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As best I can tell, the National Electrical Code 2011 allows electrical non-metallic tubing ("smurf tube" or ENT) to be used for running 120 V electrical wire. But everywhere I look, I see remarks to the effect that its use is mostly confined to low-voltage wiring (communications, alarms, audio/video), and some localities forbid its use for 120 V electrical purposes. Why?

I am planning to install a couple exterior weatherproof boxes. The wires will be run from the basement, through the sole plate, into the wall space, and out through a hole drilled through a brick veneer into the back of the box.

To make the bridge from wall to box watertight, I plan to run the wire in conduit. Options here seem to be liquidtight flexible metallic/non-metallic conduit, PVC, or some of the heavier metal conduits like intermediate metallic conduit. All of these seem like a pain to run through finished walls aside from the liquidtight flexible stuff, and that seems like a pain because of its weight and fittings.

ENT seems very attractive next to these: lightweight, easy to flex, easy to attach, works great with non-metallic boxes. It's allowed in damp locations (362.10(4)); the conduit run will be entirely within the wall and terminating in the rear of the box, so none of it will be exposed to the exterior or direct water. What am I missing here?

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It is often put into poured cement walls and floors. – Brad Gilbert Jul 16 '12 at 19:54
@BradGilbert When encased in cement, is it frequently used for low-voltage wiring, 120/240V wiring, or both? – Jeremy W. Sherman Jul 16 '12 at 21:11
I doubt ENT would meet code where impact or weather resistance is required. wrt low voltage wiring, for platform framing it's usually most cost effective to use wire with the appropriate stamps to meet code when installed bare. For exotic cabling (e.g. RF) where the right rating isn't available to meet code, I've seen ENT used in preference to EMT. – alx9r Aug 9 '12 at 20:57
up vote 3 down vote accepted

I don't know why it might be disallowed by local codes. As mentioned in the comments to the question, this stuff is often put into poured walls and floors. It is used quite heavily for 240V wiring and phone/data wiring in Israel where almost all construction is concrete. We also used it for both 120V/240V and data in WaterShed here in the USA.

As another answer mentioned though, it's not good for outside use.

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This provides the additional info I needed: "rarely" is relative, and I'd probably have a lot different point of view if I saw any recent slab construction. Thank you. – Jeremy W. Sherman Jul 18 '12 at 15:15

I am not an electrician, but as I'm currently in the middle of some renovations myself, I was curious and did some searching. I came across "Mike Holt’s Illustrated Guide to 101 Essential NEC Rules" (that's chapter 3 in PDF form), which, in article 362, it explains that it can NOT be used for wiring systems over 600V. To me, that appears to be saying that it is fine for 120/240V. The main concerns appear to be leaving the conduit exposed, particularly to sunlight, as UV can cause it to become brittle. I also came across a forum post discussing using ENT for 120V, and they were discussing how many wires you could run in a specific size of conduit, so it appears that it's done. Given that it meets NEC code, I can't come up with a good reason why specific localities might forbid it, other than possibly politics.

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Glad to see you are as perplexed as I was. I picked up the "rarely used" feel from Ray C Mullin's Electrical Wiring: Residential, 16/e, where it basically says, "People don't normally use this for house wiring, but it's commonly used for structured wiring" (p. 136). I guess it's rarely used because running Romex without any tubing/conduit is far easier. – Jeremy W. Sherman Jul 18 '12 at 15:20

ENT is not UV-resistant, so it won't work for your application. On top of that, it's ugly.

PVC is quite easy to run, and if you need an awkward angle, you can achieve that using a heat gun. I don't see how using PVC would be difficult in your scenario. If you can get to the sole plate with a drill bit, you should be able to get into the drilled hole with PVC.

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OP wants to run it entirely inside a wall. What does UV resistance or appearance have to do with the proposed application? – bcworkz Jul 18 '12 at 2:38
@bcworkz By "a couple of exterior weatherproof boxes," I assumed that the conduit is going to be run along the outside of the building. – Michael Jul 18 '12 at 3:19
@Michael Sorry for the confusion. There will be multiple boxes, but they will be on different circuits, so they will each be wired independently. I appreciate the assurance that PVC is easier than it sounds -- cutting and gluing PVC seemed more troublesome compared to using ENT. I am aware of UV-resistance issues and am fully respecting them in all I do. – Jeremy W. Sherman Jul 18 '12 at 15:12

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