Transformers ______ kVA and smaller, with a primary rating of 8600 volts and below, will be rated additive.

• A. 100
• B. 200
• C. 300
• D. 400

Answer: (B)

The concept being tested is how transformer banks are rated when paralleled, using additive versus subtractive ratings. When two transformers share a load on the same voltage ratio, the way their impedances interact determines whether the combined rating adds up or subtracts. If the transformers contribute current in the same sense to the bank, the kVA capability is rated additively; if the currents oppose each other in some conditions, the rating is subtractive.

For transformers with a primary rating of 8600 volts (8.6 kV) or below, the standard practice is that those up to a certain size can be considered additive. The specific boundary is 200 kVA. So, 200 kVA and smaller units with primary voltage at or below 8.6 kV will be rated additively. This reflects the typical impedance and sharing characteristics of smaller, low-voltage transformers, which tend to share load in a way that allows their individual kVA ratings to sum safely when paralleled.

Thus, the filled statement reads: Transformers 200 kVA and smaller, with a primary rating of 8600 volts and below, will be rated additive.