A 25 KVA transformer has a ratio of 27.5 to 1. If the primary voltage is 13,200 V, what is the rating of the secondary in KVA?

• A. 1.89
• B. 3.28
• C. 25
• D. 7620

Answer: (C)

The key idea is that a transformer’s power rating in kVA is a capacity limit that applies to both windings. In other words, the amount of apparent power it can transfer doesn’t change just because the voltage is stepped up or down. The relation S = Vp Ip = Vs Is must hold, ignoring losses.

Given a turns ratio of 27.5 to 1 and a primary voltage of 13,200 V, the secondary voltage is Vs = 13,200 / 27.5 ≈ 480 V. The transformer is rated for 25 kVA, so on either winding you can’t exceed 25,000 VA. That means the secondary current would be Is ≈ 25,000 / 480 ≈ 52 A, but the important point is the rating itself remains 25 kVA on the secondary side as well.