Calculate Load kVA
Single-phase kVA = V × A ÷ 1000
Three-phase kVA = √3 × V × A ÷ 1000
Use voltage and current from the same winding side. For three-phase, use line-to-line voltage and line current.
Turns and Voltage Ratio
Vp ÷ Vs = Np ÷ Ns
An ideal transformer changes current inversely so apparent power is conserved. Real transformers have losses and regulation drop.
Load Characteristics
Continuous loading, future growth, motor starting, transformer inrush, nonlinear current, harmonics, duty cycle, and load diversity can alter the required standard size.
Installation Conditions
Ambient temperature, altitude, ventilation, enclosure, sound level, insulation system, grounding, overcurrent protection, fault current, and efficiency requirements must be checked.
Calculated load kVA is not automatically the correct transformer nameplate size.