Wire fill calculation

[nissan_crawler]

Ok, I think I'm being mildly retarded here, but...

I have an dual gang switch box meant for 1/2" conduit (pvc). I'll be running it up the wall and through the ceiling.

I'm running 14-2 with ground. I'm trying to figure out what I'm allowed to do. It *looks* like I can run 9 conductors which I'm assuming is what they count 3 runs of 14-2 ground, BUT...That's for THHN, I can't find anything about NM-B, which I would *assume* would put me out, due to insulation.

So, what are my options?

1. Go up to 3/4" (would rather not, would complicate things greatly, but I can if needed).

2. Can I strip the insulation and run all 9 legally?

3. Do I put a junction box in the attic, connect the grounds, and only run one to the switches (is this legal?), and connect the neutrals in the box so all I have is a ground and 3 hots in the conduit?

I'm not an electrician, just know enough to get myself in trouble, and this has me wondering the correct/code way.

 

[boiler7904]

First question is why do you want to run Romex (NM-B) in PVC conduit?

 

[nissan_crawler]

because it can't be exposed legally...

I could buy other wire, but I have 1000' of this, plus from what I can find, it's allowed by the 2005 NEC code. It appears the 2002 code didn't, but was published in error.

Either way, I'm really not worried about that part of it. I just want to know the best/legal way to pull the rest of it off. NM-B in PVC isn't going to burst into flames (assuming of course, wire fill is right), hence why I want to be sure I don't over do the wire fill.

I may be able to put in an old construction box and run wires with fish tape, but would rather go this route, if I can.

Switching back and forth between thhn and nm-b between all this would be nuts.

 

[walrus]

Not sure what you're doing but the wire inside a piece of NM-b isn't listed to be used without the jacket, its a cable asembly. I doubt there will be any labeling on the individual conductors even though its probably thhn. Somewhere on the WWW you should be able to find the fill for that cable, fill the conduit to 40% and you're done.

IF you put 9 current carrying conductors in a pipe you have to derate those conductors, not sure what the derate factor is for 9 conductors without looking.

 

[LoneGunman]

First thing, how are you going to get three 14/2's down a 1/2" conduit without stripping the insulation? I know you asked if you could strip it but you'd have no choice.

What I would do is your 3rd option, run your romex to a junction box, down to your switch box run a hot, one ground and however many switch legs you need. It is a code compliant way to do this.

I'd like to see the 1/2" conduit ran to the junction box, strip some romex and use the single conductors to go down to the switch box. If you cannot run the conduit to the junction box, I'd use a piece of 14/3, run it from the junction box to the conduit, use a female adaptor so you can use a romex connector. Strip the insulation beyond the romex connector and then run the single conductors down the conduit.

Now we could get into the whole "It's not rated for that use", realistically, you are not getting an inspection (even though I have never seen an inspector have a problem with stripped romex in a conduit), you are not packing the conduit with excess insulation, it would be a safe insulation.

Many times we sleeve MC cable through EMT, in commercial buildings where it can't be fished. We strip the insulation and use an MC to pipe connector, this has passed multiple inspection in MY area.

 

[Charles (in GA)]

Chapter 9 (Tables) of the NEC says..... "A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have eliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter"

This describes Romex to a T. Elliptical, measure the width across the widest way, convert that into the area of a circle that size, and use that cross sectional area to determine your conduit fill. The tables that follow give you the inside diameters, areas and max fill at various percentages, for 1/2 electrical nonmetallic tubing (RNC) in 1/2 inch trade size, 1 wire is 53%, the max fill area is 0.151 in²..... for two wires you can fill to only 31% which is 0.088 in²....... and for over two wires the max fill is 40% which is 0.114 in².

For 3/4 RNC the max fill values are, 1 wire 0.269 in.², 2 wires 0.157 in.², and 3 wires 0.203 in.²

Remember one Romex is one wire, as what I posted above says to treat it as a single conductor. I have some 14-2 Southwire Romex and it measures out to 0.350 in across the widest way. Using the formula area = PI * (diameter / 2) * (diameter / 2)........ (or area = PI * (radius) * (radius)......... or simply put, PI*r²) we get 0.07306 in² for the cross section area of 14/2 romex, calculated the way the code says to do it.

You can put one complete 14/2 romex in a 1/2 RNC conduit, and two 14/2 romex in 3/4" conduit.

60% max fill for ******* of 24" or less is 0.171 in.² for 1/2" and 0.305 in.² for 3/4". so you can put two 14/2 romex in a 1/2 ****** and four in a 3/4 ****** (sounds crowded however).

As far as stripping out the wire, I opened up some of my 14/2 southwire NM-B and did not find any markings on the wires inside. I doubt you could legally use it stripped out of the jacket entirely. Thats my take on it, but I'm not an electrician, I'm just an A&P mechanic.

Charles

 

[Aceman]

Charles,

for future reference pvc is RNC, not ENT. On another note, 1/2" pvc is just about worthless, we run 3/4" at a minimum.

 

[nissan_crawler]

Well, there are two horizontal boards in the wall in addition to the header for some reason, so the wall is a definite no.

I'm just going to put a junction box on the wall near the ceiling, run a short pipe into the attic to run the wires into, and a pipe down to the switch box.

Since I have to get a j-box anyway, I'll just grab 15' of thhn to run down to the switches.

This house is such a royal pain because all the old wiring is ran around the exterior walls in the attic and stapled like you wouldn't believe. The only way to run new wire in place of old would be to pull the damn roof off. I've been slowly rewiring the place, but you have to leave most of the old wiring in, which *****.

As for the pvc, I prefer 3/4 also, but was using this due to lack of room for 3/4" in the lights. This way, I'll only have 1 ground, the hot, and two switched wires in it, so no biggie.

 

[Charles (in GA)]

Charles,

for future reference pvc is RNC, not ENT. On another note, 1/2" pvc is just about worthless, we run 3/4" at a minimum.

OOPS, had I kept looking thru the tables in Chapter 9, I would have discovered the correct table a couple of pages further on. Thanks, I'll rework the numbers in the above post later this evening.

Charles

 

[nissan_crawler]

Well, it's all done. I just ran a ground and 3 #14 thhn wires down to the switches, well in spec. I put in a junction box to wire the rest up and keep it all happy.

 

[Bevis]

This house is such a royal pain because all the old wiring is ran around the exterior walls in the attic and stapled like you wouldn't believe.

That sounds like the house my parents just bought (built in 1966) what a pain to run for light/fan switches.