I am looking at a project that has two 480V MCC's being fed from a single 750kVA utility transformer making both of these MCC's "service entrance" MCC's. Each MCC has an integral ATS on the load side of each MCC main breaker to accommodate an emergency generator feed to each MCC.
Each MCC will only feed 3-Phase motor loads with the majority of the load consisting of VFD fed motors, so there will be no L-N loads fed from the MCC. Any auxiliary loads requiring L-N will be serviced from a CPT located in the MCC.
Since these MCC's will not serve any L-N loads I am trying to determine what size "grounded conductor" will need to be run with the service entrance conductors. NEC 250.24(C)(1) requires that the grounded conductor be no smaller than listed in Table 250.102(C)(1). 230.42 gives the requirements for sizing the service entrance conductors with 23042(C) giving the requirements for the grounded conductor which references back to 250.24(C). So far to me both of these references are straight forward.
The confusion comes in when we consider the requirements in 220.61 and choosing a grounding conductor that results in the largest conductor between 220.61 and 250.24(C). My question here is weather or not 220.61 would even be applicable here for considering the neutral/grounded conductor size since there are no L-N loads served by the MCC? To me it seems like 220.61 would only come into play if there were L-N loads served by the MCC and since there are not in this case we can simply go with the values given by 250.24(C)? I have seen many designs where the engineer sizes the grounded conductor the same size as the phase conductors even when only supplying 3 phase loads so I'm not sure if this is perhaps just a standard practice? I also notice that 220.61(C)(2) prohibits the neutral reduction for 3-Phase 4-Wire loads but again the 3-Phase VFD's served by this MCC are not considered 3-Phase 4-Wire loads.
The second part that I'm seeking clarity on is weather or not these MCC's need to have a neutral bus running throughout the entire MCC? I know that to be service entrance we need a ground bus and neutral bus in order to have the main bonding jumper but can the neural bus only be included in the incoming MCC section to accomplish this or does it need to run through the entire MCC even know there will ne no L-N loads?
Each MCC will only feed 3-Phase motor loads with the majority of the load consisting of VFD fed motors, so there will be no L-N loads fed from the MCC. Any auxiliary loads requiring L-N will be serviced from a CPT located in the MCC.
Since these MCC's will not serve any L-N loads I am trying to determine what size "grounded conductor" will need to be run with the service entrance conductors. NEC 250.24(C)(1) requires that the grounded conductor be no smaller than listed in Table 250.102(C)(1). 230.42 gives the requirements for sizing the service entrance conductors with 23042(C) giving the requirements for the grounded conductor which references back to 250.24(C). So far to me both of these references are straight forward.
The confusion comes in when we consider the requirements in 220.61 and choosing a grounding conductor that results in the largest conductor between 220.61 and 250.24(C). My question here is weather or not 220.61 would even be applicable here for considering the neutral/grounded conductor size since there are no L-N loads served by the MCC? To me it seems like 220.61 would only come into play if there were L-N loads served by the MCC and since there are not in this case we can simply go with the values given by 250.24(C)? I have seen many designs where the engineer sizes the grounded conductor the same size as the phase conductors even when only supplying 3 phase loads so I'm not sure if this is perhaps just a standard practice? I also notice that 220.61(C)(2) prohibits the neutral reduction for 3-Phase 4-Wire loads but again the 3-Phase VFD's served by this MCC are not considered 3-Phase 4-Wire loads.
The second part that I'm seeking clarity on is weather or not these MCC's need to have a neutral bus running throughout the entire MCC? I know that to be service entrance we need a ground bus and neutral bus in order to have the main bonding jumper but can the neural bus only be included in the incoming MCC section to accomplish this or does it need to run through the entire MCC even know there will ne no L-N loads?
