Residental mains wiring questions (USA)
in [Design]
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From: JosephKK on 3 Jul 2010 12:29 On Fri, 2 Jul 2010 04:32:47 -0400, "Rich." <rcres(a)XXcomcast.net> 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. > >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. 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. 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. Thanks, i was about to try expressing that clearly. You already did it well.
From: JosephKK on 3 Jul 2010 12:43 On Fri, 2 Jul 2010 08:14:02 -0700 (PDT), bud-- <budnews(a)isp.com> wrote: >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? In actual practice, it means you negotiate with AHJ based on what the existing construction (all properties, not just the wiring) is and which fire / life safety properties and cost get balanced.
From: John Larkin on 3 Jul 2010 16:24 On Fri, 02 Jul 2010 09:09:26 -0600, m II <c(a)in.the.hat> wrote: >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. > > IF the loads are sine waves. John
From: m II on 3 Jul 2010 17:53 Fester Bestertester wrote: > Your example is for 3-phase, right? For residential split-phase, the vectors > would be 180 degrees apart, right? Well, in a single phase service, the two sides of the phase seem to be 180 out if you use the common tap as reference (black lead) and MOVE the lead (red lead) you are measuring with. In polar notation that would be (120 V at 0 degrees) plus (120 at 180 degrees) equals zero volts. The rectangular equivalent is: (+120 +j0) + (-120 + j0) = 0 (swapped meter leads) In reality, it's the same current flowing through the entire secondary winding. There is NO phase angle difference between the two sides. No amount of centre taps will add a phase. Think of two car batteries in series. Measuring from between the two batteries to the free terminals on either side will give you the same numbers. (+12v) +(-12v) = 0v. Measure between the two outermost terminals and you get 24 volts. (12v +12v) = 24v. The currents are sure not out by 180 degrees in the batteries. In a single phase, centre tapped transformer it's identical. One reason they ground that centre point in your panel is that it limits the maximum available fault voltage to ground to 1/2 of line to line voltage. That is the absolute maximum fault voltage to ground you can get. Start moving that ground around and fault voltage rises. The other reason the ground is in the middle is that it clamps the voltage of the phase sides to 120. If it weren't there, the two sides of the transformer secondary would act like a series circuit, with each side having a different voltage, depending on what was plugged into each. The two voltages would still add up to 240, but one side may be 80 volts and the other 160. That situation is called a 'floating neutral'. These guys explain it a lot better: http://www.allaboutcircuits.com/vol_2/chpt_2/5.html mike
From: m II on 3 Jul 2010 18:04
John Larkin wrote: >> In 3 phase, You have to have all three hots feeding a load equally >> before the neutral balances out. > > IF the loads are sine waves. I'm guessing that if all the waves were the same shape, it wouldn't make a difference. Start putting a different shaped wave or one of a different period on each phase and then it's a different ballgame. mike |