Transformer Size Calculator

Find the transformer kVA for a load, with primary and secondary current.

What Do You Want to Find?
Values from the load schedule.
–
Transformer size
Load kVA
With growth allowance
Primary full-load current
Secondary full-load current
Work

Saved Calculations

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Transformer Sizing Formulas

Single-phase kVA = V × A ÷ 1,000   Three-phase kVA = V × A × √3 ÷ 1,000   kVA = kW ÷ power factor
Full-load amps = kVA × 1,000 ÷ (V × √3 for three-phase)

Standard dry-type sizes (three-phase): 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 kVA; single-phase: 5 to 500 kVA.

Worked Example

Tips

Overcurrent protection and conductor sizing for transformers follow NEC 450.3 and 240.21(C). Harmonic-heavy loads may need a K-rated transformer. Confirm with an engineer or electrician.