Why is the insulation rating for some distribution transformer set at 220°C and for others the rating is 200°C?
Most standard ventilated distribution transformers use a 220°C insulation system. This insulation system provides a 150°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.
However, if a transformer is wound using copper wires, a few of the smaller frame sizes (15kVA and 30kVA three phase, 15kVA and 25kVA single phase) utilize a 200°C insulation with a 130°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.
The differences in these smaller kVA sizes using copper wire are the result of U.L. ratings of the wire’s insulation temperature rating. Copper wire has a significantly smaller diameter than an equivalent ampacity aluminum wire.
Small copper wire with 220°C insulation is not always available, so insulation systems are limited to 200°C (Note: U.L. does not cover 200°C insulation systems for units greater than 1.2kV, they will use 180°C insulation systems).
Small control and potted transformers have insulation systems well below 220°C because of the resin used. As a result, transformer manufactures generally rate transformers using copper conductors 30kVA and below as having a 200°C insulation system.
Manufacturers compensate for this by building the transformers to run cooler at full load and as a result have a lower 130°C temperature rise and can operate in a 40°C environment.
Because both units can be operated in a 40°C ambient, we say copper transformers 30kVA and below with a 130°C temperature rise are equivalent to larger units with a 150°C temperature rise.