Main breaker feed meeting code

[mikeengurs]

We are feeding a main breaker in a MV switchgear line-up by a secondary side of a transformer. We want to use the same feed to feed an other breaker. In doing that, we would be running a cable from source side of main breaker, to the new breaker, instead of feeding the new breaker directly from the transformer. I couldn't find anything in the code agains this design. Does anyone know of any code requirement that is applicable to this situation? Thanks in advance!

 

[rcw retired EE]

What overcurrent or other protection is provided for the conductors?

 

[mikeengurs]

There is overcurrent and ground protection on both main breaker and new beaker. There is also differential protection for transformer. Also I forgot to mention, the main breaker is rated for 2000A, so existing aluminum termination bars where existing cable terminate is also rated for 2000A. We will add new lug/phase on each bar for new cables. Do we have to replace the whole bars for 4000A rating? Can they add more copper plates whe cables terminate to increase ampacity?

 

[jraef]

There is overcurrent and ground protection on both main breaker and new beaker.

I think he meant what is the overcurrent or other protection for the LINE side conductors? The main and new feeder are protecting on their respective load sides.

Also, it appears you have a 2000A main, but what is the size of the conductors feeding that main? If you want to tap off for 400A, your conductors must be capable of 2400A, both for the ones feeding the main and the ones feeding the 400A breaker. Then you have to look at whether or not the 400A breaker can hold those conductors.

By the way, this is what switchgear is for.

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[mikeengurs]

jraef, the switchgear consists of 1200A feeder breakers feeding MCCs. The feeder to the meain breaker, which is connected to the secondary side of the transformer doesn't have over current protection on the source side (the secondaty side of the XFMR). This was installed over 30 years back. It's less than 25ft though, and there is a differential protection.
Going back to the feeder size issue, the new breaker is 1200A. The feeder to main is rated for 3600A (3-1/C #1000 MV-105 copper). The new breaker is 1200A. The feeder to the new breaker is rated for 1200A. Did you mean it has to be rated for 3600A? The load this new breaker is feeding is only 800A.

 

[jghrist]

If the main breaker is included in the transformer differential zone, then you cannot run a cable from the source side of the breaker to a new breaker without adding the new breaker into the differential zone. Otherwise, load on the new breaker or a downstream fault will trip the differential.

 

[mikeengurs]

jghrist, you are correct. We added a CT on the new breaker load, and will parallel that with the one in the main breaker.