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When roughing in thermostat wire, how many pair wire would you use to cover most thermostats?

I'm asking because keep seeing questions about C wire and wifi.

  • WiFi is actually quite the opposite - cuts down on the need for wires. But typical is a WiFi thermostat in the location of a regular thermostat - same number of wires as "normal". Also, unlike phone/ethernet/etc., thermostat wire is a bunch of separate wires, not actually pairs. – manassehkatz May 19 at 19:35
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    What is normal,? No less than 5. – Robert Moody May 19 at 23:08
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    @manassehkatz Same number plus one typically, for C. – Harper May 20 at 0:54
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For traditional Heat + Fan + AC systems, you need 4 for the system to work at all, and 5 if you want to support the C wire for smart 'stats.

However when you get into heat pump and multistage systems, just throw an 8 in there - it's readily available and not that much more expensive. You're not going to get very many latté's with the cost savings from using 4-wire.

  • I personally would run an 18/10 or 18/12 depending what was available to me, if I didn't already have some ENT roaming around the job that is ;) – ThreePhaseEel May 20 at 3:31
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If you're building a new building and having new HVAC installed, I would simply put 1/2" conduit in the wall and be done. That way, you let the installer run the wire they need. I don't know that I would do this without at least consulting an HVAC installer to see where they would put the unit and the air return (typically the thermostat is as near the return as possible).

If you're going to just pre-run wire and dictate where the return will be and the HVAC unit will sit, I would run 18/5 wire if you know you will have a AC + gas heat, and 18/8 if you know you will have a heat pump (if you're in doubt, more will not hurt). Your most basic setup on most modern units needs 4 control wires plus the fifth blue "C" wire that provides power to the thermostat. It won't cover anything elaborate like a heat pump, but your greatest risk would be the installer uses your wire to pull the wire they actually need.

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    If running a cable instead of putting in conduit, I would err on the side of "extra". We're talking about literally just a few dollars for a typical run between 18/5 and 18/8. If the installer ends up needing more than a basic 5 wires, it will cost a lot more to fix it later. – manassehkatz May 19 at 19:34
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    Is extra bad? 5 seems to be ok 8 seems to cover the unknown. – Robert Moody May 19 at 20:08
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    @RobertMoody Not at all. Costs a bit more is all – Machavity May 19 at 21:46
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I'd use 20/8 which is pretty standard today. That will cover anything up to a 2-stage heat pump system. I'm not sure how WiFi factors in here. An smart/electronic thermostat like a Nest does support WiFi but not between the HVAC unit and the thermostat.

  • When using 20/8 does it leave extra wires in case one gets nicked? – Robert Moody May 19 at 19:28
  • It's not common to run extra wires in anticipation of damage. If you expect it, run conduit. – isherwood May 19 at 19:30
  • Not so much anticipation of damage.But for the future. – Robert Moody May 19 at 19:38
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    "FOR THE FUTURE", RUN CONDUIT NOW. Conduit is the most future-proof technology there is for wiring. Pull out the obsolete, pull in the up to date, done. – Ecnerwal May 19 at 21:07

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