On the subject of nm-b romex inside conduits and derating

tk3000 · Oct 8, 2015

Chapter 9 of the NEC lacks any tables with acceptable dimensional areas for romex cables for the purpose of area filling inside conduits since most people don't use romex inside conduits, as follows:

Chapter 9 Tables
Notes to Tables
(5) For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.
(9) A multiconductor cable, optical fiber cable or flexible cord 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.

So, one has to calculate the relevant dimensional area, in this case the cross-sectional area of a romex cable, and the romex cable is mostly flat one has use the maximum width of the cable as a diameter (for nm-b 14-2 it would be 7/16” of an inch based on rough measurements)

=> Cross-sectional area of 14-2 calculation: ((7/16”)/2)^2*pi = 0.1503”, thus the cross-sectional area for a 14-2 romex is about 0.1503”.

=> According to NEC's chapter 9 tables the inside cross-sectional area of a 1/2” EMT conduit is equals 0.30385. Considering that the 14-2 romex is seens as a single conductor one can use the 53% for the conduit fill area which then renders: 0.30385(.53)=0.161”

And since 0.161” > 0.153” one can use nm-b 14-2 inside 1/2” EMT conduits. But similar calculations indicate that the nm-b 12-2 would not be acceptable inside a 1/2” EMT conduit.

Having to use 3/4” conduit in some of my exterior walls would be difficult due to clearance issue (concrete wall and wood strips). So, I was wondering if there is any conduit with a higher fill rate.

Also, on the subject of exterior walls; I got two different types of what seems electrical boxes with a vapor barrier but none of them would work out for the small cavity in the wall without any modification:

Would there be any type of electrical boxes with a vapor barrier built-in that would fit the bill?

thanks

Jim Port · Oct 8, 2015

Keep the cables 1.25" away from the framing and skip the conduit.

tk3000 · Oct 8, 2015

Yeah, maybe I will do that in some situations with the exterior walls. thks

InPhase277 · Oct 12, 2015

There are stand-off available that will keep the cable away from the stud. Caddy CJ6 comes to mind but I'm sure there are others.

tk3000 · Oct 12, 2015

Thanks for the suggestion. But after looking up CJ6 caddy it seems a possibility, but still the clearance between the furry strips and the rigid insulation is minimal (maybe 1/2” ), so the wires still would very close to the drywall. The previous old wire installed at least had some braided sheathing on it and it was basically running down the wall and attached by staples at some points. I probably could use 1/2” EMT conduit by carving a lengthy opening in the rigid insulation and thus having enough clearance for the conduit. Below is a pic depicting the situation:

The above pic has the electrical box cavity in the concrete wall filled with a fiberglass electrical box. But for this situation I would need an electrical box with a bracket sidelining the electrical box, as the following:

http://www.alliedmoulded.com/catalo...-products/electrical-boxes/wall-boxes-for-switch-or-receptacle-devices/1096-z2/

But it seems difficult to find it in most home centers and the like.

brric · Oct 12, 2015

How long is that wall? If you put a recep near each corner on the adjacent walls you may not need a recep on that short wall.

tk3000 · Oct 13, 2015

I am not at the house at the moment to measure it, but I believe that is less than 6 feet in length (but I am not sure). Hmmm... I looked up a rule that is related to your point: power outlet must be present in a room so that no point along the wall space is more than 6 feet from a power outlet, measured horizontally along the floor line, from a receptacle outlet [210.52(A)]

Does that mean that I could install the power outlet in the adjacent walls as far as I do not have a linear space without a power outlet within 6 feet of any point on the wall. I assume that there is a requirement for a minimum of power outlets per linear distance along a wall, but nothing should stop me for having more than necessary (like out power outlet every 3 feet, for instance).

Yeah, installing a power outlet in the adjacent walls may be a solution, but there is already an opening in the concrete wall for a power outlet and I was wondering whether or not an emt conduit would suffice to protect the romex and thus eliminate the requirement of a clearance. Besides I have lots of concrete walls, some without a wood framing wall nearby; so I would face this situation in the future.

Thanks

stickboy1375 · Oct 12, 2015

Personally, I would just staple the wire to the insulation keeping the cables 1.25" away from the furring strips... if I had to run the wire horizontally, then just nail plate as required.

tk3000 · Oct 14, 2015

Thanks for the pic! Since there is no 1.25" clearance in depth, you are are saying that I could install it within 1.25" of horizontal distance from the furring strips and it would comply with code. In this case the wire would face the drywall even though it would be 1.25" further from the strips. Stapling it to concrete would be difficult, and stapling it to the insuation would be a sort loose fitting. Since it complies with code, it certainly would be an option. But it seems that a 1/2" emt conduit would not be that might time consuming, and it would be safer and easy to upgrade in the future.

On the subject of nm-b romex inside conduits and derating

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