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Transformers are designed to carry their normal rated load in specific ambient temperatures with a maximum stated temperature rise for normal life. If ambients or temperature rises are exceeded under operating conditions, the transformer life may be decreased. If lower temperatures occur, the transformer life may actually be increased. Overload capabilities of transformers are indicated in ANSI/IEEE C57.91, ANSI/IEEE C57.92, and ANSI/IEEE C57.96.

(1) Ambient temperatures. The ambient temperature for an air-cooled unit should not exceed 40 degrees C; and the average temperature for any 24-hour period should not exceed 30 degrees C; with

a minimum ambient temperature for dry-type units of not less than minus 30 degree C. These restrictions apply if the transformer is to provide its normal life expectancy.

(2) Insulation temperature ratings. Liquid-immersed transformers are rated 65 degree C rise or 55/65 degree C rise. Dry-type transformers are rated 150 degree C rise, 115/150 degree C rise, or 80/115 degree C rise. The lower the temperature rise the lower the rated full-load capacity.

(3) Altitude. The dielectric strength of transformers, which depend in whole or in part upon air for insulation, decreases as the altitude increases due to the effect of decreased air density. This applies to liquid-immersed transformers as well as dry-type transformers.

e. Location. Transformers can be classified by their location, but only those which are most often installed on military facilities are covered below.

(1) Outdoor transformer.

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