Using 2" PVC electrical conduit

RonLNY · 01-06-2013, 01:11 AM

Hi everyone,

I have a new house and the builder left an empty 2" PVC conduit from the basement to the attic for future electrical expansion. It provides a nice straight unobstructed path. I'm planning to install some new lights and am trying to figure out the best way to make use of this conduit.

From what I've read, running NM or AC inside a PVC conduit is a no-no because of heat build-up. Given a PVC conduit, the consensus seems to be that you pull separate THHN stranded wires through it. I'm fine with that, but of course there are runs on either side of the conduit (to the panel on one side, and the load on the other side) and I'm not sure how to handle this.

I was actually thinking about putting in a 50A sub-panel in the attic so I only have to worry about this once. So the plan would be something like:

1. NM from the main panel to a j-box near the bottom of the conduit
2. Transition to the conduit run - how?
3. Run 3x6ga THHN (plus ground) through the conduit
4. Transition from the conduit run - how?
5. Connect to another j-box near the top of the conduit
6. Run NM from there to the sub-panel

This leaves the question of the transitions to/from the conduit run. Do I just reduce the PVC from 2" down to 3/4" (plenty big enough for 4 wires) and use a standard PVC j-box?

Or is there a better way to do this entire thing? Opinions seem to be split on whether NM through PVC is a code violation or simply not recommended... in my case, I'm only using the PVC because it provides a clear cable path. If I'm only doing one run, perhaps a single 6/3 NM through a 2" conduit is really not that big a deal?

Any advice appreciated. :-)

NJMarine · 01-06-2013, 06:24 AM

You can run 6/3 thru the pipe.

rjniles · 01-06-2013, 07:20 AM

Running cable through conduit is not a violation, the conduit acts as a sleeve. Run NM or SER cable. Eliminates all those splices.

RonLNY · 01-06-2013, 11:10 PM

That would be much easier! I can't tell you how many threads I found with people arguing about whether NM is allowed in conduit. It's forbidden, allowed by the NEC but forbidden by local ordinances (most people seem to be concerned about excessive heat), possibly allowed based on the size of the conduit, allowed but not recommended, etc. Very confusing!

Last question: what's the proper way to secure the 6/3 NM at the top of the 20' vertical conduit run? Can I just staple it to a stud and then continue up the stud to the sub-panel? Will a single SEU staple be enough to hold the weight of 20' of cable? I may need to jog to the side a bit to find a suitable place to install the panel, and I assume it needs to be fully secured on the vertical before bending.

(BTW, how strict is the no-more-than-4-bends-between-jboxes rule... I have to assume on a large cable like this, a few extra bends would be better than a splice, no?)

Oops, one more: from the responses, I assume there is no problem with the NM just being exposed on either side of the conduit? Both spaces are unfinished. Other than sanding it down, do I need to somehow protect the cable from scraping against the cut ends of the conduit? Is it enough to just secure the cable (with staples) on the top and bottom, forcing it to stay perfectly parallel to the conduit, or is there some kind of protective cap or bushing for 2" PVC?

I've found plenty of other threads on how to wire up the sub-panel (use separate ground and neutral bus bars!) but getting this cable to the attic is looking to be much more difficult than I expected, given that I have the stupid conduit already. Thanks for all your help! :-)

kbsparky · 01-06-2013, 11:11 PM

It's forbidden to install NM cable in a PVC conduit that is outside.

Not so, if the conduit is inside.

Maybe that is the source of your confusion?

You can install a terminal adapter with a bushing or bell end on the ends of the conduit for abrasion protection.

rrolleston · 01-06-2013, 11:26 PM

You can either install a bell end on the conduit ends or just a threaded fitting with a screw on bushing. Even a coupling glued on each end would give a smoother edge for the wire to be pulled inside.

rrolleston · 01-06-2013, 11:34 PM

Once you get to 6 gauge it makes for a tough splice since it's such a large stranded wire I use a mechanical butt splice with heat shrink for anything over 10 gauge but it's such a pain and time consuming.

I would either use all NM or SER or continue the conduit to both panels and use THHN or maybe even Aluminium wire.

k_buz · 01-06-2013, 11:34 PM

Along with what KB cited, there is also an issue if you would pull in multiple runs of romex thru the sleeve. Derating is necessary for anything more than 9 current carrying conductors.