Transformer Sizing Calculator
Transformers are rated in kVA — apparent power — while loads are usually described in kW, real power. The difference is power factor, and ignoring it is how a transformer ends up undersized on day one. Once you have the kVA figure you cannot simply order it: transformers are built to a standard series of nameplate ratings, so the practical answer is always the next size up.
Run the numbers
The working calculator is free and needs no signup.
The formula
FLA = kVA × 1000 / (1.732 × V) [three-phase]
FLA = kVA × 1000 / V [single-phase]
| kW | Real power drawn by the load |
| PF | Power factor, between 0 and 1 |
| V | Line-to-line secondary voltage |
| FLA | Full-load amperes at the nameplate rating |
Worked example
A 400 kW load at 0.85 power factor:
kVA = 400 / 0.85 = 470.6 kVA
The standard series runs 300, 500, 750, 1000 — so the unit specified is 500 kVA.
At 480 V three-phase: FLA = 500 × 1000 / (1.732 × 480) = 601 A.
Which standard governs this
Standard nameplate ratings come from ANSI/IEEE C57.12.00. Loading practice beyond nameplate is covered by IEEE C57.91, which governs how far and how long a unit may be run over rating before insulation life is affected.
What this calculation does not account for
This sizes for steady-state load only. It does not account for future growth, harmonic heating, non-linear load, or motor starting, all of which argue for headroom. It also says nothing about impedance, which is the other half of a transformer specification and the one that sets downstream fault current.
Common mistakes
Sizing on kW and never dividing by power factor, which under-sizes by however far PF sits below unity. Sizing exactly to calculated load with no growth allowance. Choosing on kVA alone and leaving %Z to the supplier, which is how switchgear ends up under-rated for the fault current the transformer can deliver.