[Tfug] CMP *in* ductwork?

[Adrian]

On Tuesday 11 September 2012 01:03, Bexley Hall wrote:
> Hi Will,
>
> --- On Tue, 9/11/12, William Stott <will at stottland.net> wrote:
> > CMP in ductwork? I'm sorry, but I hope this isn't your only option.
> > This is a bad idea, and violates codes (not all). I can look up the
> > exact codes if you want, but really, just go a different route if
> > at all possible.
>
> I scoured the code and couldn't find anything that imposed
> *different* restrictions on CMP in "air handling spaces" vs.
> *ductwork*.  But, I'm looking at an old code book -- I'll
> have to make a run to the library tomorrow.
>


Silly me.... I read your initial email as installing in ducts and put my 
electrician hat on... Of course there isn;t anything wrong with that! 
Scowling at all these hostile responses I was considering writing long 
missive back as to why it is reasonable and proper... only to now realize you 
meant "air ducts"... not electrical ducts/conduit.

So yes, with very few exceptions (namely something directly associated with 
the air handling system, i.e. an airflow sensor, or perpendicular crossings 
of a duct), you can not install any type of cable within an "air duct". You 
can install properly rated cables within an air handling space, a plenum, but 
not within a dedicated ducts.

The relevant section would be the NEC 300.22, specifically section (B) -
"Duct or Plenums Used for Environmental Air: Only wiring methods consisting of 
Type MI, Type MC..., EMT, FMC, IMC, or RMC without an overall nonmetallic 
covering shall be installed... FMC and LFMC shall be permitted in lengths not 
to exceed 1.2m (4ft)...".

So if you wanted to run all that CMP inside EMT, inside the duct, with 
air-tight connectors, you might squeak by on a technicality as far as the 
NEC. Section 300.22(C) goes on to list a different kind of space you can 
install cables, but that specifically excludes spaces "...specifically 
fabricated for environmental air-handling purposes...".

The NFPA 90A 4.3.3 states it a bit more explicitly: "Wiring shall not be 
installed in air ducts" unless it is "directly associated with the air 
distribution system and does not exceed 1.22m (4 ft)." Cables installed 
within conduit within the duct is not considered with in the airflow of the 
duct. 


Adrian