I may (or may not) be pulling new wires to support current-generation thermostats.... Upgrading the thermostats themselves seems a good time to upgrade the wire too, though I think I could get by on what's already there (which looks like old telco four-conductor, or something similar).

But since I'm not sure I trust the local home center to give me the right cables. I've seen anything from three conductors (four with AC. five if forced-air rather than hydronic so the fan can be switched separately) to more-than-I-wanted-to-count. I suspect the latter is serious overkill for residential...

So: What's standard these days for a gas/radiators system which may have minisplit AC added someday? Or does the minisplit have its own thermostats?

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    When selecting thermostat cable here are my top 3 reco's: 1. Extra wires. 2. Additional conductors. 3. More colors of wire. Seriously they make 8-wire thermostat cable, just go with that. – Harper - Reinstate Monica Dec 9 '16 at 15:30
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    I agree more is better and allows for future updates or repair. Make sure to have a small service loop at each end (service loop= extra wire at each end). + – Ed Beal Dec 9 '16 at 16:23
  • Good thoughts folks; thanks. Replacing the current upstairs thermostat may actually be easier if I just do a completely new run and leave the old one in place. – keshlam Dec 9 '16 at 21:07

24VAC HVAC control circuits are a type of class 2 low voltage control circuit, and can use any cable suitable for that application. This means that any UL listed type CL2 (optionally CL2R/CL2P) cable can be used, provided the wires inside are correct -- in practice, 18AWG is adequate.

As to how many wires to have in your cable, the correct answer is as many as possible -- big-box stores may only carry 5 or perhaps 8 wire thermostat cable, but you can get CL2 cables with more 18AWGs in them if you're willing to reach out beyond the big box. Talk to your electrical supply house, or check electrical/electronic catalog suppliers, as both Carol/General Wire and Alpha Wire make CL2 cables with more wires still -- in fact, the Alpha Wire 881812 (CL2, 12wires/18AWG stranded) is available through Newark as a stock item (in 100' spools).

  • Nothing exceeds like excess, hm? ... I have to admit, I do lean toward running additional conduits and conductors on principle; they will eventually be used. My only concern is how hard it'll be to snake through, and I'm already considering alternatives for that. – keshlam Dec 10 '16 at 4:55
  • Yep -- if you have the dough, you might as well put the extra wires in the wall now. Haven't heard of any configuration that needs more than 8, but if you can get the 12 wire cable, I'd go for it for the sake of whatever's next in HVAC controls :) – ThreePhaseEel Dec 10 '16 at 5:04
  • This is on the edge of being a new question: the home center gave me a choice between 7-conductor thermostat wire, or 7-conductor irrigation wire. I couldn't see any difference in the labelling, gauge, or color coding, and the irrigation wire is cheaper. Will anyone be upset if the jacket is black rather than brown? Is there likely to be a difference in fire cert, and what should I look for on the package to check that? Or should I just play safe and pay the difference? – keshlam Dec 22 '16 at 2:18
  • @keshlam -- they're both listed as CL2 or CL3, correct? Or what are their listings? – ThreePhaseEel Dec 22 '16 at 12:46
  • "UL AWG 18 CU 7 CDRS 30 VOLTS SPRINKLER SYSTEMS WIRE ... HC/JC". Also says RoHS compliant. Manufacturer website doesn't seem to say much more but says insulation is PVC with "overall poly jacket". – keshlam Dec 23 '16 at 3:08

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