Low voltage in 1" FMC running 75' in plenum space

[Warmsmeallup]

Hi Guys:
I'm new to this forum. I'm hoping to get the real answer to this question. If any of you have seen this question postes elsewhere, I apologize for the redundancy. I'm just getting conflicting information. After watching a couple of Mike's Youtube video's, I'm feeling that I'll get a solid answer here. Here's the project;

We have to install a low voltage snow melting system on a membrane roof on a new school. There are 8 zones, each using (1) pair of #2thhn (total 16 cables) as "cold leads" from the transformer in the "control room" on the 3rd floor fed down through the floor into a plenum ceiling area on the 2nd floor where they travel accross the ceiling area through a sheetrock wall into and through another room without a plenum ceiling and then into a third room that is plenum celings. They then run up through the ceiling of the 3rd room plenum to the roof above. (I hope that's clear)

Here's where it gets funner...the lead pairs stay together as two runs of 8 cables (or 4 zones each) through the floor of the control room down to 12"x12" x4" work boxes where, once they penetrate the floor need to branch off the pairs to their respective coverage areas. The pairs stay together until they have to penatrate the 3rd room's ceiling up to the roof where they then connect to each of two ends of the element that is laid out on the roof.

Here's what we would like to do: pull each pair of #2's through (8) separate 1" FMC (steel) out to where each run ends at 4" junction boxes. The then leads split up out of the box and are run using aluminum 1/2" FMC up to where they penetrate the roof.

We don't want to use EMT because 1)of all the equipment that is already up there blocking our paths that would create a minefield of issues for bending and C) We are not allowed to remove the dropped ceiling grid which would make it alomost impossible to send 10' lengths up into the ceiling plenum area with all the obstructions. Also, we have to pull it into the greenfield because the supplier(s) only make 3 conductor MC as standard when it comes to #2thhn, AND, there's no way to get the 1000' reel of 1" MC up to the third floor of the school.


All that for the question; I understand that FMC can only be run in 6' runs when connecting to the equipment installed up in the plenum but are there a length restrictions to just running FMC through the plenum areas when not connecting to a fixture inside the plenum?

Thanks in advance for taking the time...

 

[PEDRO ESCOVILLA]

section 348.30 2008 nec, securing and supporting FMC, w/in 12 inches of every box and every 4-1/2 feet. if you've got that many obstructions, what will make it easy to pul the conductors through the fmc ? suck a pull string through to give you a better chance ( a good strong one) i've not ha d ereal good luck fishing through fmc. too rough. make a good head on your wires prior to pulling. oh yeah, before someone else gets to me, there is an exception on supporting where fmc is fished in concealed places in buildings, and its "impractical" more sags will only make the pulls harder. have fun

 

[augie47]

i believe the lenght limitation is for FMC less than 1/2".
i may be overlooking something, but I see no problem. My only concern would be assuring the single conductors don't pass thru metal such as decking.

 

[Warmsmeallup]

We tested pulling it. We sucked a good nylon pull string through and wrapped it around one of our trucks twice! It pulled fairly easily as long as someone at th eother end fed it. The deck will be slotted between the separate penatrations, as will the boxes.

Thanks for the replies.

 

[mike7330]

We tested pulling it. We sucked a good nylon pull string through and wrapped it around one of our trucks twice! It pulled fairly easily as long as someone at th eother end fed it. The deck will be slotted between the separate penatrations, as will the boxes.

Thanks for the replies.

Mule tape is easier and has less chance of damage to bends in longer runs.

 

[iwire]

FMC is still limited to just 360 degrees of bends between pull points, you can't just run it like a cable.

 

[mike7330]

FMC is still limited to just 360 degrees of bends between pull points, you can't just run it like a cable.

correct.

 

[MichaelGP3]

Of course you are correct as far as the 360 degree pull, but is there a problem with laying the FMC flat on the ground, pulling the wire in, and then running it like mc or bx to get around the 360 degree limitation?

 

[Twoskinsoneman]

Of course you are correct as far as the 360 degree pull, but is there a problem with laying the FMC flat on the ground, pulling the wire in, and then running it like mc or bx to get around the 360 degree limitation?

300.18 Raceway Installations.
(A) Complete Runs. Raceways, other than busways or exposed
raceways having hinged or removable covers, shall be
installed complete between outlet, junction, or splicing points
prior to the installation of conductors.

 

[iwire]

Of course you are correct as far as the 360 degree pull, but is there a problem with laying the FMC flat on the ground, pulling the wire in, and then running it like mc or bx to get around the 360 degree limitation?

There is no legal way to install FMC with more than 360 degrees of bends between pull points regardless of what order you do it.

 

[Warmsmeallup]

Yea, it won't have more than 180 degrees of turns in any single run. The bend down and out of the main box and then the bend out of the 4" up to the roof. No problems there. We'll probably run the greenfield first, tie them to unistrut cross supports and then pull.

I'll check out the mule tape.

Thanks again, keep it comin'. It's all good information.