Residental mains wiring questions (USA)
in [Design]
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From: bud-- on 2 Jul 2010 11:08 On Jul 1, 11:23 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > On Thu, 1 Jul 2010 10:40:48 -0700 (PDT), bud-- <budn...(a)isp.com> wrote: > >On Jul 1, 6:10 am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > >> On Wed, 30 Jun 2010 00:49:16 -0700, Bob E. <besp...(a)invalid.tv> wrote: > > >> >Can I use a single neutral wire between these 2 boxes? How do I size > > >> No. The NEC rule is one neutral per breaker. > > >Nope, but the NEC may not allow a single neutral as proposed. > >Multiwire branch circuits (1 neutral for 2 or 3 hots) are allowed and > >have been widely used in the past. > > The NEC has cut that back pretty sharply recently. A single neutral for > a single multipole breaker is allowed and very common. It may be allowed > in some other cases, provided none of the load current is presented to > the neutral conductor. 250.4-B requires "simultaneously disconnect all ungrounded conductors". That does not necessarily require a multipole breaker - it can be multiple breakers and a listed handle tie. -- bud--
From: m II on 2 Jul 2010 11:09 Rich. wrote: > > "m II" <c(a)in.the.hat> wrote in message news:4c2d812c(a)news.x-privat.org... >> >> >> Last I heard, a multipole breaker was only required if the hots went >> to the same device. Where two or three hots, on alternate phases, feed >> their own individual loads, a single neutral wire was allowed for that >> group of breakers. >> >> If this has been changed, when did it happen? I haven't opened a Code >> book in a couple of years. >> >> Look at office receptacle circuits. You can have three receptacles in >> a row, on phases a, b and c. A single white goes back to the panel. If >> only one receptacle is being used, the white is certainly 'presented' >> with the load current. >> >> If TWO of the receptacles are being used, the neutral is STILL >> carrying load current. Only when all three hots are carrying an equal >> current is the neutral current free. > > Multiwire circuits are now required to have a common trip (multi-pole) > breaker feeding all circuits that share a neutral. In your example of 3 > (20 amp) outlet circuits in an office, yes you can run 3 hots and one > neutral back. But now, instead of 3 1-pole 20 amp breakers you are > required to install one 3-pole 20 amp breaker. The reasoning behind this > is if you turn off one of the 1-pole 20 amp breakers to work on the > circuit. Yes that one hot wire is dead, but if you were to break the > neutral splice, the other two circuits sharing the neutral can backfeed > the white wire and kill you. OK. I see that they have actually changed it. > > BTW, your load carrying neutral example is incorrect. The neutral > carries current anytime there exists an imbalanced load between any two > hot wires sharing a neutral. If you have two hots and one neutral with > one hot carrying 10 amp and the other carrying 13 amp, then the neutral > carries the difference of 3 amps. Yes, the difference between the loads. But only in a single phase, three wire circuit. Much like the old Edison three wire circuit. > Assuming the 2 hots are correctly > install to not be on the same phases. If they were on the same phases, > then the neutral carries the combined load of 10 and 13 amps, or 23 > amps. I agree. The have to be on alternate sides of the single phase, or on separate phases in a 3 phase system. > It's basically the same with a 3-phase setup except you throw the > third hot into the mix and the unbalance load calculations are a bit > more complex. If all hot carried the same current load, then the neutral > is carrying nothing. Three phase is a bit weirder. *Two* distinct hot phase wires with a shared neutral will see the neutral carry the SAME line current even when the two phases are evenly loaded. That's why in, say, a residential apartment building, with 208/120 feeds to the units (two hots and a neutral), you can not derate the feeder neutral. In 3 phase, You have to have all three hots feeding a load equally before the neutral balances out. mike >
