ANSI standards collection for the C57 series sets forth the general requirements for the design of power
transformers. With respect to loading, the requirements of concern are those that define the transfor-
mer’s thermal characteristics. The average ambient temperature of the air in contact with the cooling
equipment for a 24-hr period shall not exceed 308C, and the maximum shall not exceed 408C (IEEE,
1994). For example, a day with a high temperature of 358C and a low of 258C has an approximate
average ambient temperature of 308C.