Basic Conditions
1. The basic loading of a transformer for normal life expectancy is continuous loading at rated output when operated under normal service conditions.

It is assumed that operation under these conditions is equivalent to operation in a continuous ambient temperature of 30 C for cooling air or 25 C for cooling water.

Normal life expectancy will result from operating with a continuous hottest-spot conductor temperature of 110 C (or equivalent variable temperature with 120 C maximum) in any 24-h period.

2. The hottest-spot conductor temperature determines loss of life due to loading. This temperature cannot be directly measured on commercial designs because of voltage hazard when placing a temperature detector at the proper location.

The hottest-spot allowances are based on tests of laboratory models.

3. The hottest-spot temperature at rated load is usually taken as the sum of the average winding temperature and a 15 C allowance2 for hottest spot.

For mineral oil-immersed transformers operating continuously under the foregoing conditions with normal life expectancy, this temperature has been assumed to be a maximum of 110 C.

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