From: bud-- on 2 Jul 2010 11:14 On Jul 1, 11:08 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > On Thu, 1 Jul 2010 09:43:45 -0700 (PDT), bud-- <budn...(a)isp.com> wrote: > >On Jul 1, 6:08 am, PeterD <pet...(a)hipson.net> wrote: > >> On Wed, 30 Jun 2010 22:04:59 -0700, Bob E. <besp...(a)invalid.tv> wrote: > > >> Huh? Your outlet wires go over the roof? Uh, no you don't take > >> seperate runs for the ground wire, all need to go together. You > >> mention 'passing inspection'. That won't... > > >Existing ungrounded wiring can be grounded by adding a ground wire > >which does not have to be run with the power wires. It is in 250.130, > >which also details where the added ground wire is to be connected at a > >'source'. I believe the ground wires does not necessarily have to > >connect through boxes on the way back to the grounding 'source' , but > >the connections probably have to remain accessible. > > > >> >The ground conductor doesn't have to run along side the > >> >power conductors, does it? > > >> Yes, they do. > > >For existing ungrounded circuits see 250.130. > > For the level and kind of change contemplated it is near certainty that > upgrading to current code can be required. It is an Authority Having > Jurisdiction thing and if they say upgrade you upgrade. From the description given that means essentially rewiring the building. I doubt many jurisdictions would require that. If they did, the OP may well decide to keep his classy Zinsco panel and leave everything alone instead of improving what he has. Kinda counterproductive. Does "upgrading to the current code" mean, for example, receptacles have to have the spacing in the current code? -- bud--
From: Fester Bestertester on 2 Jul 2010 21:32 > All neutrals are white, but not all whites are neutrals. You have to > have 2 (or 3) hots, from different phases, sharing the white in order > for it to be a neutral. Ah, it's beginning to make some sense to me. Thanks! > A real neutral carries only the *difference* in amperage between the > hots. ** > ** could be called sum. It depends how you mark the vectors in the > diagram. (+120 v at 0 degrees) plus (+120 v at 120 degrees) plus (+120 > at 240 degrees) equals zero. Your example is for 3-phase, right? For residential split-phase, the vectors would be 180 degrees apart, right?
From: JosephKK on 3 Jul 2010 12:18
On Thu, 1 Jul 2010 23:55:00 -0400, "Rich." <rcres(a)XXcomcast.net> wrote: > >"JosephKK" <quiettechblue(a)yahoo.com> wrote in message >news:q1mq26ttj43uvauhlu7gkdgo4hgvl98adm(a)4ax.com... >On Thu, 1 Jul 2010 12:07:59 -0400, "Rich." <rcres(a)XXcomcast.net> wrote: > >> >>>"JosephKK" <quiettechblue(a)yahoo.com> wrote in message >>>news:m2uo26965hjc6sdg6lv3ns21ci81runl4g(a)4ax.com... >>>On Wed, 30 Jun 2010 13:01:20 -0400, "Rich." <rcres(a)XXcomcast.net> wrote: >>>> >>>>Save yourself a world of headaches and don't use conduit. Instead just >>>>match >>>>the size and type of each cable there and run a matching romex cable over >>>>to >>>>the new location. Better yet, for any cable runs that are exposed, >>>>unstaple >>>>them and run them towards the new location to help keep the length of the >>>>run to a minimum. You will still need to put the splices in a j-box, but >>>>you're going to avoid the neutral issue, conduit bending and fitting, >>>>plus >>>>you won't have to figure and make allowances for derating of the >>>>conductors. >>> >>>>That is just asking for multiple code violations. >>> >>>Um, it's completely legal and approved. Why are you suggesting it's not? > >>How about that there is not appropriate bus transfer? Nor is the >>occupancy and some other special applications properly addressed. There >>were big changes between the 2005 and the 2008 NEC for all classes of >>backup and alternative power systems. > >What the heck are you talking about. All he's doing is relocating the panel >and extending the homeruns over to the new location. There's no back-up or >alternate power sources being used here. So i check the thread and i can't find any such either. I must have wigged over he new panel and j-box. |