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I've got a switch box on a wall, it was originally meant to be a three-way with the switch outside, but that is not necessary and right now it is just functioning as a pass-through. I know you can't plaster over a junction box as it needs to be accessible. But I've read that you can plaster over the box if you permanently join the wires together by either crimping or soldering with heat shrink. Is this correct?

Further, if I do solder the wires and heat shrink them, can I apply plaster directly over the wires? Or should I put in some styrofoam/drywall as a "backing" for the plaster over the wires inside the junction box (we have fully plaster walls over brick).

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No, only the Power Company can make running splicing without junction boxes and that are not accessible. Electricians are bound by a different set of Codes which they're none that say it is permissible to splice wires and cover them up so they are non accessible. Probably an urban myth you heard somewhere.

Update! According to NEC Art 334.40 (B) Devices of Insulating Materials -- a listed nonmetallic-sheathed cable interconnector can be used in exposed or concealed repair work.

But fair warning -- I've tried these before and though many may claim they work fine, I threw it in the trash. The tiny 4 screws make it almost impossible to assemble especially when normal repair work is not in the ideal conditions. Would never recommend them.

  • There are NM-cable to NM-cable splice connectors that can be buried in a wall, but I doubt they'll work for his application. – ThreePhaseEel Apr 10 '17 at 22:37
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No, you can't do this -- Code requires that boxes and most kinds of splices be accessible. Under certain conditions, it's possible to bury splices made using a special connector in a wall (see this answer for what's needed) -- but the conditions aren't likely to be met because the cables at your box aren't all the same size.

  • Gave u +1 because you really deserve credit. I absolutely totally forgot about them NM splices. Easy to forget junk I guess :P – Kris Apr 11 '17 at 23:19

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