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?
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.
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.
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.)
Well, iiuc, ALL municipalities use the NEC as a "basis".
Regarding NYC, I read the opposite here on this ng in a recent thread: NYC relaxed its onerous electric code to substantially the NEC AFTER 2001 or 2003. Before that, it was quite stringent.
I know for a fact (well, at least if my bosses were correct!) that in the
1980's, you could *not* put 20A breakers on #12 wire -- at least not before inspection.
As another example, in those days, you were allowed eight #12 conductors in
1" EMT -- in fact, it was 3, 5, and 8 for 1/2, 3/4, and 1" EMT, respectively.
Nowadays, for 1" emt, NYC allows pretty much what the NEC allows -- pert near close to 20 wires in 1" emt! What a diff.
This may also reflect more modern wire insulation material.
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.)
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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.
Every three years a new set of rules comes out in the NEC. It's entirely possible that the "conductors in conduit" changes are reflective of that. I've never seen #10 used for 20 amp circuits in NYC. If the conductors were aluminum, not copper, that would be the case, although I'm not sure if NYC ever allowed small conductor aluminum wire.
If there's even an inch of #14, old or new, anywhere in the circuit, then that circuit needs to be protected with a 15A breaker or fuse, and there is essentially zero advantage to splicing in any #12, unless it's all you have and the stores are closed. Possible exception is if it's a very very long run, like to an outbuilding, in which case you maybe need to use #12 or heavier with 15A protection.
If it's #12 that's visible leaving the panel, then there's a danger that someone will assume the whole circuit is #12 and thus they might put in a 20A fuse. So if your new wire is going to go into the panel, then there's a very good reason to make it the same was what's already in the rest of the circuit.
(Besides, some will tell you that old wire is HEAVIER than new stuff of the same nominal gauge. No matter. Old #12 or new #12 should ok with 20A, unless it's damaged, in which case it shouldn't be used at all.)
You got to tell us what the heck is going on here.
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.
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.
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?
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.
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.....
Apparently you don't, so permit me to enlighten you. Code requires that all 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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