My house was built in the 1940s and has armored cable with rubber insulated wires. As I’m rewiring, I found a cable which serves two circuits. It has 3 wires, red, black, and white. It appears that it has 2 hot and 1 neutral.
I was wondering if it was a common convention, either then or still today, to have a single cable serve two circuits. Is it much of a problem and should I be rewiring all the way back to the panel or just wherever it’s easiest.
Listeners write in about deepening foundations and ask questions about HRVs, smoothing rough concrete, and finding the right workwear.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
Yes, common, but mostly in commercial. It is legal to share a neutral between two circuits, but I rarely do it any more because some DIY-er is gonna kill himself thinking he shut off every thing in a box or burn dow his house because he moved one of the two wires to the other phase in the panel.
It would be a good idea to rewire whereever you can to provide a ground in your circuit, especially in the kitchen, basement, bath, garage and any high load area.
If you haven't drawn blood today, you haven't done anything.
"but I rarely do it any more because some DIY-er is gonna kill himself thinking he shut off every thing in a box"
Now who would do something stupid like that??? Actually, I gotta confess to being one of those DIY-ers who luckily discovered my error.
Just so I don't shock myself again, or burn the house down, do I have this right:
Run the 3-wire, armored cable into a metal box. Run 2 new 14/2 Romex into the box, connecting all white neutral wires together, and one black hot wire to the old armored cable black and the other black to the old armored red. Ground both to the box. Those two new Romex cables will then service their respective circuits.
I'm sure this is one of those dumb DIY-er questions, but just trying not to make the headlines of the local papers.
Edited 8/18/2005 8:52 am ET by KenL
That is correct.One other thing is to check and make sure that the two hot are on different legs so that there is 240 volts between them. On most panel that is on adjacent full size breakers (or a 2 pole breaker). I am asuming that the panel as been upgraded at least once and it might have been moved.BTW, it is called a multi-wire circuit.
Edited 8/18/2005 9:20 am ET by BillHartmann
"make sure that the two hot are on different legs"
Each of the hot wires is connected to a separate single pole breaker inside the box. By rewiring the way I described, each leg will then be switched by their own breaker, right?
If you don't mind me asking one more stupid question, in looking at an existing old 2-prong outlet on this circuit, it appeared that red went to one prong, black to the other and the remaining red/black were connected together. Why? It seems like you'd be crossing over the circuits that way.
"Each of the hot wires is connected to a separate single pole breaker inside the box. By rewiring the way I described, each leg will then be switched by their own breaker, right?"Yes, that is OK. If any device has both hots on it (like it was) them it needs a 2 pole (240 breaker) so that it kills all of the power on the device. But by spliting it out in two circuit like you are each device will only be powered by one leg or the other."If you don't mind me asking one more stupid question, in looking at an existing old 2-prong outlet on this circuit, it appeared that red went to one prong, black to the other and the remaining red/black were connected together. Why? It seems like you'd be crossing over the circuits that way."I am not following you here.The way that I would EXPECT it would be for the neutral to be connected to the side side with the wide blade (silver screw on newer receptacles). On the hot side (gold or dark screws on new receptacles0 break the tab between the two screws and connect the red ot one and the black to the other. That puts the two halfs of the receptacle on different circuits.IIRC Canadian (or maybe just Ontario) kitchen coutertop receptacles are wired like that.If this circuit was exteneded them you would do the same them except connect like wire to like wire and also pigtails to connect to the receptacle. More commone would be to either split it off like you are doing to to wire the first receptacle to one side (black or red) and the next one to the other and not break out the tabs.OK, I just guessed what was going on. That is what they did, but they did not bother to keep the legs separately. In fact they might have done it on purpose is they followed the same "logic" it would balance the loads. The incoming black is connected to one terminal on the hot side. One wire for the OUTGOING cable is connect to the other terminal on the SAME SIDE. Except they used the read instead of the back. Then the read from the income is connected to the black for the outgoing cable. That is OK, if followed at each recetpacle, just confusing.But I would verify that you have 120 between each hot and the neutral. And 240 between the two hots. Just to make sure that the supply is wired correctly.However, some peole might try and use the two terminals and the junction for the neutral. That is against the code and unsafe because if one of the connections where broken then you could have upto 240 across the other receptacles.
I gather that one reason for wiring receptacles like that is so that one can be switched while the other stays hot. Personally, I am not a fan of switched recepticals and I think that is is some builders way of saving a couple of dollars, not putting in overhead boxes.
"I gather that one reason for wiring receptacles like that is so that one can be switched while the other stays hot. Personally, I am not a fan of switched recepticals and I think that is is some builders way of saving a couple of dollars, not putting in overhead boxes."No, we are talking about a completely different situation.What saves in running a 2nd cable and the ability to have more power at any one receptacle. IE, you could have a toaster and coffe maker working at the same time and pluged into the same duplex receptacle.
