Transformer Sizing Calculator — kVA Rating & Currents
Determine what kVA rating your transformer needs for a given load, the voltage turns ratio, and the primary and secondary line currents — for both single-phase and three-phase systems.
Phase configuration
V
V
kW
%
Minimum apparent power rating needed to serve this load
- 1
Load apparent power (kVA = kW ÷ PF)
100 ÷ 0.85 = 117.6The transformer must handle the full apparent power, not just real power. - 2
Input power = kVA ÷ efficiency
117.6 ÷ (98% ÷ 100) = 120.05 - 3
Turns ratio = V₁ ÷ V₂
11,000 ÷ 400 = 27.5
How does this calculator work?
Divide load kW by power factor to get required kVA. Turns ratio = V₁/V₂. For three-phase, secondary current = kVA×1000/(√3×V₂). Round up to the next standard kVA size and add a 10–25% growth margin. At 98% efficiency, a 100 kVA output needs ~102 kVA input.
Formula
How this is calculated
A power transformer couples two AC circuits magnetically to step voltage up or down. The critical sizing quantity is apparent power in kVA — the real load power (kW) divided by the power factor. Because the transformer winding must carry the full apparent current, it must be rated at least equal to the apparent load demand; undersizing causes overheating and shortened life.
The turns ratio equals the voltage ratio V₁/V₂. By conservation of energy (ampere-turns balance), the secondary winding carries higher current at lower voltage. For three-phase systems the line current equals kVA × 1000 / (√3 × V_line); for single-phase it is kVA × 1000 / V. Efficiency (typically 95–99%) accounts for core (iron hysteresis and eddy-current) losses and copper (winding resistance) losses; a 98%-efficient transformer delivering 100 kVA requires about 102 kVA from the primary.
This calculator assumes balanced load, sinusoidal waveform, and a conventional two-winding transformer. Practical selection rounds up to the next standard kVA size and adds 10–25% headroom for load growth. Derating for harmonics, altitude, ambient temperature, or continuous 100% loading requires additional manufacturer engineering factors.
Frequently asked questions
Always choose the next standard rating above the calculated minimum — common sizes are 25, 50, 100, 160, 250, 315, 400, 630 and 1000 kVA. Add a 10–25% margin for future load growth and inrush tolerance.
A transformer must handle the full apparent power (kVA), not just real power (kW). A 80 kW load at 0.8 PF demands a 100 kVA transformer — 25% more than at unity PF — because the winding current is proportional to kVA.
For three-phase, line current = kVA × 1000 / (√3 × V_line). For single-phase, line current = kVA × 1000 / V. The √3 factor (~1.732) means three-phase conductors carry lower current for the same apparent power — one reason three-phase is preferred for large loads.
Also known as
TG we-Calculate Editorial Team. (2026). Transformer Sizing Calculator — kVA Rating & Currents [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/transformer-sizing-calculator
TG we-Calculate Editorial Team. "Transformer Sizing Calculator — kVA Rating & Currents." TG we-Calculate. 2026. https://we-calculate.com/calculator/transformer-sizing-calculator.
TG we-Calculate Editorial Team, "Transformer Sizing Calculator — kVA Rating & Currents," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/transformer-sizing-calculator
@misc{wecalculate_transformer_sizing_calculator, title = {Transformer Sizing Calculator — kVA Rating & Currents}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/transformer-sizing-calculator}}, year = {2026}, note = {TG we-Calculate} }
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