When paralleling single-phase transformers for greater capacity, which conditions must be met?

• A. Voltage ratings are identical
• B. Tap settings are identical
• C. Percent impedance of one is between 92.5% and 107.5% of the other
• D. Frequency ratings are identical

Answer: (D)

When paralleling single-phase transformers, the system frequency must be identical. If the transformers are rated for different frequencies, their winding reactances at the operating frequency will differ, causing uneven current sharing and potential circulating currents between the units. Matching the frequency rating ensures both transformers present similar impedances and respond in the same way to the same AC waveform, so they share the load consistently. In practice, you also check that voltage ratings and impedance are close and taps are aligned, but the fundamental requirement to parallel safely is identical frequency ratings.