Subtractive polarity is standard for all single-phase transformers 200 kVA and below having high-voltage ratings above ________ volts?

• A. 9000 volts
• B. 7500 volts
• C. 8500 volts
• D. 8000 volts

Answer: (A)

Understanding polarity in transformers helps you see how windings relate in phase when you connect them in circuits. Subtractive polarity means the secondary voltage is opposite in phase to the primary when the windings are oriented the same way, so if you connect windings in series, the voltages subtract rather than add. This is important because misconnecting polarity can cause circulating currents, overheating, or overvoltages, especially in configurations using high voltages.

For small single-phase transformers (200 kVA and below), a standard is adopted: if the high-voltage rating is above 9,000 volts, subtractive polarity is the default. This convention gives a consistent, safer baseline for wiring, testing, and paralleling or stacking units in the field, where incorrect polarity could lead to dangerous conditions or equipment damage. So the threshold is 9,000 volts.