Equivalent Ambient Temperatures for Loading Transformers

Safe overloads for transformers can be determined by a modification of the American Standards Association rule for overloading to permit the use of an equivalent ambient temperature, which will result in the same loss of life as for continous operation at 30 degrees centigrade ambient and full load. Based on the rule that aging of insulation doubles for each eight degrees centigrade increase in temperature, methods are developed for calculating the equivalent ambient temperature based on United States Government Weather-Bureau records. The equivalent ambient temperature for each month is calculated as the sum of the greatest recorded monthly mean temperature plus a daily correction factor based on the greatest recorded daily range, plus a monthly correction factor based on the greatest recorded monthly range. Monthly equivalent ambient temperatures so obtained can be used in combination with load curves to determine the permissible safe load for a transformer for a given locality.