# Greenfield in plenum ceiling



## warmsmeallup (Jan 30, 2012)

We need to run a pair of #2thhn about 75' in 1" greenfield in a plenum ceiling. Does anyone know if there are length limitations for this use?


----------



## Dennis Alwon (May 9, 2009)

You would need an egc and you technically need to install the flex before the wire can be pulled. You also need to deal with the 360 degree rule for bends. Read art. 348

Wouldn't it be easier to get MC cable.


----------



## warmsmeallup (Jan 30, 2012)

I've always called it mc but our supplier says that mc already has wire inside of it. Hollow mc is called greenfield...he says.

This is a project specific issue. We install low voltage snow melting systems where the "cold leads" are a pair of #2thhn's. They connect to a bronze screen heating element under the membrane materials on the roof.

As you know, the #2 thhn is not plenum rated. I need to run 8 separate pairs out to different locations under the roof in a plenum from a single point. The building is a finished school with dropped ceilings in place and no room to get 10' lengths of EMT up above the ceilings with 75' between the areas the controls will mount and the area that they will then penatrate the roof.

Thus, the hopes that I can insert the pair of 2's into the 1" Greenfield (mc) to their respective points from a 12'x12'x4' job box.


----------



## Dennis Alwon (May 9, 2009)

Southwire makes MC cable with #2 already in it. Why would you want to pull it thru the greenfield and be limited to bends etc?


----------



## warmsmeallup (Jan 30, 2012)

I don't want to pull it, I assure you! I didn't know that SW had it. I won't have any issues with bends. It's 1 bend down and out of the box and another up to the roof. That's it.

Thanks again...:thumbup:


----------



## noarcflash (Sep 14, 2011)

amd it must be supported, not lying on the ceiling grid.


----------



## warmsmeallup (Jan 30, 2012)

I was thinking of extending them below the grid and letting the library double as a playground!


----------



## Chris Kennedy (Nov 19, 2007)

I think FMC is fine for the described application. 300.22


----------



## nitro71 (Sep 17, 2009)

You don't know the difference between MC and flex.. You're an electrician?


----------



## warmsmeallup (Jan 30, 2012)

Absolutely not! I never said I was. 

I'd love to have more of you guys install these systems but.. alas! We install the system when no one else will and you guys run the feeds. On this project, the electricians didn't want to do it. So it was left to the roofer, who subbed us. Go figure!

www.comfortradiant.com


----------



## noarcflash (Sep 14, 2011)

Please explain how you can melt snow with low voltage? What do you consider low voltage? What is the current rating of a low voltage system?

I installed some 240volt mats under my bathroom floor, and even some 120 volt mats in my small bath.

Then I had a tile guy carefully do a mud job over it. And even 20 years later, I still love it. I can stand on my bathroom floor barefoot, and I warm the floors even in the summer time.

MC is BX with a green ground wire. BX uses the sheathing for ground.
Greenfield is just the BX sheathing without any cable in it, typically for short runs of custom work. I couldn't imagine pulling any serious footage through greenfield, as I would buy MC instead.


----------



## warmsmeallup (Jan 30, 2012)

Low volts (10-60v), high amps (usually around 90-95a). The primary depends on the area covered and the length of the cold leads (#2thhn).

We can melt snow at 21 watts psf in asphalt or concrete because the element (#10thhn) is within 1-1/2" of the surface. Line v has to be deeper so they need more watts.

We also use low voltage for floor warming and roof de-icing (5-30v at 90a) with an element you can nail through anywhere in the design. The length of the element dictates the voltage. We always want to draw around 90-95a. So, it's installed UNDER the hardwood floor or UNDER the shingles or membrane, such as the project we're on now. It's called Zmesh.

Check out the pictures on our site. They're not photshop'ed!


----------



## warmsmeallup (Jan 30, 2012)

BTW, as I mentioned previously, I didn't know I could get #2 in MC. My local supplier didn't know either and never suggested it. With advise from Dennis in this thread, that's what we will be doing.


----------



## Southeast Power (Jan 18, 2009)

warmsmeallup said:


> With advise from Dennis in this thread, that's what we will be doing.


That Dennis, he is so nice to you guys...


----------

