All Jefferson Electric transformers are designed and manufactured with the best quality insulation available. There are classes of insulation systems for different temperatures as defined by NEMA and ANSI. Insulation classes are rated in °C rise above a specific ambient of 40°C maximum. A transformer having a specific class of insulation, for example Class 220, can have an average winding temperature rise of 150°C with a maximum hot spot temperature rise of 180°C. If the room ambient temperature is 40°C, then the total temperature of the hottest spot would be 220°C.

Jefferson Electric transformers are designed to operate at rated load and voltage in maximum room ambient temperatures of 40°C, average room ambient temperature not to exceed 30°C, and at altitudes not to exceed 3300 feet in accordance with NEMA standards.

Insulating Classifications

The designations for insulation systems are numerical classifications based on temperature ratings. Transformer ratings are based on temperature rise. The accompanying table shows the designations.

Transformer and Insulation Systems Ratings
Insulation RatingTransformer RatingMax. Ambient TemperatureHot Spot Allowance
Class 10555°C Rise40°C10°C
Class 15080°C Rise40°C30°C
Class 180110°C Rise40°C30°C
Class 220150°C Rise40°C30°C


Overloads exceeding the maximum allowable insulation temperature can be tolerated, provided the overload is of short duration and is preceded and followed by a period of operation at less than rated kVA (refer to ANSI C57.96-1989, Tables 5,6,7). Overloading should be avoided unless approval is obtained from the Jefferson Electric engineering department.

High Ambient Temperatures

Ambient temperatures above 30°C average over a 24-hour period and 40°C maximum require either a larger kVA rating or a special low temperature rise transformer. A 150°C rise air cooled transformer can also be derated using the formula of .4% kVA reduction for each degree centigrade above 30°C ambient temperature.

Altitude Corrections

For transformers above 3300 feet, reduce the kVA rating .3% for each 330 feet above 3300 feet.

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