What are your thoughts about using a cable Listed as CL2R in the return air joist space of a residential occupancy? Is this acceptable by inspectors or should this be CL2P?

Vote me "plenum".
Riser rating only has to do with flame spread, not toxicity of the smoke as far as I understand.

Is the joist space in a residential occupancy considered to be a plenum?

I suppose it would all depend on how you ran the wire. Is it passing through at a 90 or running with the bay

Greg,

Here in some of my towns the building or fire inspector will fail a job if wire, RX or other, is run through any ductwork in a resi bldg. There is one job here I was looking at and th tin knockers "Panned" a whole main girder. and when they did that, they enclosed not only wires, but there was a large waste line. "Panning" is when the tin knockers put a flat piece of tin across the bottom of your 2 X 10's and they use that area to move air instead of using a piece of ductwork itself.

I think the answer is in 300.22(C)ex

When I first read George's note I was thinking about wires running down the stud bay <the riser ref in CL2R>.
If they simply cross the bay it is probably OK but YMMV with various AHJs
I have seen it go both ways in suspended ceilings and computer floors too.
All IBM interface and power cables after about 1994 were plenum rated (AKA "DP"), tan colored. The blue ones were riser rated and tolerated until we could swap them out but the grey (unrated) ones had to go.

In other than dwellings 725.61(A) would affect low voltage wire, no matter which direction it ran.

[This message has been edited by gfretwell (edited 10-31-2005).]

Greg nailed it. I was addressing the wiring installed with the air flow and the code reference 725.61(A) talks about cables in plenums, ducts and other space used for environmental air as needing to be Listed and labeled as plenum cable. the other reference is valid if you are installing cables perpendicular to the air flow.