[KwartzLab] plenum vs non-plenum

[Kevin Martin]

Indeed the question is what is a plenum. The word (as far as building trades are concerned) originates in ventilation terminology, and applies to spaces such as that above a drop ceiling when it forms part of the ventilation system. In commercial buildings the space above the drop ceiling is often used as the air return to the HVAC unit. It also applies in residential work to the wall and floor cavities that form part of the cold air return on a forced air heating system.

Wiring in such spaces which could generate plentiful smoke would be a safety concern because it could quickly get dispersed throughout the building.

The Ontario Building Code defines: "Plenum means a chamber forming part of an air duct system."
9.34.1.5.  Wiring and Cables: (2)  "Where a concealed space in a floor or ceiling assembly is used as a plenum, electrical wiring and cables within the plenum shall conform to Sentence 3.6.4.3.(1)." Section 3.6.4.3 then goes into a long-winded description of the fire ratings of the materials that form the plenum itself as well as stuff like wiring that happens t run through a plenum.

If the space above the drop ceiling is closed and not part of the HVAC system, it would not count as plenum space, any more that the sealed cavities within a wall between each pair of adjacent studs would.
That being said I have never seen any special provision for regular power wiring when it passes through a wall cavity that happens to form part of the cold air return in a residential heating system, although it would seem to me that the same hazard would exist. This may just be a case of endemic lax application of regulations. Or perhaps this only occurs in walls, which are not covered by 9.34.1.5 (2).

In any case if you only need a few short runs in true plenum space, you could enclose them in conduit (some of the provisions of 3.6.4.3(1) allow for this). This is essentially the solution used for power cables in plenum spaces, where one sees conduit and/or armoured ("BX") cable. This might be cheaper and/or simpler than buying all plenum cable or buying two types of cable and having to splice them.

-Kevin Martin
  the Papertrail Handmade Paper & Book Arts
  New Dundee ON

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://kwartzlab.ca/pipermail/discuss_kwartzlab.ca/attachments/20120905/cfd343d8/attachment.html>