Can I run two cables through the same conduit?

Question

I want to install electrical outlets in my unfinished basement that has block walls.

It is my understanding you can't have cable on the walls--that whatever cable used must be protected by conduit.

My idea is use NM-B cable from the breaker box, run cable through joists down 3/4" conduit to box with GFCI outlet. Then run the 12/2 cable back up the same conduit and thru joists until it goes down conduit on wall again to an outlet and so on.

Is there any issue with taking cable down and back up the same conduit due to heat?

Trying to do this with the least conduit installed as possible but safely.

Answer 1

TL;DR -- go right ahead

This looks to be a sound plan -- it's definitely cheaper than running everything in conduit, and probably somewhat easier too even with having to stuff two NM cables down a 3/4" EMT length.

Fill isn't an issue

Even though the Code conduit fill criteria don't apply to protective sleeves, you're fine on fill (albeit barely). Encore Wire quotes .160" by .450" for their 12/2 W/G NM product -- multiplying and converting to mm^2 gives us a conservative area estimate of 46.5 mm^2 per cable. Doubling this to account for the doubled run gives us 93mm^2, which is within the 106mm^2 (31% fill) available for two wires or cables in the 3/4" EMT.

Nor are derates

Normally, since you have more than 3 current-carrying conductors in the sleeve, you'd have to derate as per 310.15(B)(3)(a). However, this derate starts with the 90°C column in Table 310.15(B)(16) as per 334.80. Since we are derating by a factor of 80% for the 4-6 conductor case, we get 24A derated, which is above the 20A 60°C ampacity that's actually used for the cable as per 334.80, so we're good on that front too.

And the EMT is suitable

NEC 334.15(B) states that EMT is one of the allowable means for protecting NM from damage in an exposed application. PVC can also be used, but it must be Schedule 80 PVC -- the cheaper/wimpier Schedule 40 stuff just won't do. There shouldn't be any concerns with impedance, grounding, or the likes either -- protective sleeves are not required to be grounded or bonded as per 250.86 Exception 2, and it's grounded anyway via being connected to a grounded metal box.

Answer 2

Yes, it is legal to use conduit as a protective shield (in this case you are not using it as a wiring method). In this case the conduit fill is irrelevant, it just needs to fit (and be 4 cables or less due to derate). You will need 3/4 conduit or you will lose your mind trying to fit it. Also you will need 4x4 junction boxes at the bottom because of wire-fill rules. (4 wires, grounds and yoke = 7 units or 14 cubic inches at 14AWG, 15.75 c.i. at 12AWG).

But why not just use it as a wiring method?

Put a junction box at the top of the conduit.

Probably cheaper this way. You're getting to use 1/2" conduit instead of 3/4". Replacing 8-10 feet of thick Romex with same length of single conductor THWN-2. One extra 2x4" junction box ($1.29), some ground screws ($1 for a bunch), and some Romex clamps ($3 for 5).

This part happens at a comfortable desk.

First, the upper junction box (4x4) has a hole tapped for a 10-32 screw. This takes your ground screw. Attach an 8" pigtail to that.

Assuming you've already fit and bent the conduit lengths correctly, assemble the upper and lower (2x4) junction boxes to the conduit.

Also knock out a hole (or 2 holes) for entry and exit of the Romex, and stick a Romex clamp in each of the holes, mounted so you can tighten the screws later. Some Romex clamps allow two Romex in the same hole, that's fine.

Now, cut two THWN-2 single wires the right length for the run between boxes, with ~6" slack on each end. Strip the ends, then take one wire and tape about the last 3" black on both ends. That's the hot. This type of marking is legal for white wires only. That's why you buy white.

Wire up the receptacle on the bench.

Now back to the wall.

Now, physically mount the thing on the wall. Feel free to do all the conduit drops in the same way. You can bolt in the receptacle once the box is mounted. Use short 6-32 screws in front of a conduit entry, the long ones will bottom out. Most wire strippers have a cut-a-screw feature.

Lastly, get your Romex and connect each of the upper junction boxes. Wire nut all the hots together, all the neutrals and all the grounds including the ground from the box itself. The steel EMT will carry ground to the lower box, then the receptacle will get ground via the steel ears on the yoke.

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If you want the first receptacle to be GFCI, do all the above but use a 4x4 junction box in the lower first position (this is for legally required cubic inches). Use a GFCI cover on that box. Run 2 more wires up the conduit - two for LINE, two for LOAD. Tape them creatively to distinguish them.

Answer 3

The simple answer to your question is yes according to NEC Article 334.15 (B), but it may be a little more complicated than that. First the conductors must be rated for wet location. The idea is that moisture can condense in the conduit.

Also it never addresses the 40% fill ratio, if that is required by by your AHJ you will have to use the total area of the NM cable not each individual conductor. Although it might not be part of the NEC; IEEE Standards states you need to consider this if more than 10% of the circuit is in conduit therefore all of the circuit is regulated by a 100% conduit system.

So the question is is this a conduit system or is this just a piece of tubing being used as protection for a romex system? All of this will depend on the knowledge and opinion of your AHJ. So I would run that question through them before I proceeded with that work. Personally I have no objection to calling it a piece of tubing being used to protect a romex system, but I would suggest a PVC conduit in lieu of metal since we don't get into a grounding/impedance question.

Good luck.