3-phase, 4W switchboard feeds 3-phase, 3W MCC

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morgan3520

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Dallas, TX
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Electrical Engineer

I have a situation (this is an existing system/facility that is being moved). The main switchboard is 3-phase, 4-wire. It feeds two MCC's which are 3-phase, 3-wire. Do I need to run a neutral from the transformer to the switchboard? Or from the switchboard to the MCC?
 
If all of the loads are line to line you do not need a neutral between the transformer and the switchboard, but you will need a supply side bonding jumper for fault clearing. From the switchboard to the MCC you will need an EGC based on the OCPD that is feeding the MCC.

I am assuming that this will be a grounded system with XO connected to a grounding electrode system.
 
don_resqcapt19 said:
If all of the loads are line to line you do not need a neutral between the transformer and the switchboard, but you will need a supply side bonding jumper for fault clearing. From the switchboard to the MCC you will need an EGC based on the OCPD that is feeding the MCC.

I am assuming that this will be a grounded system with XO connected to a grounding electrode system.
Click to expand...
Thank you, yes it will be.
 
don_resqcapt19 said:
If all of the loads are line to line you do not need a neutral between the transformer and the switchboard, but you will need a supply side bonding jumper for fault clearing. From the switchboard to the MCC you will need an EGC based on the OCPD that is feeding the MCC.

I am assuming that this will be a grounded system with XO connected to a grounding electrode system.
Click to expand...
The OP was not clear if this was a service or an SDS. It seems your comment was based on the assumption it was an sds? If it's a service, the grounded conductor must be run to the service equipment.
 
ron said:
Wouldn't the run from the switchboard to the MCC be a regular feeder with an EGC per 250.122? Why 250.102?
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Post #2 was not as clear as it should have been. A supply side bonding jumper from the transformer to the switchgear. The post you quoted was just a reminder that supply side bonding jumpers and EGCs are not sized the same.
 
electrofelon said:
The OP was not clear if this was a service or an SDS. It seems your comment was based on the assumption it was an sds? If it's a service, the grounded conductor must be run to the service equipment.
Click to expand...
When someone says transformer, I always assume customer owned unless there is something in the thread that indicates the poster is talking about a service.
You are correct that if this is a service the grounded conductor must be run to the service disconnect.
 
morgan3520 said:
I have a situation (this is an existing system/facility that is being moved). The main switchboard is 3-phase, 4-wire. It feeds two MCC's which are 3-phase, 3-wire. Do I need to run a neutral from the transformer to the switchboard? Or from the switchboard to the MCC?
Click to expand...


It would be my suggestion to run the neutral to the MCC. It is usually my suggestion to run the neutral to panelboard or switchgear that is rated for it. I don't know what the added cost would be or the rating of the switchgear but in most industrial establishments they sometimes have control wiring or fault sensors with CTs that might need the neutral to be present. Some relays run on 277V also.

You might not need the neutral but when the control guys come in they might need another step down transformer to power the controllers rather than having 277V available.
 
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