U.S. Code of Federal Regulations
Regulations most recently checked for updates: Jan 24, 2020
(a) The temperature rise, based on an ambient temperature of 40 degrees C, must not exceed the following:
(1) For Class A insulation, 55 degrees C.
(2) For Class B insulation, 80 degrees C.
(3) For Class F insulation, 115 degrees C.
(4) For Class H insulation, 150 degrees C.
(b) If the ambient temperature is higher than 40 degrees C, the transformer must be derated so that the total temperature stated in this section is not exceeded. The temperature must be taken by the resistance method.