An electrician is planning to parallel two single-phase transformers to increase capacity. Both transformers are 25 kVA, 600V primary to 120/240V secondary. Transformer A has a percent impedance of 3.5%, and Transformer B has a percent impedance of 4.0%. All other characteristics (voltage ratio, polarity, phase rotation) are identical. What issue is most likely to occur when paralleling these transformers with different impedances?

Question: An electrician is planning to parallel two single-phase transformers to increase capacity. Both transformers are 25 kVA, 600V primary to 120/240V secondary. Transformer A has a percent impedance of 3.5%, and Transformer B has a percent impedance of 4.0%. All other characteristics (voltage ratio, polarity, phase rotation) are identical. What issue is most likely to occur when paralleling these transformers with different impedances?

Answer options:

• Reduced overall efficiency of the parallel bank.
• An increase in secondary voltage due to circulating currents. ✅ Unbalanced load sharing, with the lower impedance transformer carrying a disproportionately higher share of the load.
• Increased voltage drop across the secondary terminals.

Correct answer: Unbalanced load sharing, with the lower impedance transformer carrying a disproportionately higher share of the load.

Explanation: When transformers with different percent impedances are paralleled, the transformer with the lower impedance will attempt to supply a larger share of the load current. This can lead to the lower impedance transformer becoming overloaded, even if the total load is within the combined kVA rating of both transformers.