Transformers' temperature rise values are rated from a maximum ambient of 40ºC with an additional 30ºC for the coil "Hot Spot" allowance. Adding the 150ºC max. temperature rise is why transformers have 220ºC rated insulation...but to operate the transformer at these temperatures will reduce is life considerably.

To overcome the transformer's max. temperature tolerance and increase its life, the transformer's nameplate load rating should be decreased by 8% for every 10ºC above 40ºC ambient.

Additionally, the transformer should be derated 0.3% for every 100 meters (330 feet) for altitudes over 1000 meters (3300 feet). And derated if there are any involved motors that require starting at least twice an hour by 20% for service factors below 1.2 and equal to the service factor above 1.2.

The benefit of derating the transformer (or versely increasing its nameplate size) is to reduce heat via larger coils and less work.

I hope this helps in understanding how the max. temperature rise is utilized.

Volts has an excellent transformer sizing module that goes into more detail about derating and operating conditions and sizing.

edited for spelling errors

[This message has been edited by DiverDan (edited 08-20-2005).]


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