Romex & Conduit-sizing question

[renstyle]

Hello all,

Had a conduit sizing question. Want to run five or six 12-2 lines from my basement to the attic. Not sure what size conduit is necessary to hold them all. Pulling out the knob & tube upstairs to ready for attic insulation.

I am planning on running these lines from the basement to the attic in the same void as the iron main/vent stack (not a flue or chimney).

From there I was going to upgrade the upstairs wiring from Knob & Tube the whole upstairs (4BR+1bath) are on 2 20A breakers now, and create multiple circuits to even out the load (plus make the bathroom code compliant, ungrounded outlet in there still)!!

No big loads upstairs other than running a 120v window A/C in the summer. This outlet (GFCI) will be on it's own circuit. The only other load I would consider "large" would be the hair dryer, possibly a curling iron. They would run off the new GFCI in the bathroom, also on a dedicated 20A circuit. The other lines would run the other outlets and ceiling lights.

The conduit will run 12' from basement to k-ceiling, then jog over about 5' between two joists and then another 12' up following the main house vent to the attic.

I'd like to use PVC electrical conduit rather than EMT, mainly for cost (bigger EMT conduit gets pricey faster than big PVC).

First question: Regarding the yellow sheathing on the 12-2... I'd prefer to leave this intact and just go with a larger dia. conduit. What size dia. would be appropriate for five or six 12-2 lines of romex (with the sheathing intact)?

Second question: am I wrong in my thinking that running this conduit in the vent void is OK? Seems reasonable on this end?

I'm lucky at this stage to be able to see most of the conduit run, and have all of the cable ready to go before the blueboard/veneer plaster goes up hiding it for the rest of my lifetime. For this same reason, I wanted to make sure I do this right the first time.

Sorry for the novella.

 

[mdlbldrmatt135]

.......

 

[pattenp]

PVC conduit is fine for what you want to do. When you say with sheathing intact, you can't remove the sheathing from romex and use the individual wire, that's against code. I don't know the fill rate on conduct off the top of my head, but I think you would need at least 1 1/2" to 2" conduit for 5-6 cables.

 

[pattenp]

Yeah... but that's single wires not romex. You have to take into account the sheathing on the romex.

Six 12-2 lines is 18 wires.... IIRC that's 1" conduit teritory.

 

[pattenp]

Plus romex is going to be a ***** to pull through conduit. So the larger the conduit the better.

And the void area is fine to pull the wire through.

 

[mdlbldrmatt135]

.......

 

[renstyle]

Six 12-2 lines is 18 wires.... IIRC that's 1" conduit teritory.

I kinda figured it would have to be at least 1 1/2" or maybe 2" for the cables to physically fit with enough clearance to allow for heat?

 

[renstyle]

Wow, you guys are fast! I don't know what I was thinking with "removing the sheathing".

 

[Aceman]

I guess I don't understand why you're running conduit in the first place? I'd simply fish all those cables in, running conduit just makes the job harder....

 

[rwreuter]

if you try and pull 6 sets of 12-2 through 1" PVC you will curse so much someone will think you have tourets syndrome.

 

[harvero]

Why not run two 2inch conduits? Think future upgrading.

 

[rabidsquirrel]

Two inch conduit is still too small. When you take the cross section of romex you take it from the widest part. Like everyone else said, just fish it through the chase.

 

[Gooch]

Two inch conduit is still too small. When you take the cross section of romex you take it from the widest part. Like everyone else said, just fish it through the chase.

you figure the total area taken, LxW=A, but i agree, just fish the wire in the wall, if you've got enough room and access to install 2" conduit, you have enough room and access to just fish the wire, don't do the work twice

 

[rabidsquirrel]

you figure the total area taken, LxW=A

Nope, here's a good post that explains it:

Chapter nine of the NEC has methods to calculate the area of sheathed cables (such as Romex) in order to determine the fill requirements of conduit, so there is nothing wrong with running romex in conduit, so long as you properly figure the area and fill requirements.

