For the parallel operation of single phase transformers it is necessary that they should have

The correct answer is C. same kVA rating.

The other options are not necessary for the parallel operation of single phase transformers.

• A. same efficiency: The efficiency of a transformer is not a critical factor for parallel operation. Transformers with different efficiencies can be operated in parallel as long as they have the same kVA rating.
• B. same polarity: The polarity of a transformer is the direction of the current flow in the primary and secondary windings. Transformers with different polarities cannot be operated in parallel because they will create a circulating current that will overheat the transformers.
• D. same number of turns on the secondary side: The number of turns on the secondary side of a transformer determines the output voltage. Transformers with different numbers of turns on the secondary side can be operated in parallel as long as they have the same kVA rating.

The reason why it is necessary for transformers to have the same kVA rating for parallel operation is that the current in each transformer must be equal. If the transformers have different kVA ratings, the current in the smaller transformer will be higher than the current in the larger transformer. This will cause the smaller transformer to overheat.