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#97657 - 03/14/06 05:28 AM Generator Question
George Little Offline
Member
Registered: 01/18/04
Posts: 1488
Loc: Michigan USA
Question was posed to me on how to handle the common conductor of a 480v. generator being used as a backup for a utility supply. The load is 480v. only with no need for a "neutral". There are no 277v. loads. The generator is being used through a transfer switch and the question is- "Do I need to bond the common conductor to the frame of the generator and/or do I have to break this conductor with the transfer switch". My answer was that I would bond this common conductor to the frame and not break this conductor with the transfer switch. Reason - this conductor is not a current carring conductor and we are only providing equipment grounding for lightning, surges etc. If we float the common conductor we will have an ungrounded SDS and need ground detectors. Need advice on this one.

[This message has been edited by George Little (edited 03-14-2006).]
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George Little
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#97658 - 03/14/06 06:01 AM Re: Generator Question
Yoopersup Offline
Member
Registered: 03/04/03
Posts: 822
Loc: Michigan
George
I Feel 250.24 5(B) applies.Then you look at the 3 or 4 pole transfer switch to decide Grounding.
Also look at 250.24 5 FPN
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#97659 - 03/14/06 09:51 AM Re: Generator Question
Bob Offline
Member
Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
George
If you do not transfer the neutral(break the neutral) you run the existing system neutral to the generator neutral bushing. You use the existing system EGC and bond it to the frame of the generator. You have the option to transfer the neutral or not. I would not.
It make the installation simpler.

[This message has been edited by Bob (edited 03-14-2006).]

[This message has been edited by Bob (edited 03-14-2006).]
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#97660 - 03/14/06 10:02 AM Re: Generator Question
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
The only issue is objectionable current on the grounding conductor. If there is, and never will be, any 277v loads I don't see a problem with George's take on this.
Bob's solution is more like what Mr Soares suggests.
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#97661 - 03/14/06 01:58 PM Re: Generator Question
Yoopersup Offline
Member
Registered: 03/04/03
Posts: 822
Loc: Michigan
George
I feel if you have a Neutral present in the Transfer switch from the existing service then a Neutral from the Generator Is required.(to maintain same type of grounding system) If its a 3 pole transfer switch then you ground(Earth Ground) at the utilty service only. Also Bond only at the utility service. If you have a 4 pole transfer switch then your dealing with a Separately Derived sytem . Then Grounding and bonding is required at both places(Utility, & Generator)See pages 180-181 Soares Book on Grounding 7th Edition.Also view McGraw-Hill's
National Electrical Code Handbook 24th EditionPages 364,365,&366,367.

[This message has been edited by Yoopersup (edited 03-14-2006).]
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#97662 - 03/14/06 02:23 PM Re: Generator Question
George Little Offline
Member
Registered: 01/18/04
Posts: 1488
Loc: Michigan USA
No neutral anyplace either at the Utility or the Generator. That's my question and my point I guess. The 480v. Utility service does mot have any neutral load nor does the generator.
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#97663 - 03/14/06 04:35 PM Re: Generator Question
Yoopersup Offline
Member
Registered: 03/04/03
Posts: 822
Loc: Michigan
George
Then its either an Ungrounded Delta or a Grounded Delta(one Phase to ground). Grounding would be the same as usual but no neutral involved.
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#97664 - 03/14/06 07:18 PM Re: Generator Question
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
How are you getting past the idea that the grounded conductor shall be brought to the service disconnect enclosure and that is where the MBJ lives?
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Greg Fretwell
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#97665 - 03/15/06 04:55 AM Re: Generator Question
Yoopersup Offline
Member
Registered: 03/04/03
Posts: 822
Loc: Michigan
On a True Delta (ungrounded)there is no Grounded conductor, On a Conor grounded Delta one of the Phase conductors is the Grounded conductor. Look it up in Soares it explains it well there, Most Electricals never heard or ever seen either.The Key is theres No Neutral (as most know it at either place the Utility feed Or the Generator!)
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#97666 - 03/15/06 05:51 AM Re: Generator Question
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
I believe that it is an error to _both_ not transfer the neutral and bond the neutral to ground at the generator.

If you do not transfer the neutral, then the neutral of the generator is connected to ground via the main bonding jumper of the primary system. Thus even though there is no bond at the generator itself, you would not leave an inadvertent ungrounded system.

If you bond at the generator and don't transfer switch the neutral, then you have a situation where the _unbroken_ neutral is bonded to ground at two points, which creates a parallel path. The fact that there are no neutral loads served does not change the existence of this parallel path.

However lack of neutral loads means that no current would be injected into this parallel path, so I suspect that having this particular parallel path is not a significant safety issue.

-Jon
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