[KwartzLab] plenum vs non-plenum

[Darcy Casselman]

Hm, that's not a bad idea.  There are a couple places I need to cross
over heading ducts, which gave the idea of fire-rated cable more
practical appeal.

PrimeSpec looks like it only sells spools of 1000', which is way more
than I need.  And $185 is somewhat more than I want to pay.

Darcy.

On Wed, Sep 5, 2012 at 9:23 AM, Kevin Martin <kpmartin at thinkage.ca> wrote:
> 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
>
>
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