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I want to install a Wi-Fi thermostat, but the existing thermostat wiring didn't have a common wire. The house was originally wired with two-conductor cable since it was a heat-only system, and then retrofitted with a 2-wire to 4-wire adapter system when the previous owner wanted A/C. Rather than messing with additional adapters, I'm just pulling new thermostat wire and removing the previous add-a-wire adapters.

The thermostat wire that was original with the house runs up from the hallway into the attic going up through the top plate of the wall, then goes through the attic to the garage where the furnace is. It's plenty easy to drop new wire down the wall where the old wire is and fish it through the hole where the thermostat is. My question: is there anything special one should be doing with thermostat wire when it's behind a wall in terms of routing or protection? The previous wire was wrapped around nails hammered into one of the studs, and I'm not sure if that was to simply prevent the cable from being "lost" behind the drywall if it got disconnected from the thermostat or if it was done for a safety reason. I'm aware of the prohibitions around running low voltage wire and 120V AC through the same boxes or right next to each other, but there's no 120V AC anywhere nearby this so that's absolutely not a concern here.

Anything I should be doing to do this "right"?

  • Optional and sometimes much easier is to just buy a 24VAC plug in transformer and plug it in an existing outlet near the new WIFI Tstat and fish the wires neatly into the wall behind the base trim board into the Tstat – Kris Feb 24 at 14:48
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The Code rules are in Parts I and III of Article 725

Thermostat wiring is the most commonly encountered example of what the NEC terms a Class 2 control circuit, and the rules for those are set out in Parts I and III of NEC article 725. In your case, there are a few things to consider:

  • 725.25 requires removal of accessible portions of abandoned control circuit cabling
  • Your work still needs to be "neat and workmanlike" as per 725.24 -- slobbering cable everywhere is no good
  • Strapping Class 2 cables to raceways for support is NFG -- 725.143 prohibits it
  • You don't need to support Class 2 cable in a wall cavity, however -- this is implied by 725.24 not mentioning any support requirements for concealed Class 2 cables
  • When the city inspector came out to inspect the work (required where I live, even though it's low-voltage work), he also encouraged to fasten the wire to the joists in some way in the attic, just so that way it wasn't loose and wouldn't end up somewhere where somebody might trip over it. Whether that's them applying "neat and workmanlike" or just their own best practice, calling it out for anybody else who ever sees this answer down the road. – Jason Malinowski Mar 10 at 17:16
  • And thanks @ThreePhaseEel for the very detailed answer with specific code citations, since it was a great resource to find other information too. – Jason Malinowski Mar 10 at 17:17
  • @JasonMalinowski -- I'd say the inspector was applying "neat and workmanlike" (and not wrongly, either) – ThreePhaseEel Mar 10 at 18:26
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Wrapping the original low voltage thermostat cable around nails was a means of securing the cable. I'd say it's one of the worst ways I could think of to do so, but as long as the wires were not damaged by wrapping it around the nails, it was good enough. The only real reason to secure that thermostat cable inside the wall was to keep it out of the way when the drywall was installed.

It is perfectly acceptable to fish a new cable unsupported and unsecured inside the wall cavity. The rules for low voltage are very relaxed.

If this was a line voltage thermostat, there would be far more rules to follow running the wire, but it would still be ok to fish a new wire unsecured in a finished wall.

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