What effect does ambient temperature have on transformer operation?
A. Other conditions that need be considered when sizing a transformer to a motor load are ambient temperature (derate the transformer nameplate rating by 8% for ever 10C above 40C), altitude (derate nameplate rating by 3% for every 300 feet above 3300 feet), and motor loads that frequently start and stop. If a motor starts several times an hour (such as an air conditioner), the calculated transformer size required should be increased by 20% to offset the effects of inrush heating. If the motor starts very frequently (such as an elevator), the service factor of the load must be used to calculate the proper transformer size. If the service factor of the load is 1.25, the calculated transformer size should be increased by 25%. Remember: Each 10oC over the rated temperature rise cuts the life of your transformer by one half.
