Definition of environmental air

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nanckekr

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Several places throughout the NEC I find specific wiring rules for ?Ducts or plenums used for environmental air?. I have trouble defining what goes into that category.


Take this example:
- A concrete steel-frame high-rise building
- Ceilings are concealed sheet rock attached to a metal frame that is hung from the steel frame of the building.
- In between the sheet rock and the steel frame of the building (approx 16?) there are large ?tubes? for air conditioning. There are also tubes for ventilation to the external.
- All the air ?tubes? mentioned above are fully enclosed, i.e. the air flowing through the tubes do not get access or in contact with the air inside the ceiling.


Is this ceiling a
- ?duct??
- ?Plenum??
- ?Plenum used for environmental air??
- ?Other space Used for environmental air??

BR,
Anders
 
1100794770_2.jpg
 
Wow, I wish I would have found that picture a long time ago....
So this means, that class 2 cable (725), as well as audio cable (640), telephone cable (800) and antenna cable (820) must be CMP and not CM/CMG etc.

It seems like the exception is if CM/CMP wire is running inside an EMT

Is this close?
 
But what you described does not match the drawing (the drawing depicts the concealed ceil space as an"other space used for environmental air"). If the ducts terminate below the finished drop ceiling, it shouldn't be considered an environmental air space. However, many places save half the cost of ducting by just putting open grids in the ceiling and using that concealed space for return air. I maintain that a fully ducted return will make this space not an "other space used for environmental air".

That being said, in the building I work, the fire code people are overly paranoid and consider that space an air handling area even though both source and return air are ducted. The ducts leak enough so they say that makes it an "other space used for environment air". I disagreed because the design is not intened to leak (it wasn't designed to be an air handling space), and I think intent is part of the definition. But I carry no weight and had to treat it like an environmental air space at a much greater cabling cost...
 
You are right, I read the picture wrongly. I did not see that that air was going through the ceiling and then into the AC.

As you point out, it is most likely best to assume that fire code people will classify the closed ceiling as "other space used for environment air" just because of the presense of AC channels inside the ceiling, as their perception is likely that even the best closed ceiling is not entirely closed, and due to that there is environmental air in the area, and then stricter rules apply.

Thanks to both of you for helping me understand this!!
 
What gets me though is that this can be carried to the extreme -- are wall cavities now environmental air areas because there happens to be a duct in it which could leak?

I think our fire people are just dumb or paranoid. I have the same issue with underfloor areas. Half the rooms in the building have air handlers that blow under the floor (which rightly makes it an environmental air area). One has an air handler that looks like that, but it actually blows over the floor and not under it. The fire people show up, see an AC, see a raised floor, and say "plenum cable". Sometimes they just see the raised floor and say "plenum". When I point out there is no AC, they say "well, you may install one later".

These fire people are company employees, so I can't treat them like an inspector ("give me a code reference to back up your denial"). It is also frustrating when they say "ok, you can have a handful of non-plunum cable". I ask then can I go find the guy with the largest hands...
 
Environmental air space is just when the space is used in place of a duct, more commonly referred to as ventilating ceilings (in the UMC). The area above a suspended ceiling is only considered "environmental air space" if the entire area is used as a plenum. If the entire mechanical system is ducted, including return air, the ducting being installed above the drop ceiling does NOT make this area an "environmental air space".
The illustration above is not accurate as to systems installed inside of a duct. A duct IS a plenum. As long as the wiring and wiring methods comply with 300.22 (which, by the way, is titled "Wiring in ducts, plenums, and other air handling spaces") you may install within a duct, but it's probably not a good idea unless you cannot avoid it.
The same is true for raised floor systems. They are only a 'plenum' if the space itself is used to convey air in lieu of installing ducting. Typically, the only part of the mechanical system to use these spaces is the return air. If this area is not used for this purpose, it is NOT a plenum space.
Oh, and Suemarkp, the stud bays or joist bays are not considered plenum spaces unless the space itself is used as a 'duct' or plenum. If a duct is installed in the space, it usually means that the duct is doing the job of a plenum, not the wall cavity.

As to the original post, the space as you described is NONE of the categories that you mention. It is simply a concealed space. Nothing special required.
 
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nanckekr said:
Several places throughout the NEC I find specific wiring rules for ?Ducts or plenums used for environmental air?. I have trouble defining what goes into that category.


Take this example:
- A concrete steel-frame high-rise building
- Ceilings are concealed sheet rock attached to a metal frame that is hung from the steel frame of the building.
- In between the sheet rock and the steel frame of the building (approx 16?) there are large ?tubes? for air conditioning. There are also tubes for ventilation to the external.
- All the air ?tubes? mentioned above are fully enclosed, i.e. the air flowing through the tubes do not get access or in contact with the air inside the ceiling.


Is this ceiling a
- ?duct??
- ?Plenum??
- ?Plenum used for environmental air??
- ?Other space Used for environmental air??

BR,
Anders
Click to expand...

I believe your discription is of a plenum. I believe if the air space is intentionally designed to act as a supply air or as return air than the area is enviromental air space. plenum is and enclosed space where the pressure is higher than the surrounding area. Duct is a enclosed area made to contain a substance.
 
earshavewalls said:
As to the original post, the space as you described is NONE of the categories that you mention. It is simply a concealed space. Nothing special required.
Click to expand...

That is what I thought as well, but it seems like the fire people have the right to overrule what I think anyway. Referring to the real examples from suemarkp, the presence of an air conditioning, no matter how well sealed it is, can trigger them to say this is a plenum.

In lieu of that, I will properly plan for EMT to be used for everything. That way I do not have to invest in CMP cables 725.61(A)
 
nanckekr said:


- ?duct??
- ?Plenum??
- ?Plenum used for environmental air??
- ?Other space Used for environmental air??


BR,
Anders
Click to expand...

None of the above.:smile: Have the fire people provide a legitimate citation. Bet they can't.
 
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