When paralleling transformers, the percent impedance of one must be within what range of the other?

• A. 80-90%
• B. 95-100%
• C. 92.5-107.5%
• D. 110-120%

Answer: (C)

When two transformers are connected in parallel, they behave like two voltage sources with their own internal impedances. The load current splits between them in proportion to the opposite of their impedances: the unit with the smaller impedance will carry more current. To share load reliably and avoid large circulating currents between the units, their percent impedances must be close to each other. A common practical guideline is to keep the two percent impedances within about seven and a half percentage points of one another, which corresponds to a range of about 92.5% to 107.5% of the rated impedance. This keeps the current sharing near equal, so both transformers run within their limits and voltage regulation stays acceptable. For example, if one transformer has 100% impedance and the other 92.5%, the load currents will be roughly in the 48% and 52% range, rather than one unit taking most of the load. Extending far beyond this tolerance would lead to significant imbalance and potential overheating from circulating currents.