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?
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.
#96099 - 11/03/0506:49 AMRe: New panel/gutter question
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.
#96101 - 11/03/0512:43 PMRe: New panel/gutter question
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.
#96102 - 11/03/0508:26 PMRe: New panel/gutter question
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.
Mark Heller "Well - I oughta....." -Jackie Gleason
#96104 - 11/05/0505:40 AMRe: New panel/gutter question
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.
#96106 - 11/06/0510:04 AMRe: New panel/gutter question