Romex in conditioned attic? (Plenum definition)
zwilhoit
| Posted in Building Code Questions on
Hi, looking into converting our vented attic into a spray foamed, conditioned attic. We have a new air handler with new ductwork up there.
I see two options:
1) Delete the return ductwork, using the entire attic as the return plenum. But, the attic is full of old romex and some newer HVDC solar wire in conduit. Would this likely be a code violation, i.e. non-plenum rated romex in what is now, technically, a plenum?
2) Add one supply and one return at opposite ends of the attic. Would this avoid the potential code violation mentioned above?
Thanks!!
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I can tell you right now the wire in conduit doesn't "count" here, because the conduit acts as a vapor/air barrier (different from our usual definition used in green building though), so the wire inside doesn't need to be plenum rated if it's inside METAL conduit.
I have never seen an inspector call out anything for plenum ratings in a residential project. For commercial projects (where I usually work), I've seen projects shut down because of non-plenum wire used in a plenum space. I can't say if this is because the same rule doesn't apply to residential structures or not.
Plenum rated wire is all about smoke toxicity, and the idea is that the smoke shouldn't be spread around the building where people could breathe it in. That's why "air handling" spaces are "plenums" for this purpose, and you have to use plenum rated wire in those spaces. In commerical buildings, the entirety of the space above drop ceilings is often the air return for the HVAC system, so all wiring in that space needs to be plenum rated. If the HVAC system uses a ducted return system, then you can use riser rated wire (a lower classification) instead.
As far as I know, there is no plenum rated Romex-type wire. In order for your system to work as a plenum rated system, all wiring would have to be in conduit, so basically commerical style wiring would be needed. You could run everything in EMT, or use MC cable, or some combination of both. That is IF the plenum rule applies here, which I'm not sure is the case in a residential structure. Romex itself (new NMB or old NM) is not plenum rated.
I suggest you check with your local building department and see what they say. If they think the plenum rules apply, then it's probably simpler for you to use a ducted return system than it would be to run all new electrical to commercial code, AND air seal with fire stop materials any barriers between the attic and other living spaces where the new cabling makes a penetration.
Bill
Thanks, I'll plan to avoid turning the attic into a plenum and instead stick to a ducted return because of all the Romex not in conduit up there.