Splicing #14 wire, hot to neutral ratios....

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Awl --
A two-part Q:
Due to some remodeling, I'm forced to splice/lengthen some old bx cable that has #14 wire in it, and re-connect to the fuse box. Should I splice the #14 with #14, or splice with #12?
The reason I would continue with #14 wire is just to remind me and future people that Hey, this old wire is a little lighter than modern #12, and to not take liberties with 20 or 30 amp fuses or breakers. Other than that, #12 would be fine with me.
Next, is there a rule of thumb for how many hot wires can share one neutral of the same gauge? If using different gauges, is there a "gauge ratio", ie, some formula for cross sectional area between total hots and total neutrals?
tia.
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EA



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Existential Angst wrote:

There's nothing "unmodern" about 14 for 15A circuits.
I'd recommend sticking w/ same gauge as the original circuit for the reason mentioned.
Remember to not bury connections in inaccessible places.

Don't follow this question. Each hot needs its own return neutral. If you're talking about 3-wire ("Edison") circuits, if properly wired the return is in effect a neutral w/ a balancing currents from the two sides if both are loaded equally; hence only the same size conductor is required for a single-sided load.
If that isn't it, I have no clue what you're thinking.
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Really? Does the NEC state that?
I thought #12 was the de-facto "standard" for wiring, but mebbe that was just NYC in its oppressive heyday, before it relented and went with the NEC. In that heyday in NYC, 20 A breakers were illegal on #12 wire! Altho after inspection, few people adhered to that limit.
What is the max breaker allowable on #14 and #12 wire with modern insulation, nowadays? Ditto, with cloth insulation?

Indeed!
I think you answered the Q: If each hot in a 3-wire bx is connected to a different leg in the panel, then they can share a common equal-sized neutral. So the "ratio" would be 2:1.
Which is why the neutral in main service can be "de-rated" rel. to the hots, because the assumption is that there will in fact be some balancing of the load between the two legs in "Net usage", reducing the demand on the neutral. In a worst case scenario -- full load occurring on only one leg -- then the neutral proly should be equal in gauge to the hot gauge. But statistically, esp. in large buildings, this is unlikely. iirc, the svc neutral to large buildings can be smaller than the gauge of either hot leg, but I wouldn't bet the farm on my memory.
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Yes. Has for decades.

15A and 20A, respectively, regardless of the type of insulation.
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wrote:

The neutral is not derated in the service. Let's take a 200 amps service. ALL the conductors in the service cable are rated for 200 amp capacity because that is the max current you can have flowing in them. If it's a pure 240volt load of 48KVA or a balanced 120volt load of 48KVA, then 200 amps is flowing in the two hots, zero in the neutral. If it's a 120volt unbalanced load, then 200 amps is flowing between one hot and the neutral.
Gee, seems I recall having this discussion here before.....

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If I'm following what he's asking correctly, I think he's trying to pigtail some existing wires within a breaker panel to extend them to their connection points. I also am guessing that this involves more than one circuit, so he wants to connect several neutrals to a single larger wire with a wire nut (or crimp cap, solder, whatever) and then connect that larger wire to the bus, rather than pigtail each individual neutral and connect them individually to the bus.
However, I have no input on whether this is even a valid configuration; unless I were out of space on the bus bar I think I would probably just pigtail each wire individually with a wire of the same gauge - BUT I have no idea if what he's suggesting is OK per code. (I suspect not; you wouldn't want a single point of failure to affect multiple circuits. But that's just a guess based on thinking it through, not actual knowledge of code.)
nate
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wrote:

If I'm following what he's asking correctly, I think he's trying to pigtail some existing wires within a breaker panel to extend them to their connection points. I also am guessing that this involves more than one circuit, so he wants to connect several neutrals to a single larger wire with a wire nut (or crimp cap, solder, whatever) and then connect that larger wire to the bus, rather than pigtail each individual neutral and connect them individually to the bus.
However, I have no input on whether this is even a valid configuration; unless I were out of space on the bus bar I think I would probably just pigtail each wire individually with a wire of the same gauge - BUT I have no idea if what he's suggesting is OK per code. (I suspect not; you wouldn't want a single point of failure to affect multiple circuits. But that's just a guess based on thinking it through, not actual knowledge of code.)
============================================== Heh, you must be clairvoyant!! :)
Yeah, I figgered I'd keep it simple, rather than go into all that, but you got it right.
NYC at one time had it in their code that you can't run a hot wire in a pipe without a corresponding neutral wire in the same pipe. Don't kinow if that's part of the NEC, or another NYC-ism.
The Q might be then, What constitutes "corresponding neutrals": one neutral for one or two hots, or one big neutral for a bunch of hots?
No biggie just to extend each wire individually -- might even be simpler, ultimately, esp.if future work will be done. Less to figger out.
--
EA



nate



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Existential Angst wrote: ...

have its corresponding neutral, no exceptions.
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On 11/6/2009 8:38 PM dpb spake thus:

