To bank or parallel transformers, their impedances must be within plus or minus ________% of each other

• A. 5%
• B. 10%
• C. 15%
• D. 20%

Answer: (B)

When transformers are connected in parallel or arranged as a bank, they need to behave like a single source with similar impedance. The impedance of each unit dictates how much current it will contribute for a given voltage. If the impedances are close, the load splits fairly evenly; if one unit has lower impedance, it tends to take more of the load, while the other carries less. This difference causes circulating currents between the units, extra losses, overheating, and unstable voltage regulation under load. To keep current sharing balanced and avoid those issues, a practical tolerance is to have the impedances within about ten percent of each other. A tighter tolerance would be harder to achieve in practice, while a looser tolerance would lead to noticeable imbalance and potential problems.