Who This Guide Is For
Use this guide when you need a practical transformer sizing check from voltage, current, phase, and load type before moving into nameplate, code, and manufacturer requirements.
Formula Summary
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.
Worked Example
A three-phase 480 V load drawing 100 A requires about 83.1 kVA.
kVA = 1.732 × 480 × 100 ÷ 1000 = 83.1 kVA
A designer would normally compare that load with the next appropriate standard transformer size after checking duty, growth, and installation conditions.
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.
Common Mistakes
- Comparing kW load directly to a kVA transformer rating.
- Mixing primary voltage with secondary current in one calculation.
- Ignoring motor starting, nonlinear load, harmonics, ambient temperature, or ventilation.
- Treating calculated load kVA as the final selected standard size.
Reference Notes and Assumptions
These formulas are standard single-phase and balanced three-phase apparent-power relationships. Final sizing should follow manufacturer data, nameplate ratings, adopted electrical code requirements, and project load criteria.