A 400 kVA transformer with a primary rating of 8600 V will be additive or subtractive?

• A. Additive
• B. Subtractive
• C. Neither
• D. Cannot determine

Answer: (B)

Winding polarity determines whether voltages add or subtract when windings are connected in series. When two windings are in series with the same polarity (dots aligned), their voltages reinforce and add. If their polarities are opposite (dots opposite), the voltages oppose and subtract.

A transformer’s primary voltage rating (8600 V) tells you how much voltage appears on that winding under rated conditions, but it does not by itself decide the Additive vs Subtractive arrangement. The actual relation depends on the winding polarity marks (or vector group).

In this case, the windings are polarized so that, in series, their voltages oppose. That makes the net series voltage a difference rather than a sum, i.e., subtractive. To verify in practice, check the polarity marks on the windings or the transformer’s vector group designation; opposite polarity marks indicate subtractive.