CAT 6 cables and undefloor ditribution

[a10jp]

Hi, has anyone heard of problems installaing premise distribution CAT 6 cables under (or within) floor slabs in open office environment? One of our customer has requested to install outside plant cables under the floor slab. His arguement anyting under the floor slab is considered exterior environment. Usually, however, when we install underfloor conduits, they are always embedded into the floor slab concrete, which is not considered exterior condition by NEC. WHat is everyone's opinion and, if any, experience relating to this type of installation? I do know that CAT 6 is more susceptaible to moisture (due to exterior)problem and interference with crosstalk issues, but I don't think it is a suitable arguement in this case.

 

[alehman]

My main concern would be for moisture. Underfloor conduits are likely to have water in them. Is the cable rated for wet locations?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rbulsara]

I believe anything in 2 inch of concrete cover is considered "outside the building" (have not checked the latest wording of the code)per NEC. Anything below slab at grade is obviously outside the building. But if below grade, moisture/wet conditions need to be considered as Alan eluded.

Rafiq Bulsara

http://www.srengineersct.com

 

[a10jp]

I read an article that mentioned a similar scenario that when conduits installed below grade (when they are in contact with the subgrade, e.g. soil) will be subjected to temeperature difference between below grade temperature and inside the building. But I wonder, and as my argument, that if the conduits are embedded inside the floor slab, is this condition for temperature gradient still true? But perhaps rather than taking a chance, it is a lot easier to install OSP cables with outdoor rating in this location than to find out later with moisture problems.

 

[rbulsara]

Conduits are routinely buried below grade, that continues to become exposed outdoors and entering buildings and what not. Never heard of issues due to temperature differences.

Whatever article you are reading, it perhaps is taken out of context. Sounds like someone is trying not to install conduit to "save" money(read cut corners).

Rafiq Bulsara

http://www.srengineersct.com

 

[a10jp]

We are installaing conduit system. Perhaps a picture will help. Here is my detail how the conduit/j-box will be installed below grade. They are in effect inside the floor slab with 6-inch embedment all around.

I guess my original intent of my question is what constitues outside exterior environment. I am reading NEC Art 300.5 and the article talks about direct buried cables and minimum cover. But when the conduit is embedded in the concrete the way I show it, is it still technically considered exterior?

The temperature difference in the article bascially talks about where the conduit penetrate above the floor slab, the conditioned air will condensate inside the below slab conduit even when they are watertight sealed. Therefore when I rad the article I pay attention to the fact that it says about temperature gradient due to conduit in contact with the soil below.

 

[PHovnanian]

You'd have to run this by the AHJ, but raceways embedded in a slab could still be considered to be within the building.

One condition comes to mind: In the event of a ground fault, the fault currents would return through the building's grounding system. While this isn't as important for CAT6 as it would be for power conductors/conduit, similar logic would apply.

 

[a10jp]

Hi PHovnanian, thanks you're probably right. As for grounda fault, you would want the current to return back to building system groundo, is that what you mean? Or are you saying conduits underground inside a building will be subjected to fault current going back to the building system ground?