Romex in SHORT run of conduit?

[HerHusband]

I know it's generally a bad idea to put Romex (NM-B) cable inside of conduit because of the increased heat build up. However, I'm curious how big of a problem it would be for a short 3' section of conduit?

When we wired our house a couple of years ago, I installed a few runs of

1/2" pvc conduit from the panel down to our crawlspace in hopes of making future upgrades easier. Now I want to run power out to a small shed using 8/3 wire. The 8/3 Romex just fits inside the 1/2" conduit and will work great if that short section of conduit is acceptable.

The cable will run through our crawlspace to a junction box. From there I'll use individual wires to run through 1" PVC conduit (already installed) out to the shed.

Can I use the conduit, or do I need to resort to fishing cables through the walls and floor?

What if I strip the outer cable insulation and feed the individual wires through the conduit (with a cable clamp at the bottom of the conduit)?

Thanks,

Anthony

[Wayne Whitney]

Well, here's what my 2002 NEC says: Chapter 9 Table 1 says the acceptable wire fill for one conductor (or one cable) in conduit is

53%. However, Note (2) says "Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring from physical damage."

So in my ignorance, it sounds like it is OK.

I've often wondered about that practice, it seems dubious to me.

Yours, Wayne

[DanG]

If you are going to run individual wires in the conduit out to the shed, why not complete the conduit run from your stub out (or down) to the pipe going to the shed and use the individual wires for the entire run? Your are restricted to 360° of bends. If you are any where close, install a junction box in your conduit run, the j box would also allow you to change conduit sizes if this is part of your problem.

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[HerHusband]

Dan,

The junction box for the shed conduit is about 40' from the main electrical panel. For cost and ease of installation, I'm opting to use romex between the panel and junction box.

Anthony

[HerHusband]

Wayne,

That's the kind of thing I was looking for. Thanks.

Since the conduit is open above and below, I would think any heat buildup could escape easily.

Anthony

[gfretwell]

I am not sure where this "heat buildup" thing comes from but the derating for a cable in a raceway is exactly the same as a cable in free air. The size of the raceway does not figure in that calculation either. It is simply the number of bundled, current carrying conductors. There is no derating for 3 or less at room temperature

[Calvin Henry-Cotnam]

If it "just fits" you may be overstuffing the conduit. Maximum fill for conduit is 53% of its cross sectional area for one conductor, but when there is more than one conductor, it is only 40%. I suspect that pulling a piece of Romex would count as more than one conductor (this limit is from the CEC, but I suspect similar restrictions in the NEC as another poster mentioned the 53% limit while quoting the NEC).

This is a viable solution, and one I've used in several instances. All you have to do is put a fitting on the end of the conduit that would be used to attach it to a junction box, and get a female-to-female pipe coupling to couple the threaded end of the fitting to a threaded cable clamp. Works very nicely.

By the way, I have free application called "Conduit Calculator" for calculating conductor fill in conduit. It's available at

[HerHusband]

Calvin,

Yep, my "Code Check Electrical" book says I can have a maximum of two 8 gauge conductors in 1/2" PVC conduit (for individual wires). But I'll have four wires (8/3 w/ground) with the cable jacket. It fits into the conduit easy enough, but I would clearly be exceeding the fill requirements for conduit.

My main concern was whether it would be acceptable for such a straight short run (3 feet). Doesn't seem like it would be all that different than drilling a hole through a bunch of studs to snake the romex through. Especially since it's a vertical section that's open at the top and bottom.

That was my plan if I have to resort to stripping the romex jacket back. The conduit ends just above a foundation wall, so there isn't really room to install a junction box there, and I can't really install bends in the conduit with it being overfilled already.

If I have to, I could drill up from the crawlspace and snake the wire up to the electrical panel. But, there's a LOT of cables in that stud bay under the panel. The conduit would be so much nicer if I can use it.

Thanks for the input!

Anthony

[Chris Lewis]

According to HerHusband :

Under Canadian codes (like Calvin. Inspected/approved too), I've run 14/2,

14/3, 12/3 and 10/2 through 1/2" PVC for 5-7 foot "drops". Putting in outlets, switches and a 30A cube heater outlet into my workshop, the conduit is on the wall surface (not buried), and "sleeved" from the ceiling to the boxes.

The inspector was fine with it.

That said, 8/3 sounds a trifle too far.

Don't run the romex jacket back for running thru the conduit. If you're going to go sheath-less, use proper unjacketed wire, with a J-box on either end.

[HerHusband]

Chris,

With usual hindsight, I wish I had installed larger conduit at the time. But, it was a last minute idea I had before installing the sheetrock and

1/2" is just what I had on hand. I didn't figure I'd ever be adding any big loads in the future. Funny what a difference a year makes... :)

I haven't checked the electrical supply stores, but if I opt to snake the cable through the wall, my next concern would be the cable clamp at the electrical box. A normal clamp fits in the hole and a nut gets tightened on the backside. But, I'll only have access to the inside of the electrical box. Do they make clamps that can be tightened from one side only? I've never paid attention because I've never needed one before.

Anthony

[Chris Lewis]

According to HerHusband :

There are plastic "snap-in" cable clamps that might help, but you'd still need to get to the back of the boxes.

[HerHusband]

What's the normal procedure then for adding a cable to an existing electrical panel? Surely there's a way to protect and secure the incoming cable other than tearing into the wall just to tighten a nut on the backside?

Anthony

[Duane Bozarth]

You got to get enough access to get th cable into the box, anyway...

But you can certainly tighten a standard connector from the inside, but can't tighten the clamp portion itself on the cable unless can get access to the outside...

Can't envision what you're after here...

[Ron DeBlock]

The nut goes on the inside of the box. Put the clamp part onto the cable before snaking it through the wall. When you pull the cable into the box, the male threaded part of the clamp pulls right into the hole. Install the nut and tighten.

[gfretwell]

Hubbell just sent me a sample bag of these. It is a Romex clamp that installs from the inside of the box

[HerHusband]

Hmm.. Interesting idea. It would mean drilling a larger hole in the wall plate that the clamp can fit through, but otherwise it sounds like a workable solution.

Thanks!

Anthony

[HerHusband]

Pop a knockout out of the electrical panel, drill a hole up through the floor from the crawlspace, and fish the wires through the stud bay. A pain in the neck, but certainly doable. No worse than any other remodeling job.

I would think the cable clamp would work just as well in reverse? Put the clamp portion inside the electrical panel? But, that would still leave the problem of fastening the clamp itself to the box.

Unless, of course, I could find a clamp designed for this purpose.

Anthony

[HerHusband]

That's more along the lines I was thinking. Thanks!

Anthony

[Calvin Henry-Cotnam]

I have done this before:

1) Strip off the end of the Romex the amount needed in the panel, plus a little extra just to be safe.

2) Attach the cable clamp on the end of where the Romex jacket is, with the clamp's nut removed.

3) Fish the cable with the clamp on it up through the wall. You will likely have to tape together the ends of the conductors, and of course you will need a large enough hole so that the clamp on the cable clears it.

4) Once the stripped ends make it through the knockout in the panel, slip the clamp's nut over the conductors and tighten it on the threads of the clamp that now come through the knockout. The tightening may not be 100% as tight as how it would be if you had full access, but it is not that difficult to be sufficiently tight.

[Duane Bozarth]

I've found it usually easier to just make the access hole, do the necessary wiring in a relatively simple manner, then patch the hole...

imo, ymmv, $.02, etc., ... :)