Busbar equipment rating

[ballenden]

Hello,

We have two 15MVA star-star transformers terminating on two 11kV busbars with a bus-tie. Full load current for the transformers are 787Amps. Assuming the load is spread evenly between the two buses, how would the bus-tie CB and bus-equipment be rated?

Thanks.

 

[rbulsara]

It depends...

If the loads are not redundant on the two bus, that if one bus fails, remaining bus does not carry all the loads then the usually the tie circuit could be rated for 2/3 or 67% of the full bus rating.

If the loads are redundant, tie should be sized for full load.

Main Busses should be sized for full load plus the margin as either main could carry the full load.