How Are Transformer Radiators Designed?

Published 27 Aug 2026Updated 27 Aug 20267 min readIntermediateDesign
Quick answer — Transformer radiators (fin cooling banks) are sized so they can reject the transformer total losses at the rated temperature rise, by providing enough heat-transfer area and a free flow of cooling air. The design balances thermal (losses to be rejected), drag/pressure (oil must flow through the bank), and mechanical/space constraints — and the cooling code (ONAN vs ONAF) sets whether fans are needed to boost the airside.
Contents
How are transformer radiators designed?

The thermal duty

The radiator must reject the sum of no-load and load losses (total loss) at the design oil temperature rise. More loss or a lower allowed temperature rise means more area. Radiator banks are rated in kW at a defined oil-to-air temperature difference.

Sizing for oil flow

For the oil to move by natural convection (ONAN) the radiator must be dimensioned so the thermosiphon flow is maintained — tube/fin spacing, height and inlet/outlet taps matter. Adding fans (ONAF) raises the airside coefficient so less total area is needed for the same heat rejected.

Auxiliaries and control

Fans and (for OFAF) pumps are sized, sequenced and often controlled by winding or oil temperature to match the load, with loss-of-auxiliaries handling and default positions for safe operation. Radiator/pipework is designed to avoid gas pockets and to give even distribution.

References & standards

Related

Book seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

Related components

Transformer cooling systems Tank & mechanical
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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