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What temperature (how hot) can the external enclosure surface of Low Voltage Transformers reach?

The temperature reached by an Energized Enclosed Transformer can be an important quantity, as it contributes heat to the surrounding environment
Product Line:
LV Transformers
Applies to Ventilated Transformers and Sealed Resin Epoxy Encapsulated Transformers 
Energized Enclosed Transformers generate heat which is transmitted to the surface of the Enclosure
Underwriters Laboratories standards limit the external surface temperature of transformer enclosures to 50degC rise above ambient for ventilated enclosures and 65degC rise above ambient for epoxy resin filled transformers. Since the transformer core loss is present even under no-load, some parts of the enclosure will come near this limit even under the condition of light loading, or even no-load. In a typical ambient of 25degC (77degF) it`s not unusual for the outside surface at the top of a ventilated transformer to reach 70degC (158degF) with no load on the transformer. Epoxy resin transformer enclosures can reach even higher temperatures approaching 85degC (185degF) even at no-load.

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