What Are Transformer No-Load and Load Losses?

Published 27 Aug 2026Updated 27 Aug 20266 min readIntermediateDesign
Quick answer — No-load (iron) losses are the losses in the core from magnetisation, present whenever the transformer is energised, dominated by hysteresis and eddy current loss. Load (copper) losses are the winding I2R and stray losses that rise with the load current, plus the auxiliary (fan/pump) losses when running. Efficiency is the output over the input, so it varies with load.
Contents
What are transformer no-load and load losses?

No-load losses

These come from the alternating flux in the core (hysteresis) and the eddy currents in the laminations. They depend on the core material (e.g. CRGO, amorphous), the flux density and the core construction, and are essentially constant at rated voltage.

Load losses

The winding conductor I2R loss plus stray loss in the tank, leads and core, which scale with the square of the current. The auxiliary losses from fans and oil pumps add when the cooling is running.

Efficiency and economics

Efficiency is output divided by input. It peaks where load loss equals no-load loss, at a load fraction of √(P₀/Pk): about 30–45% for distribution transformers and nearer 40–60% for power transformers, not at full load (IEC 60076-20 peak efficiency index). It falls at light load because the no-load losses dominate. Buyers capitalise losses over the transformer life to compare suppliers — lower losses often justify a higher price.

References & standards

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Related manufacturers

Acme Electric (Hubbell) · Central Moloney · CORE Transformers · Daelim Transformer (US) · Delta Star · Eaton

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Conductors & core steel Transformer cooling systems
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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