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What are the overload capacities of cast coil transformers?

Cast coil transformers typically come with two insulation systems. 155C is common for IEC rated transformers while 185C is required for U.L. listed units. The 185C insulation system offers additional overload ratings over the 155C system. Assuming a maximum ambient of 40C and average of 30C, a 185C insulation system is typically rated at 115C temperature rise. When built with a 100C temperature rise, the unit will have an overload capacity of 6%. At an 80C temperature rise, the unit will have an overload capacity of 17%.

A cast coil transformer using a 155C base insulation system has a base 80C temperature rise. So, at 80C, there would be no additional overload capacity.