Ambient temperatures for overloading transformers

Because of the increasing demand for full utilization of investment by public utilities, the overloading of transformers on a temperature basis is daily becoming a more fundamental problem in engineering. Several methods have been presented in engineering literature by which overloads may be calculated for specified conditions of winding temperature, load cycle, and ambient temperature. The limiting winding temperature can be determined quite definitely from the known relation between temperature and life of insulation, and the load cycle predicted from previous load readings. The major difficulty lies in the determination of an ambient temperature on which to base the calculations of overload capacity.