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I'm working on plans to convert three bays of a 2x4 stud wall with drywall into a mass wall by adding material to the cavity. The material will be something like sand/clay/gravel (to be determined depending on the answer to this question and other factors). The goal is to catch heat from a wood stove around 8' from this wall and help moderate the resulting heat as well as transmit more heat through the wall to the adjoining room.

There are three 12/2 Romex cables running together horizontally through this wall at around 2' above the bottom plate.

What are the electrical concerns and considerations relevant to this idea? Is it safe and allowed by code to pour a bunch of heavy stuff around these wires? If not, what protection is needed and how can it be put in place given that the wires have already been run?

[Edit: I left the wires alone but I boxed them off so they have an airspace a couple feet tall and the mass only begins above them. This isn't really an answer to the question so I'm not leaving it as an answer. It's just the work-around I used since I didn't have the answer and it didn't seem like it was going to turn out to be okay to surround the wiring with mass.]

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    NM is approved for suspension in the air and through generous holes in studs of width 1.5". I don't see how it could be allowed embedded in a massive wall which is intended to be heated. It seems to me that the NM cables must be rerouted. Maybe you could leave a normal empty space in the the top 1 ft of the wall for these cables or reroute the cables through the attic. – Jim Stewart Aug 29 '18 at 11:59
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    Thanks. Any reason you left this as a comment rather than an answer? – Jean-Paul Calderone Aug 29 '18 at 12:55
  • What amount of heat are we talking? I've seen Romex tolerate 120F/49C attics but if we're talking more like 150F/65C or more I wouldn't chance it, even in conduit – Machavity Aug 29 '18 at 13:10
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    Off topic: I hope you're not expecting drywall to contain said material. – isherwood Aug 29 '18 at 13:44
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    @isherwood Initially I was hoping the drywall would but this hope is increasingly being eroded. I'm looking for alternate strategies now, but indeed that's off-topic on this question. Thanks for pointing out the potential issue. – Jean-Paul Calderone Aug 29 '18 at 14:12
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This is an unusual question but I think the code does address it and prohibit what you're doing:

334.12 Uses Not Permitted.

(B) Types NM and NMS. Types NM and NMS cables shall not be used under the following conditions or in the following locations:

(1) Where exposed to corrosive fumes or vapors

(2) Where embedded in masonry, concrete, adobe, fill, or plaster

(3) In a shallow chase in masonry, concrete, or adobe and covered with plaster, adobe, or similar finish

(4) In wet or damp locations

More general answer - when you have an unusual setup it's best to err towards caution; I'd say you need to do something other than just fill around that cable.

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