Transformer kVA sizing equation
Transformer sizing starts by converting the connected load to kVA, then applies demand, growth, loading margin, and ambient derating.
where:
- Sconnected
- Connected apparent load[kVA]
- P
- Connected real power[kW]
- PF
- Power factor
- hp
- Motor horsepower input
- η
- Motor efficiency as a decimal
- D
- Demand factor as a decimal
- G
- Future growth allowance as a decimal
- Ltarget
- Target transformer loading as a decimal
- Kambient
- Ambient temperature derating factor
- Sreq
- Required transformer rating before standard rounding[kVA]
- Sstd
- Selected standard transformer rating[kVA]
- Isec
- Secondary full-load current[A]
- Vsec
- Secondary voltage[V]
For single-phase transformers, the current equation removes √3. Final selection should also check impedance, inrush, harmonics, cooling class, and enclosure conditions.
How to calculate transformer size
Transformer sizing converts the connected load to apparent power in kVA, applies demand and future-growth allowances, divides by the target loading and ambient derating factors, and then rounds up to a listed standard transformer rating.
- Choose single-phase or three-phase and the preferred rating series.
- Enter connected load in kW, kVA, or motor horsepower.
- Apply power factor and motor efficiency where required.
- Apply demand, growth, target loading, and ambient conditions.
- Check inrush, harmonics, impedance, cooling, and protection before procurement.
Transformer sizing example
For a 200 kW three-phase load at 0.85 power factor, 70% demand, 20% growth, 80% target loading, and a 40°C ambient assumption using the calculator's simplified derating, the required capacity is about 275 kVA. The next ANSI reference rating is 300 kVA.
Sconnected = 200 / 0.85 = 235.3 kVA
Sreq = 235.3 × 0.70 × 1.20 / (0.80 × 0.90) = 274.5 kVA → 300 kVA
| Input or result | Value |
|---|---|
| Connected load | 200 kW |
| Power factor | 0.85 |
| Demand / growth | 70% / 20% |
| Target loading | 80% |
| Preliminary rating | 300 kVA ANSI reference |
kW vs. kVA in transformer sizing
Transformers are rated in kVA because their thermal loading depends on voltage and current rather than load power factor alone. A kW load must therefore be converted to input kVA using power factor and, for mechanical motor output, efficiency.
| Load input | Conversion basis |
|---|---|
| kVA | Use apparent power directly |
| kW | Divide by power factor |
| Motor hp | Convert hp to kW, then divide by efficiency and power factor |
Transformer sizing factors that need engineering review
Load diversity, motor starting, cyclic duty, harmonic currents, nonlinear loads, future expansion, cooling class, enclosure, altitude, ambient temperature, voltage taps, impedance, and redundancy strategy can all change the final transformer selection.
Use transformer current for downstream design
After selecting a preliminary kVA rating, calculate secondary full-load current and available short-circuit current. These values are starting inputs for breaker, cable, busbar, and panel checks.
Short Circuit Current CalculatorCircuit Breaker Size CalculatorBusbar Current Rating Calculator
Assumptions
- Ambient derate uses an approximate 1% capacity reduction per C above 30 C
- Target loading represents normal operating margin, commonly 75-80%
- Standard rating series is a planning reference
Important Warnings
- Final transformer selection must check inrush, impedance, harmonics, cooling class, enclosure, temperature rise, and applicable standards.
- Do not use this as a certified transformer specification.