Running electrical wire through PVC pipe

[Wayne Whitney]

If the piece of conduit is just used as a sleeve, then you can put as many electrical cables in it as will fit without damage, as long as you pay attention to derating. You don't have to follow the conduit fill percentages, they apply only to conduit systems. As mentioned previously, for NM #14 and #12 cables, up to 9 current carrying conductors are OK without reducing the ampacity below the usual 15 and

20 amps, respectively.

Cheers, Wayne

[gfretwell]

You still have a fill limit on these short sections of pipe (60%) This adds up very fast because you are required to compute a cable as a circle with the diameter of it's widest dimension. Cable jackets are also not coated with the teflon jacket on THHN so they are hard to pull through, even one bend. If this isn't one straight shot, use wire lube. It is really better to run a complete raceway system and use THHN conductors for future runs. if you don't have a mixed use duct. ENT is a good, low cost method to provide future access for 1&2 family. Set a box at each end with a blank cover and you can do whatever you want with it later.

I just did a bathroom renovation in my house and every switch location was a home run to a central J box in the attic with 3/4" ENT so it will be trivial to change our mind later on what controls what. I did this on an addition and it has already paid for itself when we had a change.

[Wayne Whitney]

I don't believe the 60% fill limit applies to sleeves, only short sections of a complete conduit system. The way I think of sleeves, they are not part of the electrical system. If the installation is NEC-compliant without the sleeve, then the presence of the sleeve doesn't change anything, it was just an aid in installing the cables.

Cheers, Wayne

[HerHusband]

Well now I'm confused... :) At what point does does conduit become a full "conduit system" that restricts NM cables in the conduit?

When I tried to decipher the code a couple years ago, I understood it to mean "short" sections of conduit (i.e. 4 feet or less) where at least one end was open for air flow was OK. Wouldn't a long "sleeve" of conduit (say

20 feet long from basement to attic) encounter the heat buildup the restriction is trying to prevent?

I'm not trying to be sarcastic, I just don't understand what the deciding factor is for NM cables in conduit. It would be good to know for future work.

Thanks,

Anthony

[hibb]

Thanks for all that information.

Heck, I was going to run 4 of the 12-2 NB wires through that 1-1/2 inch by 10 ft long pipe. It is the gray pipe used for electrical runs, btw. Looks like that could be a problem, eh?

Thanks, David

[trader4]

Maybe I missed something here, but based on what Wayne and Bud said, it sounds to me like you could run four 14-2 Romex through a single conduit.

Wayne: "In terms of overheating, you need to watch how many current carrying conductors (CCCs) you run together for a significant length. If you have too many together you have to "derate" each conductor so that it carries less current, or alternatively upsize the conductor for a given current. The rules are complicated, and derating potentially starts at 4 CCCs. However, for NM cable in #14 and #12 sizes, you can run up to 9 CCCs and still maintain the usual 15A and 20A capacities, respectively"

[Steve B]

I used underground rated PVC of 2.5" to run cable to my shop. It was done by a licensed electrician. Are we talking about sprinkler PVC, or electrical rated PVC?

Steve

[Wayne Whitney]

(4) is the maximum number of 12-2 NM-B cables you can bundle together (e.g. run through a sleeve like you are doing) and still maintain the

20 amp capacity (assuming no other correction factors, like elevated ambient temperature). So you are fine, just don't put a 5th one in there. As for the sleeve, you can use whatever material is convenient.

Cheers, Wayne

[Wayne Whitney]

Oops, I need to amend that. The one wrinkle is that you normally have to support your NM cable every 4.5 feet (NEC 334.30). The allowable exceptions are (1) you are doing remodel work and fishing the cable (NEC 334.30(B)(1)) or (2) you are running the cable inside a raceway (the NEC term for conduit) that provides protection from physical damage.

So, if your wall is open, and you are installing a sleeve and running NM cable in the sleeve, then you have to use a raceway, like PVC electrical conduit. But if you are doing remodel work, and come across an existing unused pipe in a finished wall, you can fish your wire down it without regard to what the sleeve is made of.

Cheers, Wayne

[Wayne Whitney]

When you want to use that conduit system for its normal purpose, to be able to run individual electrical conductors like THHN, XHW, etc. In that case, you have to count any NM cables also in the conduit towards the conduit fill.

If you just have a sleeve or guideway for cables, like a piece of pipe with open ends, then conduit fill does not apply, in my opinion. But see my other post for the restrictions about supporting the NM cable.

BTW, there is obviously some variation in understanding about this issue. So if you are planning to do this, you might want to check with your inspector first. [If they say it is not allowed, you could ask them what NEC article it violates.] You might find yourself in a situation where your inspector disagrees and you decide it isn't worth challenging.

Conduit fill and derating are two different things. Conduit fill is about avoiding damage to the conductors while installing them (cables already have an outer sheath to help with that), while derating is about keep the conductors from overheating when multiple conductors are in close proximity.

