Under which condition is a transformer's rating additive?

• A. When the transformer is 200 kVA or smaller and the primary voltage is 8660 volts or below.
• B. When the transformer is 200 kVA or smaller and the primary voltage is above 8660.
• C. When the transformer is larger than 200 kVA and the primary voltage is 1000 volts.
• D. When any transformer is used for isolation.

Answer: (A)

When multiple transformers are connected in parallel, their kVA ratings can be added only if they will share the load evenly. This is typically true for small distribution transformers up to 200 kVA that have a primary voltage of 8,660 V or less. At these lower voltages and modest sizes, the impedances are such that each unit carries a proportional portion of the load, so the total rating is the sum of the individual ratings.

If the voltage is higher than 8,660 V or the transformers are larger than 200 kVA, unequal impedance can cause one unit to take more load than another, making simple addition unreliable. Isolation transformers, on the other hand, are not intended to be paralleled for increasing rating.