Yes, and from the OP's continued queries, it sounds more and more like they don't really don't know much about wiring, which makes what they're trying to do pretty doubtful.
To reiterate: the *only* circumstance where two hot wires can share a neutral is in a so-called "Edison" circuit. The requirement is that the hot wires must be on different phases--in other words, from different buses in the distribution panel. That way, the sum of currents in the one neutral can never exceed the wire's rating. The OP said some vague things about "balancing loads": it's a whole lot more specific than that.
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There is nothing vague about "balancing loads". What you may find vague, or perhaps are just unclear on yourself, is along what part of the run can the load be balanced.
If you have 20 2-wire bx cables stopping, say, 10 feet short of a 20 circuit panel, then, it seems to me, you can de facto *make* 10 edison circuits up to the splice points, and therefore need only 10 neutrals going between the panel and those 20 cables.
Thus, the edison circuit business is dependent on the actual *geometry of the wiring*, ie, "partial length" edison circuits are possible.
Thus, your seemingly restrictive "only circumstance" for two hot wires sharing one neutral is in fact a fairly non-limiting requirement in this splicing circumstance, as ultimately, if a 20 circuit 240 V panel is filled, you will *always* be able to make 10 edison circuits, for at least part of the run.
AND, if you grok the above, then it follows that for that length of the "edison run", you could, in principle, use one big neutral.
Not saying I will do this, just trying to eliminate some, well, vagaries.
Love your sig.
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On 11/6/2009 11:08 PM Existential Angst spake thus:

So you do understand the concept of "Edison" circuits, right? That you can't just grab any two hots, run a neutral with them and call it a day? Please tell us you understand this, or don't try it.
Do you know *why* you can use just one neutral to serve two hots with this kind of circuit?
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My first reply to dpk would indicate that I do understand edison circuits. Your previous post, and this one, indicate that you have a problem understanding that I understand.

If I didn't, would you tell me?
Here's the deal:
If the loads on each service leg are "balanced" (recall that vague reference?), ie, electrically identical ito impedance, you wouldn't need a neutral at all. The total voltage applied to the two loads (effectively in series here) is 240 V, and the voltage drop across each load on a leg is then 240/2 = 120 V, which is the appliance rating.
But what if one load is a 100 W bulb is on one leg, and a 1,000 W toaster is on another, and the neutral is lifted. The voltage drop across the bulb is 10/11 * 240 and that across the toaster is 1/11 * 240. Thus, the bulb will burn out, but not the toaster.
Thus, the neutral acts as a kind of centertap for unbalanced loads on each leg, proly best analyzed with Thevenin's theorem -- in wiki, if anyone is innerested, altho the sample problems there are not very illuminating. Thevenin's theorem is essentially Ohm's Law on effing steroids.... wow.....
This Edison business explains why, in the presence of a a marginal or inadequate neutral, adding or switching on another load will actually make lights *brighten*, because now current from one side of the svc is not being driven through a high-resistance neutral, and instead flows better through the now-balanced load on the other svc leg.
This neutral business clearly explains why 240 V appliances are inherently superior from an electrical loading pov -- both legs are balanced by definition, and no neutral is required at all. Plus, at 240 V, there is much less IR drop, and much less I^2R transmission loss. It's better all the way around.
Europe operates on 220-240 V, but according to one poster here, that is between one leg and neutral, so they have the same "balancing problem" we do, just at a higher voltage. The key is to have 240 V *between two hot legs* -- and, of course, corresponding 240 V appliances.
Having said all this, I'll proly not go Edison-crazy, and just splice all neutrals directly to the new panel. This way, there is no possibility of future incompatibilities resulting from unwitting changes.
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ungrounded (i.e. hot) conductors of a multiwire (Edison) circuit have a *single* disconnecting means -- so unless you can find a 20-pole breaker, you're going to have a tough time meeting Code with that monster circuit you proposed.
The reason Code requires this is straightforward: the neutral wire carries current. If one hot leg of a multiwire circuit is left powered on, and any load on that leg is in use, the current drawn by that load is flowing in the neutral -- presenting a risk of fatal electric shock to anyone servicing the other leg of that circuit.
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So are you saying that in a simple 3 wire bx cable, with the two hots sharing a neutral, you'd need a two-pole breaker?
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Existential Angst wrote:

yup, exactly. Or if they were two single pole breakers they'd have to be next to each other with a handle tie.
nate
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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......
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Good so far.

Code requirement as of the 2008 NEC.
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From http://en.wikipedia.org/wiki/Shared_neutral, it would appear that wiki agrees with you! The article also references the term Edison Circuit.
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.
-------------------------------------
Man, this is news to me!
It seems, tho, there are two separate issues at work here: 1. The current carrying demand on a neutral conductor 2. A personnel-type issue, ito breaker panel voltages.
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, http://www.phy.ornl.gov/divops/ESH/98-2.html ( a link from the above wiki site) apparently frowns on edison circuits, and indeed suggests double pole breakers as one remedy.
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!
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Existential Angst wrote:

well, you don't need neutrals for "pure" 240VAC circuits, such as a water heater, air conditioner, air compressor, etc...
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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 1. When the first circuit's amplitude is at its peak, the other phase is at the bottom, and the two phases cancel each other out. It doesn't seem to be common sense that by adding a load to the other half of unbalanced load that you'd actually be reducing the current in the shared neutral wire, but that's how it works. I think. (-: At least that's how my friend who designs 240VAC gear explained it me.
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