Take a caliper, measure the romex across the widest part of the ellipse (longest cross section dimension), assume this is the diameter of a circle and convert this measurement to the area of a circle (pi times r squared ? I think) armed with this number, and knowing that a multi-conductor cable (romex) or cord, with two or more conductors is treated as a single conductor for purposes of fill (53% fill for a single conductor, one Romex; or 31% for two conductors or two romex runs; or 40% for three or more conductors or three or more romex runs) and knowing that Article 352 rigid PVC has the following fill numbers, you can determine if the romex will legally fit in the conduit.

100% of internal area of 3/4 PVC is .508 inches square
53% is .269 inches square
31% is .157 inches square
40% is .203 inches square

and oh yes, if the run is less than 24 inches it is considered a ****** and the fill is 60% allowed or .305 inches square.

Charles

So 40% fill for 6 romex.

14/2 Romex is 3/8 or .375"

.375^2 x .7854 = .1104" cross section area

.1104" x 6 = .6624"

So I was wrong 14/2 would fit in 1 1/2 ENT.

CSA of 6 12/2 is 1.5" so you'd need larger than a 2" ENT.

 

[shambuda2000]

Correct me if I'm wrong, but I thought it's against code to pull sheathed wires through conduit.

 

[rockwithjason]

Correct me if I'm wrong, but I thought it's against code to pull sheathed wires through conduit.

you are wrong. there is no such provision in the code

don't forget that if you run more than three current carrying conductors in the conduit you will have to derate the current capacity of the wires according to the proper tables in article 310

 

[Charles (in GA)]

you figure the total area taken, LxW=A........

Not correct, code says you take the diameter of the "elipse", the widest way, and calculate the area as if it were a circle.

(9) A multiconductor 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.

Charles

 

[renstyle]

I guess I don't understand why you're running conduit in the first place? I'd simply fish all those cables in, running conduit just makes the job harder....

After reading all of the great responses and taking another look at exactly how much space I have in the void next to the vent stack, it's looking like even a 1.5" conduit would be a *very* tight fit.

My main thinking for the conduit was to segregate the power lines from the low-voltage cat6 and analog phone lines I'm running to the attic from the network panel in the basement. Plus I thought conduit would be necessary since the lines would run from Basement to attic.

Wish I could buy all of you guys donuts or something...

 

[renstyle]

Thanks for all of the great responses. Those calculations and thoughts on conductors and their area will be invaluable in future upgrades!

 

[kbs2244]

If you are using plastic conduit I don’t see how you are you “segregating” the power from the signal wires.
If you are thinking EMF, I believe you would need metal conduit for the power.
But if you use shielded signal wire you shouldn’t need any conduit at all.

 

[jrsulo]

Why not just run a sub panel ??

 

[renstyle]

Why not just run a sub panel ??

I had thought of this as well. It would be in an unfinished attic, so wasn't sure what types of code clearances would be required.

It's a foursquare house, 30'x30' with a hip roof, so just under the peak there is at least 5, maybe 6' headroom, enough to frame up a spot for a box.

If I went that route I'd need to determine the appropriate size mainline from the subpanel back to the main panel.

Would there be any point (or is there a code requirement) that I run a single 220 line with both hots from main to subpanel? I figured for balancing each side of the supply at the main panel it would have to be double pole (thus 220) and thus 220 all the way to the panel? Sorry if this seems obvious to some.

The upstairs is just bedrooms and a bathroom. The summer window-AC would be a heavy 110 load, but the rest is just outlets and lights.

If 220 wire, would 10-3 be sufficent? or would I need to look at 8-3?

If 110 wire, still 10-2 or 8-2 (figured 12-2 would be way undersized for sure)?

 

[renstyle]

If you are using plastic conduit I don’t see how you are you “segregating” the power from the signal wires.
If you are thinking EMF, I believe you would need metal conduit for the power.
But if you use shielded signal wire you shouldn’t need any conduit at all.

I had made an assumption that plastic conduit would be necessary. From what I'm reading its looking more and more optional.