When transformers are banked or paralleled, the impedances must be within plus or minus 10% of each other. What is the purpose of this requirement?

• A. Ensure even load sharing
• B. Increase overall impedance
• C. Decrease safety margin
• D. Increase magnetic flux

Answer: (A)

When transformers are banked or paralleled, the goal is to have them share the load evenly. In parallel operation, current splits between units according to their impedances: the transformer with lower impedance would naturally take more current, risking overloading that unit and causing uneven voltage drops among the transformers. By keeping impedances within a tight tolerance, such as plus or minus 10%, the current split stays near equal even as load varies. This minimizes circulating currents, keeps voltages balanced across the bank, and improves reliability and efficiency.

This isn’t about increasing overall impedance, reducing safety margins, or changing magnetic flux. Those effects aren’t desirable for sharing load or maintaining stable operation.