POWER TRANSFORMER TEMPERATURE RISE AND THE THERMAL CAPABILITY BASIC AND TUTORIALS

Transformer KVA ratings have been alluded to on a number of occasions up to this point without explaining how the KVA rating is determined.

The KVA rating of a transformer is simply the steady-state KVA load applied to the output of the transformer at the voltage rating of the output winding that produces an average winding temperature rise (above the ambient temperature) equal to 65°C.

For older transformers, the rated average winding temperature rise was 55°C. Advances in insulating materials allowed a 10°C increase in average temperature.

The temperature rise of the winding is caused by all of the transformer losses that were previously discussed in this chapter. Therefore, the winding temperature is a function of load losses and no-load losses.


The thermal capability of a transformer is defined in a slightly different way from the rated KVA. Thermal capability is the KVA load applied to the output of a transformer that causes the hottest area in the windings, called the winding hot spot, to reach some limiting temperature.

The hot-spot temperature determines the rate of loss of life of the transformer as a whole, which is a cumulative effect. Therefore, the hot-spot temperature limit is usually based on a loss-of-life criterion.