Transformer Sizing Calculator
Enter load kW, PF and voltages to get the required kVA with margin, plus primary and secondary currents for protection sizing.
Last updated: 2026-09-15
How the calculation works
- Load kW converts to kVA through the power factor — transformers are rated in kVA because heating depends on current, not phase.
- The margin inflates the requirement for continuous loads and future growth.
- Primary and secondary currents follow from the same kVA at each voltage, for conductor and protection sizing.
Formula
kVA = kW / PF Required kVA = load kVA × (1 + margin) I_primary = kVA × 1000 / V_primary
| Symbol | Meaning | Unit |
|---|---|---|
S | Apparent power rating | kVA |
PF | Power factor | — |
I₁, I₂ | Primary / secondary current | A |
Worked example
An 8 kW load at PF 0.85 needs 9.4 kVA; with 25% margin, 11.8 kVA → choose the standard 15 kVA transformer. At 480/240 V: primary 31 A, secondary 62.5 A — a 70 A secondary breaker and 4 AWG secondary conductors fit.
Interpreting the result
Transformers are rated in kVA, not kW, because their heating depends on current regardless of the load's power factor. A low-PF load forces a bigger transformer for the same useful power — another reason PF correction pays. Standard sizes exist at 15, 25, 37.5, 50, 75 kVA... so round up.
Assumptions
- Single-phase transformer; three-phase sizing divides by √3 in the current formulas.
- Load is steady; motor starting inrush is a separate check.
Limitations
- Does not cover inrush/energization protection (NEC 450 rules apply).
- Ambient temperature and altitude derating for dry-type units is not applied.
- Harmonic-rich loads (K-factor) need specially rated transformers.
Frequently asked questions
What size transformer for a 10 kW load?
At PF 0.9 with 25% margin: 10/0.9 × 1.25 = 13.9 kVA → the standard 15 kVA unit.
Why are transformers rated in kVA?
Transformer heating comes from copper loss (I²R) and core loss, both driven by voltage and current magnitude — not by the load's phase angle. kVA captures that regardless of PF.