When paralleling single-phase transformers for greater capacity, which condition must be identical?

• A. Frequency ratings are identical
• B. Tap settings are identical
• C. Phase rotation is identical
• D. Load capacity is identical

Answer: (B)

When you parallel single-phase transformers to increase capacity, the voltages from both units must be in step with each other. The way to guarantee that is to set their voltage taps identically, so both transformers develop the same secondary voltage under load. If the tap settings differ, one transformer can push a different voltage into the shared bus than the other, causing circulating currents between windings, extra heating, and uneven loading. Matching the taps aligns the turns ratio, allowing the units to share the load properly. Of course, other requirements like the same frequency rating and identical phase rotation are also necessary for safe paralleling, but the immediate factor that ensures proper voltage sharing is having identical tap settings.