Plenums

Status
Not open for further replies.
e57

e57

Senior Member
Location
San Francisco, CA
Regular MC and J-boxes in plenums??????? "Other spaces for environmental air" - dropped tin can and rock ceiling with a cold air return in it.

Got someone telling me that J-boxes, and MC are not allowed. Anyone got a good article for this or wish to break it all down. I'd say it all myself - but apparently my word is not good enough.....
 
e57 said:
300.22(C) Apparently doesn't mention J-boxes... (got one adjacent to a cold air return - fully accessible.)
Click to expand...

Would the JB be consider "equipment" covered in 300.22(C)(2)
I could be wrong....

Equipment. A general term, including material, fittings,
devices, appliances, luminaires, apparatus, machinery, and
the like used as a part of, or in connection with, an electrical
installation.
 
Well, 300.22(c) says that MC is allowed, as are metal enclosures, so I think a jbox would certainly qualify. Me thinks this fellow is confusing (C) with the other subsections.

I don't think I've ever seen a sheetrocked space used an an air plenum. Only the usual drop ceiling or ducts. How common is it?
 
zbang said:
Well, 300.22(c) says that MC is allowed, as are metal enclosures, so I think a jbox would certainly qualify. Me thinks this fellow is confusing (C) with the other subsections.

I don't think I've ever seen a sheetrocked space used an an air plenum. Only the usual drop ceiling or ducts. How common is it?
Click to expand...

Actually that's what's under ever FAU in every house.

I'm in the '05 and 300.22(B) says: .....Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for thier direct action upon, or sensing of, the contained air.....

Then it goes on to say: ....enclosed gasketed-type luminaires shall be permitted.

But then 300.22(C)(1) says that you can use ......a raceway or metal wireway with metal covers......

I do agree that it is a little confusing.

I did just look at the Analysis of changes for 2002 and it shows a picture that has disconnects and j-boxes in the plenum.
 
I just wanted to throw this out because of the confusion of the word plenum.
The space above the ceiling for return air is not a plenum as defined by the NEC.

"I'm in the '05 and 300.22(B) says: .....Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for thier direct action upon, or sensing of, the contained air....."

You can't have ANY wiring in a plenum unless it's for air sensing etc.
Here's a illustration from MH talking about class 2 & 3 wires but is shows what is and is not a plenum
1100794770_2.jpg

Plenum. A compartment or chamber to which one or more
air ducts are connected and that forms part of the air distribution
system.
I%27m%20in%20the%20%2705%20and%20300.22%28B%29%20says:%20.....Equipment%20and%20devices%20shall%20be%20permitted%20within%20such%20ducts%20or%20plenum%20chambers%20only%20if%20necessary%20for%20thier%20direct%20action%20upon,%20or%20sensing%20of,%20the%20contained%20air.....
 
cowboyjwc said:
Actually that's what's under ever FAU in every house.
Click to expand...

FAU- fan air handler? In residential, I've only seen either completely-ducted forced systems, or "open space return" were the furnace draws from it's immediate area and the furnace room/closet usually has louvered doors (might be forced, might be gravity-feed).

Based on the fpn in (C), a drop ceiling is (C) and I think by extension, a framed-in space that air happens to move through would also be (C), but a purpose-build tunnel for air would be (B). Thinking about it, about the only thing I can see that would come under (B) are either ductwork (which is a plenum) or the large rooms that aggregate return air from multiple ducts to feed into AHUs and whose sole purpose is moving air. By the (B) definition, a collector room with an AHU in it couldn't have junction boxes, either.

FWIW, the online M-W dictionary says: an air-filled space in a structure ; especially : one that receives air from a blower for distribution (as in a ventilation system)
 
Twoskinsoneman said:
I just wanted to throw this out because of the confusion of the word plenum.
The space above the ceiling for return air is not a plenum as defined by the NEC.

"I'm in the '05 and 300.22(B) says: .....Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for thier direct action upon, or sensing of, the contained air....."

You can't have ANY wiring in a plenum unless it's for air sensing etc.
Here's a illustration from MH talking about class 2 & 3 wires but is shows what is and is not a plenum
1100794770_2.jpg

Plenum. A compartment or chamber to which one or more
air ducts are connected and that forms part of the air distribution
system.
I%27m%20in%20the%20%2705%20and%20300.22%28B%29%20says:%20.....Equipment%20and%20devices%20shall%20be%20permitted%20within%20such%20ducts%20or%20plenum%20chambers%20only%20if%20necessary%20for%20thier%20direct%20action%20upon,%20or%20sensing%20of,%20the%20contained%20air.....
Click to expand...

Yes you are correct and that's why I also quoted 300.22(C) Other spaces Used for Environmental Air and it says pretty much the same thing. You do see in the pic that you posted that it still requires plenum rated wire in it. While they may be different they do pretty much the same thing.
 
cowboyjwc said:
Yes you are correct and that's why I also quoted 300.22(C) Other spaces Used for Environmental Air and it says pretty much the same thing. You do see in the pic that you posted that it still requires plenum rated wire in it. While they may be different they do pretty much the same thing.
Click to expand...

That's true. The difference being plenum rated cable is fine in other spaces used for enviromental air space whereas plenum rated wire is still not allowed inside a plenum (as defined by the NEC). You know unless it's sensing the air etc.
 
iwire said:
If it helps I thought your point was a good one and I also noticed that the word 'plenum' was being used incorrectly in this thread.
Click to expand...

Thank you. This was something I myself was confused about until very recently. Plenum rated cable? Not in a plenum... perfect.

I think it's a poor choice for the NEC to redefine a common use word in such a away that the definition conflicts with a common dictionary definition.


Does this happen anywhere else?
 
Status
Not open for further replies.
Top