Transformer Sizing
Size transformers from load kVA, voltage, and power factor.
Transformer Sizing Overview
Transformers are rated in kilovolt-amperes (kVA), which represents apparent power. Real power in kilowatts (kW) depends on the load power factor (PF). Sizing requires both values plus the primary voltage and phase configuration.
Sizing Formulas
$$\mathrm{kVA} = \frac{\mathrm{kW}}{\mathrm{PF}}$$Single-phase primary current:
$$I = \frac{\mathrm{kVA} \times 1000}{V}$$Three-phase primary current:
$$I = \frac{\mathrm{kVA} \times 1000}{\sqrt{3} \times V}$$Line-to-line voltage \(V\) is used for three-phase systems. Power factor ranges from 0.01 to 1.0, with 0.9 typical for mixed commercial loads.
Selection Tips
Choose the next standard transformer rating above the calculated kVA. Allow margin for future load growth and harmonic heating. Verify local electrical codes for overcurrent protection requirements.
Related tools: HP to Amps Calculator and Ton to kW Converter.
Frequently Asked Questions
What is the difference between kW and kVA?
kW is real power consumed by the load. kVA is apparent power, which includes reactive components. kVA equals kW divided by power factor.
What power factor should I use?
Use the actual load power factor when known. Motor-heavy facilities may run 0.8 to 0.85. Resistive heating loads approach 1.0.
Why is three-phase current divided by square root of 3?
In a balanced three-phase system, line current relates to total apparent power through the factor \(\sqrt{3}\). This accounts for the 120-degree phase separation.
Is the voltage line-to-line or line-to-neutral?
For three-phase, enter line-to-line voltage. For single-phase, enter the nominal supply voltage (line-to-neutral for split-phase systems).
Should I add a safety margin to kVA?
Yes. Engineers typically select a standard transformer rating 10 to 25 percent above calculated kVA to handle growth and temporary overloads.
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