EGC when 3 phase, 3 wire MCC feeds 3 phase subpanel?

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wsbeih

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Guys,

In an old existing industrial plant, I have delta-Wye TRF 480/277V feeding 3 phase MCC. the TRF Nutral and the MCC ground Bus are bonded.
This MCC feeds balanced 3 phase motor loads. Recently I was asked to add a 480V,100Amp 3 phase sub panel (no line to nutral loads required), which feeds balanced 3 phase loads, to the existing MCC. This sub panel is 680 feet away from the service.
Please note that all equipment eclousers and motors are bonded through a grounding grid/loop.
Do i still need EGC conductor from the MCC service to the subpanel (do i still need to bond the MCC and the sub panel via EGC)? If so, Do i still need to bond the sub panel enclouser to the ground loop/grid?

My understanding is that the run from service to any 3 phasel oad should have 3 phase conductors+ Nutral (if needed) + EGC.

Please advise.
 
As I understand your install you cannot depend on a ground loop that is not connected to the system. An egc may be needed or not depending if metal conduit is used as metal conduit is a suitable egc. 358.60. If not than an egc will be required.
 
Dennis Alwon said:
As I understand your install you cannot depend on a ground loop that is not connected to the system. An egc may be needed or not depending if metal conduit is used as metal conduit is a suitable egc. 358.60. If not than an egc will be required.
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Thanks for the feedback. This what I am debating with my colleagues, some insist that ground loop is adequate and no EGC from source (service) to each load is required.

I f possible can you enlighten me, of what is the difference between the 2, Grounding loops/grid vs EGC?
I mean if we take to each load an EGC? then why do we still require to have the grounding grid? is it to ensure that all have the same potential or else?
 
i'd pull one to the sub panel. if you don't, at least make sure you're conduit connections are really tight, throw a grounding bushing in at both ends, bond accordingly, and accomplish it that way. i never don't pull a ground (always do) even in rigid pipe.
 
Would your ground grid/loop comply with the restrictions in 300.20(A), assuming you run the feeder in a ferrous raceway? Even if the raceway is plastic or aluminum, does it pass through ferrous boxes? Note that a metallic conduit can be an EGC, so not sure if you are asking to completely do away with an EGC (and just use the grid) or if you are debating a separate EGC conductor inside the raceway.

300.20(A) requires the ungrounded, grounded, and grounding conductors to all be run together.
 
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Some one correct me if this is in error ,I'm not 100% on this but I think the EGC(conduit or wire) is the ground fault return and the grid is equalizing ground potential (step or touch) both required.

dick
 
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