Hi,
I just read the thread started by Kevin about sharing a single 14 awg neutral among two separate 15 amp circuits. I have a slightly different situation.
I have been rewirng my house over the past year or so, specifically the second floor. The attic is above the second floor, and is not finished and will not be. The wiring as I found it consisted of about three distinct stages of renovation. The original AC cable was installed in 1922 (according to a UL test tag that I found attached to some of the cable. This wiring basically covered about 1 lighting fixture, one switch, and one receptacle per room. There was a second renovation using the paper/cotton insulated NM wiring, which I am guessing was about 1950-1960. This brilliant job added about two or three additional (non-grounding) receptacles per room, but they were all on the same 15 amp circuit! Finally, some more recent work added some 230 V circuits for an electric range and a big mother window A/C. There is also a 12/3 shared neutral circuit that is kind of like what Kevin described. Basically, I ripped pretty much all of this out over the course of my other renovations.
So most of the wiring that I did was going up through the walls into the attic, for various receptacles, switches, and light fixtures. In the attic, I used the 3M cable stacker wire clips tacked onto 1x4s to make kind of open raceways to organize the wires from point a to point b. I found this to be much more flexible than having to hammer staples in the dusty blown-in insulation. I built 10′ long sections downstairs, and then laid them out in the attic and screwed them into the floor joists.
Now I am doing the final stage, which is running a feed back to the main panel in the basement. My plan is to install a NEMA enclosure (basically a large junction box) in the attic, and run 1.25″ PVC conduit down through a closet into the basement and back to the panel. The question (after all this background info) is whether I can run single oversize stranded ground and/or neutral conductors (I was thinking maybe 6 awg, but haven’t decided) and then run the stranded cable to a terminal strip and then out to each circuit. I would run individual hot lines for each of the 10 or 12 circuits. My rationale was that it would be easier to pull a smaller number of cables through the conduit.
I could also install a subpanel, I suppose, but I couldn’t install it in the attic because there’s no regular access, just a hatch – you need a ladder to get in. I could install it in the closet, but the problem with that is that there’s not much clearance in the closet. I know that commercial/industrial panels need 36″ clearance – does this hold true for residential subpanels?
I guess my main questions are whether it makes sense and is permissible to use the common neutral and ground, or should I just run individual ones? Also, if I do run a common, how do I size it? THHN 12 and 14 awg is like 2.5 cents/ft at Home Depot, so I am not really looking at much of a cost difference, in fact the bigger cable might cost more overall.
Thanks for reading this longwinded post and I hope to hear some interesting replies.
Newbie Alec
Replies
when in doubt, put in a larger conduit and do it right
The conduit is big enough for what I need to do. According to NEC, I can put up to 69 14awg or 51 12 awg conductors in my 1.25" conduit. I'll probably have about 8 20-amp circuits and 6 15-amp circuits in this feed. So if I did it with individual hots, neutrals, and grounds, that would be 24x12awg conductors and 18x14awg conductors.
Based on the allowable max # of conductors in the NEC, if I run 24x12awg and 18x14awg, the conduit would be 73% "full". If I run 8x12awg, 6x14awg, and 2x6awg for N and G, the conduit would be 43% full, and if I stepped up to 4awg for the N&G, I would be still only 53% full.
I just was thinking it might be easier to pull a single large ground or neutral conductor compared to all those individual conductors. But would it be legal?
Any thoughts?
You have another problem. With 14 current carrying condutors you have to derate the ampacity by 50%.
I think that you can run a single ground wire, but not sure. If you can then it only needs to be a #12. As it is not a current carrying wire, but only for carrying the fault current.
I would start by planning on running 2 or maybe 3 conduits.
Bill,
Good point on the ground wire being for fault current only. I suppose the assumption would be that only one circuit would fault at a time.
Can you elaborate on the ampacity derating? What info do you use to determine derating?
Thanks
Alec
The rating of wirings and devices is based on heat buildup and the type of insulation. For example type TFE wire is nickel-coated copper with a teflon insulation and is good to 250 C. In free air #14 is rated at 59 amps.
So there are several requirements and adjustments in the code about heat build up. That is the reason for using the stacking wire holders rather than stapling one on top of another one.
There is also derating for high ambinate temps, but that would not be a concern in residential applications.
