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#34372 02/10/04 03:08 PM
Joined: Oct 2002
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One question. I know most guys are tired of talking about generators. I am going over some of the "archives" in this forum and other forums, (still trying to figure it out)
I just have one main question at this time though, ( though there are others ).

Is my inspector right, when he tells me that if the portable generator has the ground and neutral bonded, I don't have, to have a transfer switch that breaks the neutral??? I am talking about one that is used as backup power for a house
Thanks.... Steve

#34373 02/10/04 03:36 PM
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This sounds exactly backwards to me. If the generator has ground and neutral bonded, and the transfer switch does not break the neutral, then you have a parallel path; the neutral would have two separate bonds to ground, and some of the current flowing in the neutral enter the equipment ground system at one of the bonds and exit at the other bond.

-Jon

#34374 02/10/04 03:52 PM
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Steve, Jon is correct, you need to flip flop your inspectors thinking.

For some excellent graphics of this (By Ed Maclaren) go here: http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=7;t=000094;p=1

Roger

[This message has been edited by Roger (edited 02-10-2004).]

#34375 02/11/04 08:35 AM
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I believe that most small portable generators do have an internal neutral bond to the frame. Also I believe that most small (6-10 ckt) commercially available "load side" transfer switch panels do not switch the neutral.

What to do?

#34376 02/11/04 10:06 AM
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250.6 allows disconnecting or switching of the grounds or neutrals where OBJECTIONABLE current flows through the grounding conductor. IMHO the current through the ground wire in the cord between a portable generator and the transfer switch is not objectionable. How is this situation a hazard?
During the emergency, the generator runs and supplies power to the selected loads. The neutral is connected to the frame. The generator sits outside. Why is it a shock hazard to touch this generator? What potential is there between the frame and the earth? If one is worried about paralling the neutral and grounding conductors in the cord, then use a three conductor cord. You don't need to run a grounding conductor to a portable generator. (250.34)


Earl
#34377 02/11/04 10:37 AM
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Redsy and Earl, in reality a "portable" generator is not intended for this installation in the first place.

Without switching the neutral, there is a posibility (note I said posibility) of back feeding the POCO transformer putting these guys in jeapordy.

Now if we do use this "portable" generator as an "SDS", we would have to comply with 250.30; and 250.34 couldn't be considered applicable to the situation.

Roger





[This message has been edited by Roger (edited 02-11-2004).]

#34378 02/11/04 10:38 AM
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Roger, thanks for the link. It helps me tremendously in this ordeal. I hope everyone agrees and I hope I can talk to my inspector and get it straight. What's bad is when there are so many different opinions and it makes it kind of hard to know which is right.
Redsy: Here is a list of the different generator manufactures, that I have sent emails to, that have responded back, as to how their generators are wired:

Generators with "neutral" bonded to "equipment ground" inside generator:

** Generac generators
** "Briggs and Stratton" Elite Series
** Husqvarna
** Troy Built
** Snapper
** Craftsman
** DeWalt
** Yamaha ( Yamaha told me that ALL portable generators over 5 KW, had to be bonded together, according to the NEC. I ask him for the code reference, but he didn't have it handy, he said..??? )

Below are the generators that are manufactured with a "floating neutral":

** ETQ generators (Eastern Tool & Equipment)
** Coleman PowerMate generators
** Possibly "North Star" ( I believe Coleman makes this one too, I'm not sure)
** Honda ( I took this one from information on another "forum thread"

Hope this helps someone. This is all I have at present... Thanks again for the comments and help.. Steve

#34379 02/11/04 10:49 AM
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Turn the question around, and ask "Why current flow in the grounding system is considered 'objectionable'?"

I believe that current _which should be flowing on the neutral_, which is flowing in the generator ground wire, through the building grounding system, to the building main panel and rejoining the neutral at the main bond is exactly the sort of 'objectionable' current that code tries to prevent.

Bonding ground to neutral at both the generator and the main panel strikes me as presenting the same issues as bonding the ground to neutral as a subpanel.

But I would appreciate someone else chiming in and saying exactly why this form of current flow is objectionable. I have a vague notion that the problem is not the current flow in general, but instead the current flow across junctions that might have localized high resistance (local heating), or having current flow where a junction might be mechanically opened on the _outside_ of an enclosure, presenting a shock hazard. But I really don't know the details of why to avoid parallel paths.

Parallel paths are clearly permitted in some situations (bonding of neutral at the meter can and at the main panel, for example), and in other situations were previously permitted and are still 'grandfatherd' (eg three conductor circuits for electric ranges where the ground and neutral were shared.) So a parallel path is not such a dangerous thing that it will cause instant destruction of property...but parallel paths are clearly to be avoided in many situations.

-Jon

#34380 02/11/04 11:58 AM
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Roger,

I understand that a miswired transfer switch could backfeed a utility xfmr., but I don't follow how a bonded, unswitched, neutral would do the same.

Any explanations, please?

#34381 02/11/04 12:45 PM
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