NM in Heat Duct
- Thread starter Mike Lamb
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EV607797
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- Fredericksburg, VA
Yes, there is a very good reason why you can't. An air supply or return duct is considered a plenum. Romex cable is not rated for installations in plenums.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Two problems:
1. If used in a hot air duct rather than a cold air return, the ambient temperature of the NM can be 140F or higher, affecting the derating of the conductors and reducing the allowed ampacity. To minimize this, high temperature rated insulation must be used and ampacity may be reduced anyway.
2. Even more important, in any air duct, hot or cold, flame or gasses from overheated insulation will be carried throughout the building. Any wiring in plenum spaces, as well as ducts, has to meet the specific standards of 300.22 to make sure it will not itself be a hazard when overloaded or exposed to an external fire. NM does not qualify.
There is an applicable exception for cold air returns which in some cases allows wiring to go perpendicular to the duct from one stud to the next. But that is still bad practice and I try to avoid it.
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I found this Mike Holt video: http://www.youtube.com/watch?v=hD-RdBAFbzg
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A very good video, especially for anybody who was wondering why you could not use plenum cable in a plenum!
There is one subtle point that was left out, to avoid confusion and to limit time:
If you have an air handling space which was not physically constructed in its entirety for that purpose, but took the structurally necessary and originally accessible spaces between joists or studs (as in a basement ceiling or a stud wall) and finish them off with a sheet of metal or just drywall to make them an air handling space there is an exception. You normally see this ONLY for return air. You can put a very limited amount of wiring (not even plenum rated) through that space, perpendicular to the air flow. But you may not want to if you can avoid it.
This is very fortunate, since otherwise an air handling space of this type could become a roadblock that requires you to detour around those spaces, even if it requires building a chase (bulkhead for Canadian members or Mike Holmes fans) or going the long way around. But if you do it, do not leave the wire going straight across the air flow where it will interfere with air movement and collect lint. (Some will argue about that last advice.)
I also notice that in the 2011 code, it looks like under 300.22(B) you can run certain types of raceway and cable inside a physically constructed duct.
Did this change from 2008 or was Mike just interpreting the section conservatively? In 2011 it specifies some types of wire and raceway that are unconditionally allowed and adds one more type (Flexible Metal Conduit) which can be used only for connecting to air handling or sensing devices, and only for a limited distance.
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