Rating of outgoing OCPDs

[m sleem]

I am confused, one of the projects came to me to apply value engineering, i found some of the outgoing breakers are larger than the main breaker due to parallel operation of the transformers, it's first time for me seeing that design, anything wrong here??

Appreciate your feedback.

 

[topgone]

Can't say much with so little info there. Any sketch or some representative figures?

 

[m sleem]

Please see this scheme

 

[topgone]

There's nothing wrong there! Your total feed capacity is 2,165A X 3 = 6,495A. Your outgoing feeder breaker is just 4,000A
What's important is for you to check if the AIc ratings of these breakers can withstand the total available fault that could occur on this bus.

 

[m sleem]

There's nothing wrong there! Your total feed capacity is 2,165A X 3 = 6,495A. Your outgoing feeder breaker is just 4,000A
What's important is for you to check if the AIc ratings of these breakers can withstand the total available fault that could occur on this bus.

My first concern regarding the coordination, the 4000a ocpd suppose to trip before 2500a.

Secondly, your observation is correct, the 36ka shall be updated to 100 ka.

 

[topgone]

My first concern regarding the coordination, the 4000a ocpd suppose to trip before 2500a.

Secondly, your observation is correct, the 36ka shall be updated to 100 ka.

On overcurrent, your system will coordinate if you set your relays properly.
If you can't coordinate on the instantaneous trip, please check if you can disable the Inst on the 400V main breakers (2500A)--> let the primary protection of the transformer take care about short-circuit protection!