UFC 3 -520-01
June 10, 2002
Temperature and kVA.
4-1.3.1 The kVA capacity of a transformer is the output that it can deliver for a
specified period of time, at the rated secondary voltage and rated frequency, without
exceeding a specified temperature rise based on insulati on life and ambient
temperature. Transformers can be loaded above their kVA ratings with no loss of life
expectancy only when operated within the manufacturer's stated limits. Select the
transformer based on its kVA capacity and temperature rating.
4-1.3.2 The rated kVA capacity is based on the maximum current delivered at rated
voltage. The real limit in the transformer's capability is the amount of current that it can
provide without exceeding a defined temperature rise. Dry type transformers are
designed with various insulation types and the rating and loading of a transformer are
based on the temperature limits of the particular system. Note that the transformer's
rated temperature will be reached when it is operated at full load under the
manufacturer's specified conditions, meaning that some caution is warranted in the
selection, application, installation, and loading of a transformer. The following insulation
systems are available (refer to Figure 4 -1):
Class 105--when loaded in an ambient temperature of not over 40 C, will operate
at no more than a 55 C average temperature rise on the winding conductors, with
an added 10 C allowance for a hot spot. The sum of 40 C + 55 C + 10 C
provides the 105 C designation. This insulation class is used only on very small
transformers. Older designs refer to this as a Class A insulation or transformer
Class 150--allows an 80 C rise in the winding plus a 30 C hot spot allowance.
Class 150 insulation is often used in transformers rated up to 2 kVA. Older designs
refer to this as a Class B insulation or transformer rating.
Class 185--allows a 115 C rise in the winding plus a 30 C hot spot allowance.
Class 185 insulation is often used in transformers rated from 3 to 30 kVA. Older
designs refer to this as a Class F insulation or transformer rating. Some documents
refer to a Class 180 rating also.
Class 220--allows a 150 C rise in the winding plus a 30 C hot spot allowance.
Class 220 insulation is commonly used in transformers rated in a ll significant sizes.
Older designs refer to this as a Class H insulation or transformer rating.