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.

The formula

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.

Embed this calculator

Writing about this, or teaching it? Paste the snippet below into any web page to add a working copy of the calculator above. It is free, runs entirely in the reader’s browser, sends nothing anywhere, and links back to this page for the method.

<div class="vf-calc-widget" data-calc="transformer-sizing"></div>
<script src="https://voltfield.org/embed/calculator-widget.js" async></script>

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