MCC Vertical Bus rating

[TallTimber]

Long time listener, First time caller.

I am working a job where we are replacing (2) 75Hp pumps with (2) 200HP Pumps on VFD drives. There is an existing Centerline 2100 480V 600A MCC that I was hoping to reuse, but everything I have researched points to this not being a possibility. While digging through this job I was wondering what allows a vertical bus of the MCC to be rated at 300A when protected by a 600A main on the horizontal bus? Is it 240.21(B)(1)? Just want to understand where this comes from in code. This 300A vertical bus should limit a motor that can be installed on it to 125HP as FLA=156A*175%(time delay NEC30.52) = 273A, even though the continuous load would be evaluated as 156A*125%= 195A.

Thanks!

 

[don_resqcapt19]

The internal busing in an MCC is not subject to the NEC. It would be specified by the listing standard for the product. 430.94 specifies that the MCC have OCPD not exceeding the common bus rating.
Section 5.3.3 of UL 845 set the minimum ampacity of the vertical bus at 300 amps. 5.3.2 sets the minimum ampacity of the horizontal bus at 600 amps.
I took a quick look at UL 845 and did not find any rules governing the relationship of ampacities of the vertical bus as compared with the ampacity of the common bus. The complete assembly is tested in a manner that every connection and all sections of the busing are subjected to 100% of rated current. The assembly fails if the temperature rise is excessive.
It appears to me that the 300 amp bus can be loaded to 300 amps without applying any factors for continuous loads. That is loaded to the sum of the full load currents as shown in Tables 430.247 through 430.250.

Note that UL uses the same free viewing system as does the NFPA and it is not very user friendly and it is easy to miss something.

 

[petersonra]

MCCs that are center fed by the horizontal bus are allowed to take advantage of the full bus amperage of the vertical bus in both directions. So you can have 300 amps above the horizontal bus and 300 amps below the horizontal bus.

 

[TallTimber]

The internal busing in an MCC is not subject to the NEC. It would be specified by the listing standard for the product. 430.94 specifies that the MCC have OCPD not exceeding the common bus rating.
Section 5.3.3 of UL 845 set the minimum ampacity of the vertical bus at 300 amps. 5.3.2 sets the minimum ampacity of the horizontal bus at 600 amps.
I took a quick look at UL 845 and did not find any rules governing the relationship of ampacities of the vertical bus as compared with the ampacity of the common bus. The complete assembly is tested in a manner that every connection and all sections of the busing are subjected to 100% of rated current. The assembly fails if the temperature rise is excessive.
It appears to me that the 300 amp bus can be loaded to 300 amps without applying any factors for continuous loads. That is loaded to the sum of the full load currents as shown in Tables 430.247 through 430.250.

Note that UL uses the same free viewing system as does the NFPA and it is not very user friendly and it is easy to miss something.

So I guess where I get a little confused is on feeding an OCPD from this bus. My VFD's for the new 200hp motors will have a max input current of 248A which this Vertical bus can handle no problem. However, they are required by the manufacturer to be protected by a 350A breaker. It is my understanding that this breaker is only for short circuit protection as the VFD handles overloading from the motor. Even though my breaker is 350A by starting current is only 248A at most. So I should be good on a 300A Bus per 430.52, Correct?