NM Wire in Conduit?

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Old 10-23-07, 08:48 PM
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NM Wire in Conduit?

I have a single story house with good attic and crawl space access. I currently have an interior wall in the house open for remodeling. I'd like to drill a hole in the top plate and the bottom plate of the wall, and then run a 3/4" conduit between the two for future use. Am I allowed to run NM cable in this conduit, and if necessary, how would I do the conduit fill calculations?
 
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Old 10-23-07, 09:30 PM
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Yes that is perfectly acceptable although you should seal the pipe with fire caulking to prevent the spread of smoke or fire.
While you have it open, why not put in a larger conduit?
(3/4" is not going to hold much).
You might also want to put one in for low voltage.

National Electrical Code 2005
Chapter 9-Tables-Note 9-"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 elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter."

One piece of romex is going to limit you to 40% fill.
For 3/4" Schedule 40 PVC, that is 0.203 square inches.

As you can see, legally you can't fit much in a 3/4". Go for
1 1/2" or 2 ".
 
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Old 10-23-07, 09:53 PM
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The reason I'm thinking of 3/4" conduit is that it has about a 1" outer diameter, and I really don't want to drill holes bigger than that in 2x4 top and bottom plates. I'll probably go with a couple of sections of 3/4" conduit, rather than larger diameter conduit. The low voltage one is a good idea too, thanks.
 
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Old 10-24-07, 07:42 AM
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As you can see from the above, NM in conduit is very inefficient. It simply takes up too much space.

You can install the conduit, with a juncion box in attic and basement.

When the time comes to use this conduit, simply transition from NM to individual THHN conductors at the junction boxes. Rather more efficient that threading the entire circuits length of NM through the conduit.

-Jon
 
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Old 10-24-07, 07:57 AM
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Another option would be to drill 2 or so holes in the top and bottom plate, and run a rope or pull string through. Tie it off in the attic and basement. With the pull string there, you'll be easily able to pull through a few NM wires without any problem - and without worrying about derating, etc.

Even with insulation in the wall, it's pretty easy to pull through wire if you already have a pull cord. (but being an interior wall, it will likely be easier without insulation).

Just my 2 cents.
 
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Old 10-24-07, 08:14 AM
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Originally Posted by biking_brian View Post
The reason I'm thinking of 3/4" conduit is that it has about a 1" outer diameter, and I really don't want to drill holes bigger than that in 2x4 top and bottom plates. ..
Guess you never saw what the plumbers do. A vent stack is commonly 1-2/2" and half the time they don't even bother with a centered hole drilled with a hole saw. If you stop and visualize your force vectors you will see there is very little reason to worry. What I would suggest is anti-nail plates to prevent nails into the conduit.
 
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Old 10-24-07, 09:43 AM
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Originally Posted by ray2047 View Post
Guess you never saw what the plumbers do. A vent stack is commonly 1-2/2" and half the time they don't even bother with a centered hole drilled with a hole saw. If you stop and visualize your force vectors you will see there is very little reason to worry. What I would suggest is anti-nail plates to prevent nails into the conduit.
Interesting that you bring that up about plumbers and the vent stack because I was thinking the same thing in the back of my mind. My current "wet wall" between the kitchen in the bathroom is the depth of two 2x4's, so the hole for the vent stack is clearly structurally sound. I'm getting the sink relocated to a wall that is the depth of one 2x4, so that was getting me thinking about the plumber drilling a 1-1/2" vent hole in a 2x4. I'm an engineer in my day job so I agree with you about the force vectors, but there's a part of me that gets concerned about the irreversibility of drilling holes in the structure of the house.
 
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