Residental mains wiring questions (USA)
in [Design]
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From: m II on 2 Jul 2010 01:46 JosephKK wrote: > Wire ampacity is not derated due to conduit fill. The derating for > operating temperature is usually minor. (pretty much for small > conductors #6 AWG and smaller) The number of wires in the pipe determine what percentage of their rated current they can carry. You are referring to the percentage of cross sectional area being used in the pipe. That, does NOT have any bearing on the case. I should have been more careful in my wording. mike
From: Richard Henry on 2 Jul 2010 01:49 On Jul 1, 9:23 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > 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. > Uhhhh....What?
From: m II on 2 Jul 2010 02:03 JosephKK wrote: > 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. 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. mike
From: m II on 2 Jul 2010 02:36 Fester Bestertester wrote: > Load current is *always* presented to the neutral conductor (as well as the > "hot"). That's the nature of any 2-wire system. No? > 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. In two wire circuits, the white is usually (mistakenly) called a neutral because it has a ground potential. It always carries the same current as the hot. A real neutral carries only the *difference* in amperage between the hots. ** The right term for a white in a two wire circuit is 'an identified, grounded conductor'. Note I said 'grounDED' conductor, not 'grounDING'. There's a difference. mike ** 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.
From: Rich. on 2 Jul 2010 04:32
"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. |