parrallel grounding and grounded conductors

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I have a job where the drawing are showing a service with a 500 MCM neutral and 2/0 ground. They are wanting both the neutral and ground
landed on X0 in a pad mount transformer. They are also wanting the same ground and neutral to be bounded with the building steel and ground rod at
the main. Thus parralleling this neutral and ground conductor. I know this unsafe and against code. I'm having trouble explaining exactly why or what is unsafe about parralleling them. Can any one hel? Thank you
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I would have no issue with bonding the X-0 at the transformer to its case and earth, nor with bonding the neutral at the service disco to its enclosure and earth.

If there's no disconnect at the transformer, the system neutral should be the only grounded conductor brought from the transformer to the service disconnect.

Am I wrong? :-?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Larry,

In addition to the Earth bonds at the two points (which I agree is a correct approach), there is also an EGC between them. This makes the EGC a parallel conductor to the neutral conductors, and since the EGC and neutral conductors are different sizes, this violates 310.4.

What will happen in this installation is that some fraction of the current will flow on the correct neutral conductor, and some fraction will flow on the EGC, where it _shouldn't_ be flowing.

-Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
jtester said:
Remove the 2/0 ground in the service. It probably shouldn't be there. Then there is no problem.
Right; the entire thing.

(I'll take it off your hands if you pull it out.)
 

Bea

Senior Member
connect system bonding jumper an GEC at same piont either at XO or first disconnect secondary side of transformer but not both
 

iwire

Moderator
Staff member
Location
Massachusetts
Bea said:
connect system bonding jumper an GEC at same piont either at XO or first disconnect secondary side of transformer but not both

In this case it is done in both places. The question is regarding a service. The utility will bond their transformer XO to ground and the NEC requires we bond at the service disconnect.

Because of that bonding in both places we do not run an EGC between the service disconnect and the utility transformer.

FWIW also check out 250.30(A)(1) Exception 2 it allows bonding at both the source and the disconnecting means of separately derived systems under certain conditions.
 
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