Re: Short Plenum Run Question

[robertlbass@xxxxxxxxxxx]

> I have no idea how the NEC may define it but the normal definition of plenum is "a chamber at higher than atmospheric pressure."

The NEC defines a plenum as a "compartment or chamber to which one or
more air ducts are connected and that forms part of the air
distribution system."

Regarding wire and cable in ducts and plenum spaces, the code states:
"Only wiring methods consisting of Type MI cable, Type MC cable
employing a smooth or corrugated impervious metal sheath without an
overall nonmetallic covering, electrical metallic tubing, flexible
metallic tubing, intermediate metal conduit, or rigid metal conduit
without an overall nonmetallic covering shall be installed in ducts or
plenums specifically fabricated to transport environmental air."

At first glance I didn't take exception to Mr. Olson's suggestion of
using BX to jacket non-plenum rated cable.  That advice is incorrect.
For fire safety reasons, NEC prohibits this practice if the run exceeds
4 feet and its use is restricted to cables required to operate the duct
system itself.  It's not allowed as a means of avoiding using plenum
cable.

NEC discusses the above as follows: "Flexible metal conduit and
liquidtight flexible metal conduit shall be permitted, in lengths not
to exceed 1.2 m (4 ft), to connect physically adjustable equipment and
devices permitted to be in these ducts and plenum chambers."

Also, if you do choose to use BX in those limited applications where it
is permitted you must use connectors which "effectively close any
openings in the connection."  IOW, you can't just slip a cut off piece
of BX jacket over your regular cable and run it in a duct or plenum.

There is a great deal more in tne code about running cables, not just
in plenum spaces but throughtout your home.  Anyone wanting to make
significant changes to house wiring would do well to order a copy of
the code.  It's available on CD-ROM, in bound book format and as a
downloadable PDF file.

Regards,
Robert L Bass
www.BassBurglarAlarms.com