Perhaps just put a suitable pull box where the lines will reach, and install a ground buss bar kit and an isolated neutral bus bar kit, and tie them to the ground and neutral in the panel with appropriately sized conductors. Since it's a short run, the same gauge as the panel neutral and ground connections would certainly be sufficient.
When I have done this type of extension for a panel replacement, I have normally done something like this for the grounds, but not the neutral.
OK, one more clarification: Said 3-wire bx cable, with two hots, one neutral, each hot on a sep. svc leg, and each hot powering a 120V appliance, right? Say, one hot for the microwave, and the other hot for the toaster, with each neutral from that appliance's outlet/box being pigtailed together to one neutral going back to the breaker panel. Good so far?
You would then put those two hots on a double pole breaker? So that if the toaster goes out, the microwave goes out as well? Hmmmmm......
From
Split phase shared neutral In split phase house wiring, for example, a duplex receptacle in a kitchen is typically connected with a cable that has three conductors, in addition to ground. The three conductors are usually colored red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from a ganged breaker, i.e. a breaker in which the handles are tied together for a common trip, so that if one kitchen appliance malfunctions and pops the breaker, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit.
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Man, this is news to me!
It seems, tho, there are two separate issues at work here:
The split-phase nature of the two hots addresses (1), while the double-pole breaker addresses (2). I personally never thought (2) was such a big deal, as any wire dangling in the air can be hot, *including* a plumbing connection to the street, say, when the water meter is disconnected. But,
AND, a double pole breaker also has another safety feature that just dawned on me, quite related to (1): It discourages moving one of those neutral-sharing hot wires in the breaker panel willy-nilly, possibly putting both hots on the same svc leg, thus overloading the neutral.
But here's what I have experienced: More often than not, in opening a junction box, esp. the larger 5" ones, I'll see oodles of neutrals wire-nutted together, irrespective of the phase of the hot leg. Apparently there is a wide-spread cavalier attitude toward the lowly neutral.
I suspect some of this cavalier attitude toward neutrals comes from the notion that the ground, or other mystery neutrals, are picking up some of the slack anyway, so no biggie. When in fact it could be a biggie.
And consider this: For X number of circuits, there should X number of neutrals on the neutral bar, MINUS the number of edison circuits. This would seem to be check out pretty well on bx/romex systems, but how about separate wires pulled through EMT? It seems that there could be much more abuse of the hot/neutral ratio in pulled wires. I've seen pretty scarce neutral bars in some breaker panels!
well, you don't need neutrals for "pure" 240VAC circuits, such as a water heater, air conditioner, air compressor, etc...
nate
Correct -- not only for the reason I cited above, but also for another reason which has already been discussed in this thread: to ensure that the two hots are on opposite legs of the service.
If the two hots are on the *same* leg of the service, then the neutral could be overloaded, because it will carry the *sum* of the currents in the hot wires. This is a fire hazard.
Heh, this is exactly what I was contemplating! And alluded to when I mentioned "neutral cross sectional area vs. total hot...." I woulda been more explicit, but I didn't have a sufficiently fire-retardent flame suit handy.... :)
I proly will just run the individually spliced bx cables and their neutrals to the panel, if for nothing else so the next person looking at the box doesn't scream, WTF??!!! Just more straightforward, less confusing in the electrical bookkeeping, ultimately safer.
The original fuse box happens to have 6 three-wire bx cables, so when I hook them up to the new breaker panel, I will make doubly sure the phases on each of the wires in these cables is correct. Judging from how the lites dim in some situations, I suspect they are not phased-up correctly!
Good so far.
Code requirement as of the 2008 NEC.
In article , "Existential Angst" wrote: [major snippage]
More likely IMHO a failure to understand that the neutral carries current. It's a Code violation, and a dangerous one, to connect the neutral of one circuit to the neutral of another circuit unless the two are part of the same multiwire circuit.
Indeed, I just posted that in my second response to Nate -- really very inneresting.
Altho this job is really a pita, it is also an opportunity to perhaps remedy some of this, as per my response to Pete.
This shared neutral business is a little reminiscent of the Ring Circuit, brought up in a recent thread. Altho of course different, the similarity to me is that both share a kind of clever "slickness of economy", that can unfortunately backfire -- heh, almost literally.
All in all, a super-illuminating thread, that will undoubtedly make this project better and safer -- in fact, better and safer than the existing state of this wiring.
It's perfectly normal to see several neutrals wire nutted together in a junction box. Say a switch is going to serve 3 lights that have seperate runs to them. In that box, the neutrals from each light run would typically be tied together with the neutral coming into the box from the breaker. So, you'd have 4 white neutral wires nutted together.
That is distinctly seperate from two hots from two breakers sharing a neutral in an edison circuit.
Agree completely. Also appears that the OP has some slightly odd ideas about 'modern' versus 'older' wire gauges?????
