Bus duct used as tie between switchgears

[Sean_Carb]

We have (2) 6000A switchgears, an A side and B side, fed from different sources. They are designed to have a bus duct that connects them, via a tie breaker located on the A side switchgear. This will provide redundancy in the event of a power loss - one switchgear can supply the other. With this design intent, the 6000A rating of each switchgear represents the combined calculated load of both gears. The EOR has sized the bus duct connection between the gears at 6000A as well, which seems unnecessary. By design, the bus would only ever see the load of either one gear, or the other. There is no scenario where the bus would ever take on the full load of both gears. Therefore, the bus should only need to be rated according to whichever gear has the highest calculated load. Correct?

Appreciate the input everyone.

 

[petersonra]

What protects this bus?

 

[Sean_Carb]

What protects this bus?

The tie breaker on the A side switchgear.

 

[petersonra]

The tie breaker on the A side switchgear.

And what is it rated at?

 

[Sean_Carb]

And what is it rated at?

currently rated at 6000A as well. Understand that the rating for this breaker would need to be reduced in order to reduce the rating of the bus, however this does not change the fact that the bus is always connected to a 6000A bus on the B side.

 

[electrofelon]

I don't see any reason why the bus duct can't be reduced in size based on the calculated load (greater of each switch gear) if properly protected per 368(A).....but does it have protection at both ends?

Also 368(B) exception could possibly be applied.

 

[d0nut]

When you say the switchgear is redundant, do you mean that the load on each is 3000A or less and the entire load can be fed from a single side, or is the load on each greater than 3000A and there has to be some manual load shedding before the tie breaker can be closed? The tie may be oversized in the first scenario, but not in the second. Also, the tie could be sized to match the switchgear rating for future loads that may make the switchgear not fully redundant.

 

[petersonra]

I don't see any reason why the bus duct can't be reduced in size based on the calculated load (greater of each switch gear) if properly protected per 368(A).....but does it have protection at both ends?

Also 368(B) exception could possibly be applied.

Presumably you are referring to 368.17 (A) and (B).

Why would it need protection at both ends? It is protected from one end by the CB and at the other end it is effectively a feeder tap.

368.17 (B) is an interesting provision. basically you can have 50 feet of bus with no overcurrent protection, just short circuit protection.

 

[electrofelon]

Presumably you are referring to 368.17 (A) and (B).

Why would it need protection at both ends? It is protected from one end by the CB and at the other end it is effectively a feeder tap.

368.17 (B) is an interesting provision. basically you can have 50 feet of bus with no overcurrent protection, just short circuit protection.

Good point, didn't think about using the tap rules for one of the "directions".