Say someone retrofitted a new panel and installed a gutter above the panel to splice the short cables.
Assume they feed all the spliced circuits into the panel via a single big nipple.
Can they just put an isolated neutral bus bar and a ground bar in the gutter feeding back to the panel with two appropriatly sized conductors?

Yes.

I do it all the time with EGC's, however, I'm not sure about doing it on the neutral...if you lose that large jumper, you may fry some of your loads. Maybe it would be better to parallel 2 or 3 conductors for redundancy?

If you look at "column width" panelboards, they do this as well, with a large j-box at the top with a wireway going down to the panelboard.

The inspector might be interested in how full the nipples are and what the neutral load is and what size the wires are that you are putting in parallel.

Actually "fill" was one of the reasons I though this would be done.
If you took the example of 20 120v 20a circuits, you would have 20 blacks, one 2ga neutral and one 4ga EGC. (~.55 sq/in) That fits in a 1.25" @ 36.33%. You can load a short nipple more than that and a 2" nipple would give you lots of breathing room.
Any 240v circuits without a neutral load could come down in another nipple.

I agree you can't parallel anything smaller than 1/0 but if you wanted "belt and suspenders" for the open neutral concern, you could parallel a 1/0 neutral, adding an extra .14" to the fill.

Code has explicit permission for multiple sets of feeders sharing a single suitably sized neutral, and has explicit permission of multiple ungrounded conductors _with voltage between them_ sharing a single neutral (multi-wire branch circuit), but I don't recall anything that either permits or prohibits multiple circuits sharing a single suitably sized neutral.

I would be rather uncomfortable, for example, with 2 #12 conductors on the same supply leg fed through 2 20A breakers, both sharing the same #8 neutral. The installation being discussed here is essentially an extension of this sort of concept.

However the strongest reason that I can come up with for not sharing the neutral in this nipple is that it would not work with any sort of breaker that needs to sample the neutral current, in particular GFCI and AFCI breakers.

On the parallel conductors, while you are not permitted to parallel conductors smaller than 1/0, I be comfortable with smaller conductors paralleled for redundancy, with 2 provisos: the conductors are all in the same conduit or raceway, or are in non-metallic conduit, and that each individual conductor be suitably sized to carry the entire load. Basically, if no hazard would be created if the parallel connection would fail, then I would be comfortable with more relaxed requirements for parallel conductors.

-Jon

My 2 cents:
As winnie said, GFI/AFCI excluded, I see no problems with what you proposed, and your spec.

Now, as to sizing the ground.....would you not base the ground conductor on the size of the OCP, or the largest OCP for the branch circuits involved??

(That's an opinion someone laid on me, I tend to agree with Greg's specs)

John

See 300.3(4)
http://www.markhellerelectric.com/300.3(4).pdf

You can do it... But something tells me, at least, at one point or another the auxilliary gutter was to be listed for the purpose, but if you are allowed to do it in any junction box, I might start doing it myself more often, just to get rid of some fill in some of the tighter panels.

I challenge anyone to point to a code section that prohibits using one properly sized grounded conductor for multiple circuits as Greg described or for any branch circuit wiring.

The 2002 NECH even shows how it is done.


Exhibit 225.3 A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).



FWIW I do not understand the worry expressed in loosing the neutral, if we worry about that than how would we install feeders or services?

Bob

Bob- you've selected a very unique example for your point and I have never seen this type of installation installed. It is only usable outside as 225.7 states. All other type of installation would have us sizing the neutral to match the individual branch circuit.

George please cite a code section that prohibits doing this in any branch circuit.

I have been down this road before so you have an up hill battle.

Bob

I belive that there is only implication that this is not allowed, but no actual code to back it up.

The physics certainly works.

Bob pointed out the explicitly permission for a common neutral in 225.7 for _lighting_ equipment installed _outdoors_.

215.4 explicitly permits feeders with common neutral. You are explicitly permitted to install 6 ungrounded conductors (for 2 separate three phase feeds) with a _single_ suitably sized common neutral. If I read this correctly (and I've not delved too deeply), I don't even see a requirement that these feeders originate from the same panelboard or even the same system. Imagine the possibilities for ugliness

210.4 permits multiwire branch circuits, and article 100 has a definition (Branch Circuit, Multiwire) which specifies that the ungrounded conductors in a multiwire branch circuit have a voltage between them.

300.3 has explicit permission for the shared neutral bus in a gutter attached to the panelboard.

One could argue that the lack of explicit permission for common neutral branch circuits in article 210 implies that they are not permitted,...but IMHO this is a rather weak argument that would not stand up legally. It most this lack of explicit permission or prohibition suggests that this is an extremely uncommon practise.

-Jon

Winnie- I agree with you. I have read this post over a coupla times and I have been enlightened by your response and Bob's. I don't see this as a routine installation and all it really amounts to is an extention of the neutral bus. If I were to inspect this type of installation I would be asking for some calculations, you can bet your bippy on that. See, Inspectors are human (some of them)

George if you ever do see this installation let us know.

I have never seen it and can not think of a way to pull it off for less money than the standard wiring methods.

By the time you use boxes with enough CI and buy connectors to terminate the large common neutral I think you will have spent much more money.

Bob

If you have the Feeder, and all the branches running through the gutter. (For other reasons, like accomadating entrance to the panel anyway.) Why not just pull the nuetral bar from the panel and re-locate it in the gutter. For Resi AFCI's, or GFI breakers that would require a neutral connection in the panel, I guess it would not work...

As for codes to back it up, does this work?


FYI: I do this with Grounds all the time... Terminate them and bond accordingly to the panel ground with a single conductor. Not neutrals, but if allowed I might start to rethink of doing it for them too.

[This message has been edited by e57 (edited 11-06-2005).]

As Bob said, we've been here before.

Is it a violation wired this way?