To possibly keep discussion as simple a possible; recommend: a) If existing wire is 14AWG, use 14AWG same as previous this will help to remind anyone else who in the future works on the system that the circuit incorporates 14AWG. (i.e. 15 amp). b) Use only 15 amp fuses or breakers, especially if there is any doubt that each/any circuit may contain any 14AWG. See personal note. c) Likely that the OP would not understand an 'Edison' wired outlet circuit. Let's omit and keep it simple. Personal note: Realised some months before an insurance inspection that we had wired a simple attached shed circuit comprising a single bulb, a hanging 3 wire (L,N & G) and an external GFI garden outlet to an existing lightly loaded 12AWG house outlet circuit that's been there since 1970. We used a length of armour covered wire through the outside wall and up to some 40 inches above floor, inside the shed, which 'might' have been 14AWG! We promptly changed the breaker for the whole circuit, back at the main panel, from 20 to 15 amp. Inspector never looked at it! But at least we know it's safer and to code!
It's perfectly normal to see several neutrals wire nutted together in a junction box. Say a switch is going to serve 3 lights that have seperate runs to them. In that box, the neutrals from each light run would typically be tied together with the neutral coming into the box from the breaker. So, you'd have 4 white neutral wires nutted together.
That is distinctly seperate from two hots from two breakers sharing a neutral in an edison circuit. =======================================
Good point.
And, I just checked my original fuse boxes. Of the six 3-wire bx cables coming, the 4 that I checked had both hots on the same leg! So I'm assuming all six were wired in this way. Which may explain why the lights sometimes dim precipitously. So someone didn't know about Edison circuits, eh??
So here's another Q:
Is a 3-wire bx cable leaving a panel *necessarily* an edison-type deal? Do the two hots *always* have to be connected to separate phases? I would think so, but I'm not absolutely sure. But I am going to put them on sep phases regardless.
Agree completely. Also appears that the OP has some slightly odd ideas about 'modern' versus 'older' wire gauges?????
To possibly keep discussion as simple a possible; recommend: a) If existing wire is 14AWG, use 14AWG same as previous this will help to remind anyone else who in the future works on the system that the circuit incorporates 14AWG. (i.e. 15 amp). b) Use only 15 amp fuses or breakers, especially if there is any doubt that each/any circuit may contain any 14AWG. See personal note. c) Likely that the OP would not understand an 'Edison' wired outlet circuit. Let's omit and keep it simple.
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Appreciate your succinct advice, but I had already posted these conclusions. As for (c), apparently you have not read all my replies. Keep this simple:
As far as odd notions of wire gauge, prior to about 2000, I don't think NYC made any general provision for #14 wire, except perhaps in special circumstances.
In new construction, I'd be curious to know the ratio of #14 wired houses vs #12 wired houses. Given our energy gluttony, I'd hazard that #14 is used in more rural areas, while #12 dominates in urban areas.
Heh.... could always breaker the neutral!
That's perfectly fine -- as long as they're all part of the same circuit. Wire-nutting together the neutrals from two different circuits is not fine.
How do you plan to do that?
Existential Angst wrote: ...
From the cable alone, "not necessarily" -- it _could_ be a 240V w/ third conductor ground. Of course, it would still require the hots be on opposite buses or there would be no potential between them and, of course, the third conductor wouldn't be neutral but ground.
But, if'en this-here supposed cable is serving 110V circuits, then yes. (Or at least at the moment I can't think of any alternative that would be Code-compliant).
Agreed. While that's technically true, most of the 240VAC equipment in an average home is not pure 240VAC. In my panel they run neutrals to power the oven timers, water heater igniters and other circuits needing only 110VAC.
I think the primary reason NEC demands "tied" breakers, is, as said before, to prevent electricians from thinking the entire split-receptacle is disconnected. Another problem is that if they are on separate breakers, someone could move the wire from one side of the panel to the other and thus overload the single neutral. Usually, the best and easiest fix is pulling two runs of Romex to the receptacle so that each circuit has its own neutral. Not sure, but believe the NEC considers that "parallel circuits." But it still doesn't get around the issue of an outlet being hot when you think it's not. Even testing an outlet is no guarantee that it's dead. A broken wire or bad connection could make an outlet read dead but once you got your fingers down past the break --- ZAP!
I believe the problem most people have with shared neutrals is that it seems like they are adding 1 plus 1 and getting 1 as a result. However, since they are out-of-phase, you're really adding 1/2 plus 1/2 and coming out with
-- Bobby G.
On 11/7/2009 11:51 AM Robert Green spake thus:
That's 'zactly right.
Being 180° out of phase, any current running in one leg of the circuit will be cancelled by any current in the other leg. The amount of cancellation depends on the amount of current being drawn in each leg. The highest current possible in the shared neutral will be when only one leg is drawing maximum current. If both legs draw maximum current, then the current in the neutral is close to zero. In this case, the majority of the current flow is through the two "hot" wires.
Very clever idea. However, for reasons given here many times, I think Edison circuits are to be avoided, on account of the potential problems they can cause.
Spend the extra 25 cents and use paired hots and neutrals.
On 11/7/2009 10:49 AM Existential Angst spake thus:
I know your tongue is in your cheek: however, while that would be a clever way of making the circuit safe, at least fire-wise, I'm sure you realize it could make it very, very unsafe, since it would possibly leave an energized hot wire but a disconnected neutral.
But hey, it's a fun thought experiment.
(This is a similar problem to those very badly designed fuseboxes of yesteryear that had fuses for both hot and neutral. My friend's house has one of those, which can leave the hot energized but the neutral disconnected. Wonder what genius came up with that design? His simple but elegant solution is to overfuse the neutrals (w/30-amp fuses), leaving 15 A fuses (actually circuit breakers in the form of a fuse) on the hot side.)
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