What the nameplate rating means
A transformer kVA rating states how much apparent power the transformer can carry continuously under its specified conditions. It is based on voltage, current, temperature rise, cooling method, and insulation class.
Because transformer heating depends heavily on current, kVA is more useful than kW for rating capacity across different load power factors.
How to calculate kVA
For single-phase transformers, kVA equals volts times amps divided by 1000. For three-phase transformers, kVA equals line-to-line volts times line current times 1.732 divided by 1000.
After calculating load kVA, compare it with the transformer rating and include margin for future loads, ambient temperature, and continuous operation.
- Single-phase: kVA = V x A / 1000
- Three-phase: kVA = V x A x 1.732 / 1000
- Real power: kW = kVA x power factor
Sizing considerations
Transformer sizing should consider harmonics, nonlinear loads, duty cycle, ventilation, installation environment, and code requirements.
Undersizing can cause overheating and nuisance trips. Oversizing can increase upfront cost and may reduce operating efficiency at very light loads.