A 200/400 V, 20 kVA, 2-winding transformer is connected as an auto-transformer to transform 600 V to 200 V. Calculate the kVA rating of the auto-transformer.

• A. 60 kVA
• B. 50 kVA
• C. 30 kVA
• D. 25 kVA

Answer: (C)

Here auto transformer change voltage from 600 V to 200 V.
Voltage ratio (a) = 600/200 = 3
kVA rating of the auto-transformer = (a/(a - 1))*(kVA rating of auto transformer)
= (3/2)*20
= 30 kVA