So for cables stacked together or wires in a conduit over 24" the derating is
4-6 80%
7-9 70%
10-20 50%
Now these are for current carrying conductors. The ground is not a current carrying conductor and thus does not count. A neutral that is only carrying the unbalanced between the two hots is not counted. So if you run a 2 120 circuits you have 4 current carrying conductors. However, if you run it as a multi-wire circuit you only have 2 current conductors.
Now the most common wire for this application would be THHW and that has an ampacity rating of
#14 20
#12 25
#10 35
Thus they could use the 80% derating and the #14 would be 16 amps and #12 is 20 amps so they could be used on 15 and 20 amp circuits.
But if you have the 70% derating then you need to up the size of the wires.
Ok, I understand your derating schedule. But is it only based on the number of current carrying conductors and not the diameter of the conduit? I think you'd have more problems with ampacity with 6 10awg in a 1/2" conduit compared to those same 6 in a 2" conduit.
That is the limit of any code. They give you tables of maximum this and minumum that that will cover about 90% of the applications.
They also don't adjust for the type of conduit, but I would think that a metal one would conduct the heat better than PVC.
But like the other codes there are provision of actually engineering the requirements. That means computing the actual heat rise under that specific condition.
common ground = ok
common neutral with this many circuits sounds like a no-no
Derating due to quantity is a good point to keep in mind too.
Also, remember that once you've pulled a bunch of wires into a conduit, code does not allow you to pull any more in later (very easy for rubbing somewhere to overheat the insulation and very quickly wear right thru). You would have to pull all the existing wires out, then re-pull the whole thing with the new wires included. Not hard, just annoying.
Of course, 'code' is not there watching you while you do this so you can try to do any thing you want, but feeding a new wire into a partially full conduit is a drag, wires get tangled and everything.
So, running two conduits, one with all your planned wires and one that is empty for the future, and a dedicated neutral for each circuit (if you ever want to use any GFI on any of those circuits you will HAVE to run dedicated neutrals for sure) is the most "perfect" way to go!
Norm
Having a similar situation, this is what I would suggest doing if permissable in your area. In the closet, open up a space near the top and the bottom of the wall that is big enough that you can grab wires with your hand. In the attic, drill 4 to 6 (3/4") diameter holes through the top plate above this hole. In the basement, drill 4 to 6 similarly located holes in the ceiling.* Above where the holes are drilled, place a number of the 2 1/4" deep 4x4 J-boxes. Now, one at a time, pull either (1) 12/3 or 14/3 through each pair of holes, or a couple of 12/2 or 14/2 through each pair of holes, from the basement up to the attic. Then wire 2 circuits into each J-box in the attic fed by a single X/3 or pair of X/2. If you plan ahead, it is really easy to fold and organize the wires this way in each J-box for future reference or adding more wires. I was lucky in that there was unused ducting from the basement to a vent in the second floor wall. It was then trivial to cut open the top of the vent, and then drill a number of holes in the top plate over the vent, and pull pairs of 12/2 up through each hole.
IMHO, running conduit would be a pain unless you were going directly from the service panel to a legitimate sub-panel using 6/3. You are going to have a whole bunch of loose *identical* wires to keep track of. Or you are going to have to run a bunch of conduits with a handful of wires each, and if using single wires then you will have to allign a whole bunch of pipes from the service panel to the third floor since they will have to be contained the whole way. From service panel to J-box in this case is best done with Romex.
Your comment about trying to nail staples into the brown fluff insulation is one reason why there isn't brown fluff insulation in my attic anymore. Well, that and the poorly terminated K&T wiring randomly spread throughout the attic within said insulation.
* Actually, in thinking this through, we seem to be missing a floor. Are you going through similarly located closets on both the first and second floor?
You might also look for plumbing stacks, since they usually provide an unobstructed path between the basement and third floor attic.
I ran two inch and a quarters from my panel to the attic to do just what you describe only,you can't make a common"super"neutral.Just cut the correct number and length of whites,one at a time,and label each end.When you're done pulling them in,the labels won't be exactly where you want them,so re-tag as you're making joints.I ended up using one pipe for 220v loads and one for 110v loads.I ran a #6 insulated(don't use bare in a metal pipe) ground to a ground bar in the j-box.I believe there's a code requirement that you have to use a fire rated sealer in the pipe when you're done to prevent the "stack"effect in case of a fire.Maybe that's what Norm meant,otherwise, yeah you can pull extra wires in after the fact.And yes the three foot rule does apply to panel clearances residentially.