When to Use This Calculator
Use this calculator when voltage and current are known on the same side of a transformer and you need the apparent-power load in kVA. It is useful for loading checks, preliminary sizing, and comparing measured current with a transformer rating.
Enter Values
Formula Summary
Single-phase kVA = V × A ÷ 1000
Three-phase kVA = √3 × V × A ÷ 1000
For three-phase systems, use line-to-line voltage and line current.
Practical Examples
480 V panel load
Three-phase, 480 V, 100 A
kVA = 1.732 × 480 × 100 ÷ 1000
kVA = 83.14 kVA
Single-phase secondary load
Single-phase, 240 V, 50 A
kVA = 240 × 50 ÷ 1000
kVA = 12.00 kVA
How to Apply the Result
Transformer ratings are normally expressed in kVA because heating depends largely on voltage and current rather than the load's power factor. Select the phase type, then enter the rated voltage and current for the same side of the transformer.
For a three-phase transformer, enter line-to-line voltage and line current. Do not mix primary voltage with secondary current.
Sizing Considerations
- Account for continuous-load requirements and expected future load growth.
- Check nonlinear load, harmonic, temperature, altitude, enclosure, and ventilation requirements.
- Verify primary and secondary protection, conductor sizing, grounding, and available fault current separately.
The calculated kVA is the load requirement at the entered operating point, not automatically the correct standard transformer size.
Common Mistakes
- Mixing primary voltage with secondary current.
- Using line-to-neutral voltage in the three-phase formula instead of line-to-line voltage.
- Comparing real power kW directly to a transformer kVA rating.
- Ignoring continuous load, ambient temperature, harmonics, ventilation, or future load growth.
Reference Notes
The formulas are standard apparent-power relationships for single-phase and balanced three-phase systems. Final transformer selection should follow manufacturer data, nameplate ratings, applicable electrical code requirements, and project load criteria.