Bill, I think your last description of the wiring is right. When I go home, I'll take a photo and shoot some wires and post tonite.
Thanks for such a helpful explanation. I feel smarter this afternoon than I was this morning (but that doesn't take much for me, haha).
Bill, just as you described. Attached is a sketch of how the plug is wired.
I still don't understand, though, why one would wire this way.Thanks again for the explanation!
Edited 8/18/2005 9:19 pm ET by KenL
Ken,
Here's a schematic of what you've shown and told.
View Image
Looks ok to me.
Whatcha got at the other end of the three wires? ? ?
SamT
Bill, for the benefit of those who have not seen your great explanation of this problem before you might tell them why doing otherwise is dangerous. I am having trouble understanding how kenl's solution you are responding to would be much different than the status quo he has. If you were to be on the wrong leg back at the panel would it not be the same?
One correctly wired multi-wired circuits the currents in the neutral will cancel. So that maximum amount of current is you have one leg fully loaded (15 amps in this case, IIRC) you would have 15 amp in that hot leg and in the netural.If you have both legs load then you with have 15 amps in the black, 15 in the red, and 15-15 = 0 in the neutral leg.However, if they are connected to the same 240 leg the currents in the neutral ADD. So if both legs where fully loaded the neutral would have 15+15 = 30 amps and would be overloaded.On MOST panels each full size breakers are on althernate legs, so any two adjacent breakers would be OK. However, there are tanden breakers where both are on the same leg, and pancake breakers (on older zinsco) where they may be either way.So unless you are sure about the panel verify that you have 240 between the black and the red.The other "issue" with multi-wire circuits is if the neutral becomes disconnected taht you have have upto 240 volts on the receptacles depending on what loads are connected at the time.
If you were using 3 wire for separate circuits, would it be better to use a 2 pole breaker rather than two single poles? This would make it more difficult for someone to move things around & end up with both hots on the same leg (thus doubling the current on the neutral rather than cancelling it out).
Yes. But on the other hand it would cause both circuits to trip when there is only an overload on one.And don't forget that there are alwasy "better idiots". Some one could pull the 2 pole and replace it with 4 tandems because they need to add more circuits. And end up with both on the same leg and it "still works".
make sure that the two hot are on different legs so that there is 240 volts between them
Scary memories of 240V room a/c circuits "converted" to 110 . . . Occupational hazard of my occupation not being around (sorry Bubba)
"Just so I don't shock myself again, or burn the house down, do I have this right:"
I'm happy to hear that you are still in the living world.
Be sure that you are not using the flex of the armored cable as your equipment grounding conductor. If the AC cable has only a white a black and a red (no green or bare) then you need to run a green or bare #12 wire to a sure source of ground. The panel would be ideal, but a water pipe would work if it is continuous metal from your point of connection to the earth.(no plastic or dielectric unions)
Oh, and "In dwelling units, a multiwire branch circuit..." (that's what you have) "...supplying more than one device or equipment on the same yoke..." (a split-wired duplex receptacle) "...shall be provided with a means to disconnect simultaneously all ungrounded..." (hot) "...conductors at the panelboard where the branch circuit originated." (a 2-pole breaker) NEC 210.4(B)If you haven't drawn blood today, you haven't done anything.
"Be sure that you are not using the flex of the armored cable as your equipment grounding conductor. If the AC cable has only a white a black and a red (no green or bare)."I think that this is another local rule. In the 99 NEC AC cable is still listed as an approved EGC. 250-118 (9). I know that read of local areas that don't allow it."then you need to run a green or bare #12 wire to a sure source of ground The panel would be ideal, but a water pipe would work if it is continuous metal from your point of connection to the earth.(no plastic or dielectric unions)"The NEC does not allow connecting to a water pipe, except for the first 5ft.
"The NEC does not allow connecting to a water pipe, except for the first 5ft. "
You are right. Our local AHJ lets us go to a water pipe even though it is technically not part of the grounding electrode system. It would be better to get to a grounded electrical box, the panel, even an EMT.
"I think that this is another local rule. In the 99 NEC AC cable is still listed as an approved EGC. 250-118 (9). I know that read of local areas that don't allow it."
If you look at the definition of AC cable and its construction specifications you will find that the old stuff that was used in the forties and fifties does not meet the definition of AC and it would be better to apply the code for flex to this situation and not rely on the flex as a EGC.
If the original poster wants to rely on the flex for the EGC then I would suggest that he be sure to use proper fittings on both ends and make sure they are tight.If you haven't drawn blood today, you haven't done anything.