800 A Branch Circuit

[YoungElectricalEng]

I have a 600V MCC Section that is fed from an 800A MCB in a 600V Switchboard. Let's call the existing 600V MCC Section 'MCC1'. MCC1 has an 800A Non-Fusible disconnect to isolate the MCC section from the 600V Switchboard. MCC1 was built with a second 800A Non-Fusible disconnect, connected to the horizontal bus downstream from the incoming Main 800A disconnect. The second 800A disconnect was installed in the event we wanted to connect PFC. The max load on MCC1 is 270A and the MCC section has no room for spares. I have a NEW 600V MCC Section (MCC2) being installed beside MCC1. MCC2 has an 800A Breaker (adjustable). The max load on MCC2 will be 290A and MCC2 has no room for spares. I want to feed MCC2 from the spare 800A disconnect in MCC1. Combined load would be 560A. QUESTION: Does the branch circuit feeder from MCC1 to MCC2 require it's own protection, or is this branch circuit feeder considered protected by the 800A MCB in the Switchboard? Thanks in advance.

 

[petersonra]

you could run enough conductor for 800A from the disconnect in MCC1 to the MCB in MCC2.

you could also run less conductor as a tap conductor and set the MCB in MCC2 to a lower value.

 

[YoungElectricalEng]

Additional Info: I intend to run two per phase, 500MCM 3C TECK, and turn down the 800A MCB in MCC2 to 80%x800A.

 

[YoungElectricalEng]

you could run enough conductor for 800A from the disconnect in MCC1 to the MCB in MCC2.

you could also run less conductor as a tap conductor and set the MCB in MCC2 to a lower value.

Thanks Bob. Are you saying that as long as the branch circuit feeder conductor from MCC1 to MCC2 is sized for the total available current, the 800A MCB in the 600V Switchboard continues to act as sufficient protection for the branch circuit feeder? My electrical contractor, whom I have the utmost respect for, wasn't certain if the second 800A Non-Fused Disconnect would have to be switched out to either a fusible disconnect or a breaker to provide protection to the branch circuit feeder conductor.

 

[GoldDigger]

Thanks Bob. Are you saying that as long as the branch circuit feeder conductor from MCC1 to MCC2 is sized for the total available current, the 800A MCB in the 600V Switchboard continues to act as sufficient protection for the branch circuit feeder? My electrical contractor, whom I have the utmost respect for, wasn't certain if the second 800A Non-Fused Disconnect would have to be switched out to either a fusible disconnect or a breaker to provide protection to the branch circuit feeder conductor.

The principle applied to tap conductors is that the upstream short circuit protection will also protect the tap wires against short circuit as long as the tap rules are followed, while the downstream overcurrent protection will protect the tap wires against longer time overloads which would never trip the upstream breaker.
There cannot be any loads on the tap wires between the two ends which might generate a slow overload.

 

[petersonra]

Additional Info: I intend to run two per phase, 500MCM 3C TECK, and turn down the 800A MCB in MCC2 to 80%x800A.

Teck? sounds like Canada. What I suggested would be NEC compliant, but I don't know if it complies with CEC. In the US two CU 500 cm would be good for 780A, while two AL 500 kcm is good for 620A. I am guessing you are looking at AL.

I don't know that I would run cable of any kind as long as the MCCs are adjacent, or there is some other reason to do so. Probably simpler and less expensive just to use regular single conductors.

 

[dhkane]

I take it that these two 800A MCCs can't be bussed together (since there mounted adjacent to each other) or same mfgr.
Otherwise, I would change out the second 800 disconnect in MCC1 to a fused disconnect sized smaller, with smaller conductors for MCC2. Then that tap would be protected similar to what you have with MSB to MCC1.

 

[YoungElectricalEng]

Teck? sounds like Canada. What I suggested would be NEC compliant, but I don't know if it complies with CEC. In the US two CU 500 cm would be good for 780A, while two AL 500 kcm is good for 620A. I am guessing you are looking at AL.

I don't know that I would run cable of any kind as long as the MCCs are adjacent, or there is some other reason to do so. Probably simpler and less expensive just to use regular single conductors.

Yes I'm in Canada. I am using two CU 500MCM, with the MCC2 MCB turned down less than 780A. In many cases the CEC is a duplicate of the NEC, except for a few instances. It was my understanding that the 800A MCB in the Switchboard should be sufficient protection for the Branch Circuit, but as I mentioned my electrician questioned it, so I wanted to get some level of reassurance from the community here.

 

[YoungElectricalEng]

I take it that these two 800A MCCs can't be bussed together (since there mounted adjacent to each other) or same mfgr.
Otherwise, I would change out the second 800 disconnect in MCC1 to a fused disconnect sized smaller, with smaller conductors for MCC2. Then that tap would be protected similar to what you have with MSB to MCC1.

The two 800A MCCs are the same mfgr, however the room size doesn't allow for them to be connected at the bus. The two lineups are facing each other with 42" between them. I agree that it would be ideal if I could change the second 800A disconnect in MCC1 to a fused disconnect, however we have what is considered a 'live biological' process, backed up with 10MW of Generation in the event of a power outage. I cannot arrange the shutdown of MCC1 to change out the second disconnect. Unless it can be deemed to be a code violation, I'm probably going to proceed as planned. Thank you for your advice.