
- Select single-phase or three-phase operation.
- Enter primary and secondary voltages in volts or kilovolts.
- Choose the sizing mode that matches your design task.
- Enter a quick total load or build a detailed load schedule.
- Mark continuous loads and identify motor loads.
- Enter realistic motor starting multipliers and simultaneous-use percentages.
- Apply demand, diversity, safety, future-growth, and spare-capacity values.
- Add ambient temperature, altitude, harmonics, and custom derating.
- Review the recommended kVA, currents, loading, losses, and warnings.
- Verify final selections against codes, utility rules, and manufacturer data.
Example data
| Input | Example | Purpose |
|---|---|---|
| System | Three-phase, 11 kV / 415 V | Defines current formulas and transformation ratio. |
| General load | 30 kW, PF 0.90 | Represents process or facility demand. |
| Motor load | 15 kW, PF 0.85, 6× starting | Checks voltage-sensitive starting duty. |
| Demand / diversity | 90% / 1.10 | Estimates coincident maximum demand. |
| Growth / safety / spare | 20% / 15% / 10% | Provides planning and expansion allowance. |
| Environment | 35°C, sea level, 20% nonlinear | Applies simplified derating guidance. |
Transformer sizing guidance
Transformer selection begins with the maximum coincident apparent power, not only the sum of equipment nameplate watts. Power factor converts real power into apparent power, while efficiency affects horsepower loads. A three-phase system also uses the square-root-of-three relationship between line voltage, current, and kVA.
Continuous loads can require additional capacity because transformers and associated conductors may operate for long periods. This calculator applies a 125 percent multiplier when a load row is marked continuous. The final code treatment still depends on the connected equipment, duty cycle, jurisdiction, and installation method.
Motor starting can create a short, severe kVA requirement that exceeds normal running demand. Direct-on-line starting usually produces the largest current, while soft starters and variable-frequency drives can reduce the transient. The calculator adds the largest motor starting increment to the diversified running load for a practical planning check.
Demand factor represents the portion of connected load expected at the same time. Diversity factor recognizes that individual peak loads may not occur together. These values should come from measured data, design standards, utility guidance, or an experienced electrical engineer rather than arbitrary assumptions.
Ambient temperature, altitude, enclosure ventilation, and nonlinear loads can reduce usable transformer capacity. Harmonic-rich loads may also increase winding, neutral, and stray losses. A K-rated transformer, larger neutral, harmonic filter, or other mitigation may be necessary after a detailed power-quality study.
Oversizing can improve growth capacity but may increase capital cost and no-load losses. Undersizing can cause overheating, nuisance trips, poor voltage regulation, and shortened insulation life. The recommended standard size therefore balances calculated demand, motor starting, derating, future expansion, safety margin, and spare capacity.
Primary and secondary protection estimates are preliminary values based on full-load current. Actual fuse, circuit-breaker, cable, grounding, and fault-duty selections depend on applicable electrical codes and coordination studies. Always check manufacturer impedance, temperature-rise, inrush, efficiency, sound, and enclosure data before purchase.
The annual loss estimate uses a simplified efficiency relationship at the calculated real load. Real transformer losses vary with voltage, loading, temperature, core construction, and harmonics. Manufacturer no-load and load-loss curves provide better lifecycle cost comparisons for procurement decisions.
Frequently asked questions
What does kVA mean?
kVA means kilovolt-amperes. It measures apparent power before power factor is applied.
Why is transformer capacity rated in kVA?
Transformer heating mainly depends on voltage and current. Therefore, the rating is generally independent of the connected load power factor.
How do I size a transformer from kilowatts?
Divide kilowatts by power factor, then apply demand, motor starting, derating, and design allowances.
Should continuous loads receive extra capacity?
They often require special treatment. This calculator uses 125 percent when a detailed load is marked continuous.
How is three-phase current calculated?
Current equals kVA multiplied by 1,000, divided by line voltage and the square root of three.
What transformer size should be selected?
Select the next standard rating at or above the final adjusted requirement, subject to engineering verification.
Why does motor starting matter?
Starting current may be several times running current and can cause excessive voltage dip or protection operation.
What is transformer derating?
Derating reduces usable capacity because of temperature, altitude, harmonics, ventilation, or other operating conditions.
What is a K-factor transformer?
It is designed to tolerate additional heating caused by specified harmonic current content from nonlinear loads.
Can this calculator replace an electrical engineer?
No. It provides planning estimates and should not replace code review, protection coordination, or manufacturer selection guidance.