IT room floor plenums and the IMC

[klarenbeek]

We keep running into the problem of rated IT rooms having non plenum rated wiring in the underfloor plenum. Typically this wiring (or in my current scenario, Sealtite) is ok per the NEC because its an IT room, but as a mechanical only inspector I'm not enforcing the NEC, just the IMC. The IMC does not address IT rooms at all--a plenum is a plenum wherever it is at, so everything in that underfloor IT room plenum has to meet the same requirements of any other plenum. Just wondering if anyone else was running into the same issues or how you were addressing this situation. Thanks.

 

[Mule]

I guess you are calling it a plenum because there is return or supply air within the chase???

 

[Dr. J]

I agree, if it is being used as either a supply or return plenum, it is a plenum, and IMC 602.2.1 applies. However, not all IT space underfloor areas are plenums (plena?). Some are just wiring paths.

 

[klarenbeek]

The air handler discharges supply air directly into the cavity between the concrete floor and the elevated floor, the same space where all the cables are located. some of the removeable flooring tiles are perforated, basically acting as supply registers. This setup makes the underfloor space a plenum. The conflict we run into is that because it is a rated IT room per electric code, with independent fire suppression and AHU shutdown within the room, there are exceptions within the electric code allowing some products to be in the underfloor air plenum (its not even a plenum per the ELECTRIC code) that aren't normally allowed in such a space. The IMC makes no exception for IT rooms. They are not even mentioned in the mechanical code. Most electrical contractors and suppliers, and even manufacturers, can't seem to understand that its not ok if it meets the electric code, but not the mechanical.

 

[Mule]

Doesn't this cover you?

From the 2006 IMC under Section 602 Plenums

602.2.1.1 Wiring. Combustible electrical or electronic

wiring methods and materials, optical fiber cable, and

optical fiber raceway exposed within a plenum shall have

a peak optical density not greater than 0.50, an average

optical density not greater than 0.15, and a flame spread

not greater than 5 feet (1524 mm) when tested in accordance

with NFPA 262. Only type OFNP (plenum rated

nonconductive optical fiber cable) shall be installed in

plenum-rated optical fiber raceways. Wiring, cable, and

raceways addressed in this section shall be listed and

labeled as plenum rated and shall be installed in accordance

with ICC Electrical Code.

 

[Dr. J]

If only under the provisions of NFPA, then a room complying with NFPA 70 Article 645 would not need any special cabling. This would be consistant with the rest of the NFPA family. However, you are enforcing both NFPA 70 AND I-codes.

A good argument could be made that an Article 645 compliant system should be all that is needed, however this would requre a variance from adopted code. The argument is that such a room is dedicated to IT, is isolated from the rest of the building, and any smoke generated under the floor will not spread beyond the confines of that space. The reality is that in this situation, there really is no difference between smoke under the floor and smoke above the floor - either situation results in only one smoke filled room.

 

[klarenbeek]

Dr. J,

I agree, this seems to be one of those areas not adressed well in the IMC, which leads to overkill. Because of the requirements in NFPA and NEC for a room to be an IT room, I personally think there is adequate protection there, but I can't go off my personal opinion, just the code. Hopefully in the future someone with more sayso than myself will address some of these issues. Goes right along with combustibles in any plenum. if there is duct detector shutdown for the air handler, it seems the noncombustible requirements could be relaxed, because once the air handler is shut down all of the issues caused by the space being a plenum go away.