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#96601 - 12/11/05 09:49 AM NM in buried PVC Conduit  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
I know NEC explicitely allows NM through SCH 80 conduit, but doesn't mention SCH 40. Can I run NM through SCH 40 conduit? What if it's #12 NM on a 20A breaker (120V) and GFIC protected?

This will be buried and will pass through concrete.

2017 / 2014 NEC & Related Books and Study Guides

#96602 - 12/11/05 10:04 AM Re: NM in buried PVC Conduit  
DougW  Offline
Joined: Jun 2003
Posts: 1,143
North Chicago, IL
NM or UF? Putting NM underground constitutes a "wet" environment - a prohibited application for NM.

#96603 - 12/11/05 11:23 AM Re: NM in buried PVC Conduit  
Speedy Petey  Offline
Joined: Aug 2003
Posts: 173
Upstate, NY
Why would you even want to run NM cable through conduit, legal or not?
Unless of course it's just a sleeve.

The answer to your question though is that Sch40 is not suitable for protection. Sch40 is no more protection than NM itself.

Speedy Petey

"Anyone who has never made a mistake has never tried anything new." -Albert Einstein

#96604 - 12/11/05 01:22 PM Re: NM in buried PVC Conduit  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Application is to a kitchen island for one conduit and to a detached shed for the other; one end of each conduit run will be through grouted concrete block up into the walls, and the other will be through the slab to the island in one case, and into the shed in the other. The rest of the house wiring is NM and NM through the conduit would allow me to eliminate the conduit boxes and just pull the NM straight through. UF would be the same, but I thought watertight conduit constituted a "dry" environment even underground? I guess I could put in boxes and pull THW, it's not that big a deal. Speaking of which, is there any prohibition to burying conduit boxes behind drywall when I transition back to NM? Dealing with the boxes was something I wanted to avoid.

Pardon if these are stupid questions, I don't normally deal with things like this [Linked Image] And I don't have any conduit bending or tapping tools so I'm reluctant to use IMC, which would certainly be the better option. We're pouring the concrete this week so I have to make that decision fast, but I won't actually be wiring it up for a couple months at least.

[This message has been edited by SteveFehr (edited 12-11-2005).]

#96605 - 12/11/05 01:40 PM Re: NM in buried PVC Conduit  
Tiger  Offline
Joined: May 2005
Posts: 706
Crystal Lake, IL USA
In my experience and in the experience of an engineer/inspector I learned from...all underground conduits get wet or fill with water.


#96606 - 12/11/05 01:53 PM Re: NM in buried PVC Conduit  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Steve, why don't I just start from the beginning, and give you ONE approved method?

Our pipe is NOT water-tight; even if it were, water is sure to enter, as condensation if nothing else...and then, where will it go?

Burying NM is a 'no-no.' Sure, you could use a length of UF for this run.
If you're going to run pipe, then why not just run the individual wires? The pipe ought to be connected to the boxes at either end anyway- what you're doing isn't a simple "sleeve" for protection, it's more than that.

The boxes must remain accessible, and not covered by drywall.

There is an idea floating around that placing NM inside a pipe is somehow 'better,' giving better 'protection.' This really isn't so. You might even create a heat dissipation issue. And it sure is a lot of work getting NM through the pipe in the first place- which negates the one advantage to pipe; you can replace or add conductors later.

Plactic boxes, of the sort used in homes, are not suitable for attaching pipe. For these two boxes, I suggest the traditional metal box with mud-ring. You will need to run a ground wire, and tie it in with the other grounds, as well as tie it to the boxes at either end. Over this you can either place a blank cover, or add a receptacle.

A final note....Though code allows pipe to be in, or immediately under a slab, I reccomend burying it a couple inches below the slab- and covering it with a couple inches of dirt!. I suggest this, as it is likely that at some point the plumber will cut into the slab; when that happens, he will cut your wires, unless there is some small clearance between the pipe and the slab.

#96607 - 12/11/05 07:29 PM Re: NM in buried PVC Conduit  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Thanks for the 101! I spent a good half hour flipping through NEC on Friday, but these specs are so disjointed regarding conduit, it's hard to find anything unless you already know where to look. In retrospect, I guess I already knew the "right" way to do it and knew watertight conduit isn't dry but didn't know for sure and was looking for an excuse to take shortcuts. (Which is awfully hypocritical of me- if this was something I was going for work, I'd have insisted on rigid metal!)

Is SCH 40 considered "Nonmetallic Conduit" or just SCH 80? The local big box has an excellent selection of SCH40 conduit, but no SCH 80 outside of the plumbing section.

[This message has been edited by SteveFehr (edited 12-12-2005).]

#96608 - 12/11/05 08:38 PM Re: NM in buried PVC Conduit  
Speedy Petey  Offline
Joined: Aug 2003
Posts: 173
Upstate, NY
Nonmetallic conduit is just that, not metal (steel, aluminum, brass).

Sch 40 & 80 are simply the wall thicknesses.

*edit- typo*

[This message has been edited by Speedy Petey (edited 12-11-2005).]

Speedy Petey

"Anyone who has never made a mistake has never tried anything new." -Albert Einstein

#96609 - 12/12/05 12:35 AM Re: NM in buried PVC Conduit  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
I would just use UF for this section, using sched. 40 just as sleeving, since the slab is the protection, and land both ends of the UF in the usual receptacle boxes like NM.

Larry Fine
Fine Electric Co.

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