What Is the K-Factor for a Transformer?

Published 27 Aug 2026Updated 27 Aug 20265 min readIntermediateDesign
Quick answer — The K-factor (a nameplate rating on distribution transformers) quantifies the harmonic content a transformer can handle. Nonlinear loads (VFDs, rectifiers, IT) draw a current with significant harmonics, which raises the winding eddy-current loss; a K-rated transformer is derated in its linear load capacity and designed with lower eddy loss so it can serve a load of a given K-factor without overheating.
Contents
What is the K-factor rating of a transformer?

Why harmonics matter

Nonlinear loads produce harmonic currents that flow in the windings and cause additional eddy-current and stray losses. A transformer rated for a pure sine (K-1) may overheat on such a load.

The K-factor

It is a rating describing the harmonic spectrum the transformer can serve: K-1 for a sine wave, K-4, K-9, K-13 and up for increasingly distorted, high-eddy-load currents. It is derived from the harmonic current spectrum.

When it is used

For data centres, VFD-driven motors, industrial rectifiers and similar nonlinear loads, a K-rated (often dry-type) transformer is specified to avoid derating and overheating.

References & standards

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Prolec GE (GE Vernova) · Eaton · Virginia Transformer Corp · Delta Star · WEG Transformers USA · LS Electric America

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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