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#35665 03/19/04 04:09 AM
Joined: Jan 2004
Posts: 91
R
rizer Offline OP
Member
you have a transformer inside the blding and you create a separately derived system(480-208 step down ) you run the ground wire from the primary and secondary to the chassi and the bonding elecrode from the xo to the chassi and on to blding steel. do you bond the neutral to the can at the first means of disconnect. if so, doesnt this create 2 paths to ground?

#35666 03/19/04 05:36 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
The code allows a few options here.

Bond at the Transformer
or
Bond at the first panel/disconnect


Part of 250.30(A)(1)
Quote
this connection shall be made at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device,

or

Bond at both places if this will not create a parallel path.

Part of Exception 1 to 250.30(A)(1)
Quote
A bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded circuit conductor.

From your description you will have a parallel path (the grounding conductor and maybe the raceways) if you bond at both.

I bond at the transformer and never again, it is a policy of the company I work for. [Linked Image]

Bob

[This message has been edited by iwire (edited 03-19-2004).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#35667 03/19/04 07:56 AM
Joined: Jan 2004
Posts: 91
R
rizer Offline OP
Member
even if i use a plastic raceway dont i still have a parallel path ?

#35668 03/19/04 12:49 PM
Joined: Dec 2003
Posts: 751
E
Member
Exactly, that's why I bond only at the Tformer, and have an insulated neutral bus at the 208 volt panel.


Earl
#35669 03/19/04 04:29 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
I can not think of any normal installation that the exception would apply to.

Like I said I do my bonding in the transformer only, I was just trying to give you all the allowable options. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#35670 03/19/04 05:35 PM
Joined: May 2002
Posts: 1,716
R
Member
Rizer,
Quote
even if i use a plastic raceway dont i still have a parallel path?
only if the first means of disconnect is mounted on a conductive material common to the Transformer or Grounding Electrode.

Roger

#35671 03/19/04 05:45 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Roger Even with plastic raceways I still see the EGC and GEC as parallel paths to the grounded conductor.

How would you avoid this?

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#35672 03/19/04 06:54 PM
Joined: Jan 2004
Posts: 91
R
rizer Offline OP
Member
i agree that you are creating a parallel path to ground. this was explained to me many years ago by an inspector who stated you can bond it at the xfmr or the first means of disc but not both. none the less my boss insists he has " done it that way all (his) life". he then tried to find it in one of his electrical books and after 5 minutes said " it won't make any difference so do it my way".

#35673 03/19/04 08:03 PM
Joined: May 2002
Posts: 1,716
R
Member
Bob and Rizer, maybe I'm not understanding the discription of what's being done here.

What I'm talking about would be the diagram on the left below.

[Linked Image]

even bonded at both locations there would only be one path for the load to return to the source.

Roger

#35674 03/20/04 12:10 AM
Joined: Jan 2004
Posts: 91
R
rizer Offline OP
Member
maybe ive been told incorrectly but i was told that the ground wire from the service is connected to the chassie not the xo. and that the bonding electode should be grounded to the chassie on its way to the xo. i would think a ground fault could travel back to your bonding electode via the neutral or the ground wire thereby having 2 paths to ground.

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