ECN Forum
Posted By: sparkync Help!!! - 02/10/04 07:08 PM
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
Posted By: winnie Re: Help!!! - 02/10/04 07:36 PM
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
Posted By: Roger Re: Help!!! - 02/10/04 07:52 PM
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).]
Posted By: Redsy Re: Help!!! - 02/11/04 12:35 PM
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?
Posted By: earlydean Re: Help!!! - 02/11/04 02:06 PM
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)
Posted By: Roger Re: Help!!! - 02/11/04 02:37 PM
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).]
Posted By: sparkync Re: Help!!! - 02/11/04 02:38 PM
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
Posted By: winnie Re: Help!!! - 02/11/04 02:49 PM
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
Posted By: Redsy Re: Help!!! - 02/11/04 03:58 PM
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?
Posted By: Redsy Re: Help!!! - 02/11/04 04:45 PM
Also, see here:
https://www.electrical-contractor.net/ubb/Forum1/HTML/003620.html
Posted By: sparkync Re: Help!!! - 02/11/04 06:08 PM
Ok, it's me again... Good thing I don't have much work this week. It gives me time to figure these generators out, just in case someone calls and wants one hooked up [Linked Image]
I appreciate Roger's link, but that gives me some more questions.
Say I got a 200 amp. single phase service; feeds out of the back of the meter into a 200 amp. 42 circuit panel. I want to come out of this panel and set a generator transfer switch, with circuits that can be switched on during power outage. The generator to be used is a portable generator that has a bonded neutral, therefore requiring the neutral to be switched. I am wanting to use a 16 circuit Square D panel and use one of the Square D "interlock" devices for the "utility" and "generator" main. I come out of the main panel with 2 --100 amp. phase conductors, 1 neutral, and 1 equipment ground, all sized per code. I separate my neutral from the equipment ground, since it is a "sub-panel". NOW [Linked Image] at this point, do I feed off of the generator main breaker out of my sub panel with the interlock, to a 3 pole disconnect

(1 for each "phase" conductor, and 1 for the "neutral" conductor, carrying with me the equipment ground, bonded to the 3 pole disconnect, but isolated from the neutral)

Then off the "load side" of the disconnect, set a box with a "4 wire" male plug (male because the power will be coming from the generator side)
Then from this point, of course, will be the 4 wire cord to the "bonded neurtral generator". Does this sound like an acceptable way to do this, provided that the amperage ratings of disconnect, plug, etc.... are met?

Still in the "sub-panel" the neutral and equipment ground are still bonded [Linked Image] But at some point they will be bonded anyway, right...? Thanks again... Steve....getting closer [Linked Image]
Posted By: Roger Re: Help!!! - 02/11/04 11:59 PM
Redsy, it would be a long shot meaning there would have to be some coincidences take place, hence the reason I said possibility. (which I misspelled)

It could be a fault through the bond jumper backfeeding the neutral (unswitched) to do this.

The fault couldn't be enough to open the "portable" generators breaker, or there would have to be a problem not allowing the breaker to open.

Even then, one of the normal ungrounded conductors on the secondary winding would have to be grounded (a tree maybe) to provide a potential difference from the backfed neutral to the end of the winding.

I did say possibility. [Linked Image]

Roger
Posted By: sparkync Re: Help!!! - 02/12/04 05:15 PM
Also as a continuation of my question about coming out of an existing panel, how about if on a new installation, you have a 200 amp meter base outside. Can I come out of the side of the 200 amp. meter base, and set a 200 amp double throw transfer switch, then come out of the bottom and set a 30 amp. or 50 amp. twist lock receptacle for the generator? The neutral and ground will have to be bonded for the household main electrical panel, but if the portable generator to be used has a bonded neutral, then the neutral at the double throw switch will need to be isolated [Linked Image]

Or would I come out of the meter base,then set a 200 amp.Main breaker disconnect, then feed the double throw transfer switch [Linked Image] Then the neutral and ground would be separated in the transfer switch, because I would have to run a 4 wire feed to my inside panel, because it would then be a sub-panel?? Since the 30 amp. generator receptacle under the transfer switch would be fed from the generator side, then the 30 amp wire off the load side of the 200 amp. transfer switch wouldn't have to be fused [Linked Image] The transfer switch I'm referring to, I assume, would just be "knife blade" non-fusible. I'm just trying to get all the scenario's straight, so I'll know what to do, if I need to do it for a customer.... I thought I knew the code and wiring pretty good, til I got into this portable generator wiring [Linked Image] Thanks again, Steve
ps. The only problem I see with the above,
(of course there may be more),is trying to get the 30 amp. wire to tighten up, under the 200 amp. lugs on the transfer switch.
This don't sound exactly right... someway..
So I would appreciate your input [Linked Image]
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