In an open transformer bank, the capacity is reduced to what percentage of the combined KVA ratings?

The capacity of an open transformer bank is impacted by the configuration and operation of the transformers. In an open bank, typically one transformer is taken out of service for maintenance or replacement, which affects the overall capacity you can safely utilize. When calculating the effective capacity of an open bank, it is known that the capacity is reduced to approximately 86.6% of the total combined KVA ratings of the transformers in the bank.

This percentage is derived from the understanding that, in a three-phase system, if one transformer is removed, the current is redistributed among the remaining transformers, and some balancing loss occurs. This results in a reduction in overall capacity due to the lack of full-load sharing and the fact that the transformers may not be able to handle the same loads without overheating or stressing them. Thus, in practical applications, 86.6% reflects a safe operating level while still allowing for adequate functionality during maintenance or unexpected outages.

In contrast, the other options represent either higher or significantly lower capacities that do not align with standard practices for operating an open transformer bank configuration.