BTW, you could have a derating issue due to bundling of cables without any sleeve. E.g. if you drill a series of holes in a stud wall and pack a bunch of cables tightly together through the holes (such as more than 4 12/2 or 14/2 NM cables), that could be derating problem. Certainly it would be if you also cable tied the cables together in between the studs.

Cheers, Wayne

[Wayne Whitney]

In fact, in the 2008 NEC, you have to derate any time you have more than one cable going through a hole in framing that will be sealed with thermal insulation, foam, or caulk. NEC 334.80 says, in part:

Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.

Cheers, Wayne

[HerHusband]

Wayne,

Thanks for the clarification. I appreciate it!

So far, the only conduit related issue my inspectors have been concerned with is bushings on the open ends of the conduit to prevent the conduit from chafing the cables.

On a related note, I wonder how an open two story vertical run of conduit would be viewed from a fire spread point of view? Our building inspectors were strict about sealing all openings at floors and ceilings to prevent fire spread. It seems like an uninterupted vertical run of conduit would act a lot like a chimney, allowing a fire on the lower level a straight path to the top floor (or attic).

I was surprised to learn on my last inspection that it was actually OK to run two cables through the same clamp on the breaker panel. It may pass inspection, but it didn't seem right to me. I prefer one cable per clamp.

Anthony

[Wayne Whitney]

Yes, that is a good idea, and in certain code sections is called out explicitly.

I would expect it would be required to be fire caulked. My building inspector required every penetration of top and bottom plates to be fire caulked. Where I used EMT to sleeve NM cable for protection from physical damage in a thin (2" framing) wall, I used bushings and caulked the openings.

Are you talking about clamps that you installed in a KO on the panel? The manufacturer's packaging or instructions should specify what combinations of cables it is to be used with.

Cheers, Wayne

[gfretwell]

The real question is what your AHJ thinks. It says "nipples less than

24 inches", not conduit systems. In this jurisdiction that is what you call a sleeve. If this is a conduit system you can't exceed the fill in any of it.

Also there are plenty of AHJs who say if you pull wire in it, you need to use a listed raceway. I have seen 4" white PVC pipes under driveways failed because they pulled a 14/2 UF in it. Personally I don't agree with that one. It doesn't matter. If that was my AHJ I would be using RNC. The attitude is that they make the appropriate raceway and that is what you should be using.

Be careful to call these things ducts when you are talking to an inspector since that is acknowledged in the code but not defined or regulated. It might give you a better case.

As for sealing, find out if they want "draft stopping", "fire blocking" or"Fire stopping" (3 different things). They would require different materials for each. Draft stopping could just be spray foam, Fire blocking might be as simple as fiberglass insulation packed in the cavity or closely fitting wood members but that is up to the opinion of your AHJ. Fire stopping is a listed assembly of components with a fire resistance rating based on time.

[hibb]

OK, so running four 12-2 NM B cables in the 1-1/2 inch conduit. Again, I already have the gray conduit specifically meant for electrical.

I am not sure how I can support the wire every 4 1/2 feet inside the conduit. How is that done? The whole of the 10 feet of conduit will be vertical, btw.

Also, what is a good way to tag the wires so I would know what each wire goes to as they exit the conduit?

Thanks, David

[bud--]

352 Rigid PVC conduit "352.22 Number of conductors .... Cables shall be permitted to be installed where such use is not prohibited by the respective cable articles. The number of cables shall not exceed the allowable percentage fill specified in Table 1, Chapter 9."

Identical language was added to most raceway articles (xxx.22) in the

2002NEC, probably to make existing practice clear. I would expect inspectors would apply the same fill restrictions to PVC water pipe (if they allowed water pipe). I haven't tried, but it would probably be difficult to pull in Romex that did not comply with the fill restrictions.

The NEC doesn't specify a length or reason for using PVC conduit.

Fire stopping might actually be enforced by the building inspector (not electrical inspector).

[bud--]

"344 Non-metallic sheathed cable: Types NM, .... [commonly called Romex]

344.30 Securing and Supporting. ...4 1/2 ft...12 inches.... Sections of cable protected from physical damage by raceway shall not be required to be secured withing the raceway."

I would think an inspector would view a raceway used to fish cables the same as one used for physical protection.

Can't remember if "raceway" has been defined in this thread, but it is a hollow wiring method installed without wires that wires are added to - like PVC conduit.

[JIMMIE]

Polypropylene is Dupont Dacron, used for ropes on sailboats.

Jimmie

[HerHusband]

That seems like it would make future additions more difficult, having to dig out the caulk before a new cable could be run. I wonder if a junction box at the top and bottom would serve a similar function, but still allow easy access in the future. Just thinking out loud...

Yeah, sorry for the vague description. I always use one clamp per cable in the knock-outs, so it didn't even occur to me you could run more than one. I was really surprised when the inspector said I could if I needed to.

Anthony

[HerHusband]

A sharpie pen works great for writing identifications on the outer covering of NM-B cables. If you're feeding multiple cables at once, you can write the ID on the cables first, then feed them through the conduit, something that wouldn't work well with adhered tags or labels.

Anthony