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I need to pull new wire from my furnace to my thermostat and was wondering if there would be any problems in using an Ethernet cable instead of standard thermostat wiring.

The reason being is I have left over cat6 cable and figured the wire should suffice in place of spending cash on a 4-wire thermostat cable.

fwiw: the run will be approximately 50'.

If the cat6 wiring will suffice, should I use both twisted wires as one? Or just a single wire from each twisted pair? i.e.,

Orange/OrangeWhite to 1 hvac connection or Orange to 1 hvac connection

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    @MetroSmurf - You haven't stated where the run will be going, but if you're replacing an existing in-wall cable, you should make sure the cable is rated for this (Usually termed "Plenum-rated"). Most Ethernet cables (which belong on desks or in computers) aren't rated for in-wall installation with the requisite fire resistance and such.
    – Kevin Vermeer
    Jan 13, 2012 at 21:03
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    @KevinVermeer - the Ethernet is plenum rated.
    – Metro Smurf
    Jan 13, 2012 at 21:52
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    CAT6 Cable is #24 AWG, HVAC cable is #18 AWG. Check the amperage of the HVAC control system before doing this!
    – Tester101
    Jan 13, 2012 at 22:54
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    Low voltage system. No line voltage.
    – Metro Smurf
    Jan 13, 2012 at 23:15
  • I'm no expert, but I'd agree with others that doing something non-standard is not a good idea. But one final note - in addition to the plenum vs non-plenum rating issue, Ethernet cable also comes in two-types: stranded and solid-core. Stranded is used for Ethernet cables from wall-to-PC. Solid-core is used for infrastructure: punchdown block to jack.
    – wadesworld
    Jan 14, 2012 at 18:08

7 Answers 7

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Ethernet cable should work fine since thermostats use low voltage and low current. For simplicity, it's probably easier to use each pair as if it were a single wire. That makes is easier to remember what is what at each end and clearer to anyone looking at it years later.

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    I've seen 230 V AC used for thermostats. Would CAT 6 do fine even for that?
    – AndrejaKo
    Jan 13, 2012 at 18:56
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    @AndrejaKo: You'd have to look up what the insulation rating for your wire is. Here in the US I think thermostats, like doorbells, have to use low voltage, although I'm not totally sure about the rules. I've never encountered a high voltage thermostat, but that doesn't mean they aren't out there.
    – Olin Lathrop
    Jan 13, 2012 at 19:34
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    Appreciate the confirmation. I'll give it a go and will mark the answer if everything works.
    – Metro Smurf
    Jan 13, 2012 at 19:46
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    @Curd - That's true, but you only get that advantage if you're using differential signaling. AFAIK, all the signals in a thermostat are single-ended, so you don't get any benefits from it.
    – Kevin Vermeer
    Jan 13, 2012 at 21:00
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    Out of curiosity, does anyone have a reference for the current draw by a typical furnace relay? Newer furnaces may have electronic controls, but those I've run into have control relays that the thermostat must hold closed when calling for heat, fan, or cooling.
    – TomG
    Jan 14, 2012 at 2:16
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I wouldn't do it. The next poor sod (who may be you) trying to fix something (or perhaps changing the system!) is expecting to find properly colored #18 AWG wire.

Instead they'll find #24 wire in all these weird (to them) colors. You're saving yourself some money now, and making someone else's life difficult down the line.

And of course, if you or someone else later change to a system that requires the #18 wire, then it's got to be ripped out and re-run, which is another pain in the neck for someone.

I advise against doing non-standard things that are going to cause someone extra work in the future!

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    I second standard wire because its heavier and it has the right colors.
    – BrianK
    Jan 11, 2015 at 2:32
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The big question is if your thermostat/furnace is line voltage or low voltage (24V usuaully). If it is line voltage, then no, this might be outright dangerous and a fire hazard (ethernet is not rated for this type of voltage).

If it is low voltage, there is no reason why this wouldn't electrically work, at least in the short term. I would however worry about if the cat6 can stand up to the higher temperatures near your furnace, and whether the cable could itself be a fire hazard.

Ethernet can be quite fragile compared to bell wire (for example); it is of higher gauge and the insulation is not as robust.

The high-speed properties of ethernet (twisted pair, pair ratios, wire arrangement, etc.) is of no importance here so there is no real reason to use more than a single conductor. If anything this might just confuse the next person working on it.

If it were my house, I would not use this. Thermostat wire is cheap, safe and proven. The savings by using leftover cable is easily exceeded if this setup fails (needs replacement), or worse yet, causes damage to your furnace, thermostat or house.

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    CAT6 = #24 AWG, HVAC cable is #18 AWG. Are you sure #24 can carry the load?
    – Tester101
    Jan 13, 2012 at 22:56
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    Your concerns are noted. The Ethernet has been wired up and working like a charm. There's not much heat to worry about since all the wiring is in an open attic and the furnace sits in the garage which is always cold. Also, I spliced into the existing t-stat line near the furnace and just tied into there so the Ethernet isn't even in the furnace.
    – Metro Smurf
    Jan 13, 2012 at 23:18
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    @Tester101 - the wiring is plenum rated. Does that matter?
    – Metro Smurf
    Jan 13, 2012 at 23:25
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    @Tester101, This table gives the ampacity of 24AWG as 0.577A in a bundle, compared to 2.3A for 18AWG, and 3.5A in free air. So the question is whether the low voltage loads (probably relay coils) switched by the thermostat are drawing as much as 577 mA if a single conductor is used, or about 1.1A if two are used in parallel. In any case, typical 24V relay coils are usually designed to draw well under than 100mA, so even a single conductor is likely to be safe, but if in doubt it would be easy to measure with a multi-meter.
    – RBerteig
    Jan 13, 2012 at 23:55
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    Just FYI, the power over ethernet spec (of course primarily pertaining to I.T. equipment but still applies here) is rated for 15.4 W of power, minimum 44VDC @ 350mA for that but the fact remains 24V shoudln't be a problem as long as it's kept under 640mA (I would keep it under 320 to be safe but that's just me).
    – MDMoore313
    Jan 15, 2013 at 17:10
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Here is the real deal. National Electrical Code. CL1 and CL2 rated cable is the only approved method.

Yes, almost every heating & cooling unit is electronically controlled, 24vac and extremely low current. Even the thermostats are battery powered and only signal heat/cool and fan. 5 conductors is the standard for heat/cool unless someone fingers the controller board around a bit. Keep in mind that thermostat could actually be switching the A/C condenser unit also. Those little contacters NEED full voltage and current to completely pull in and prevent contact failure.

Just my 2 cents.

Thanks

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    FWIW: 4 years later and no issues. Even with the T-Stat running Heat and A/C.
    – Metro Smurf
    Nov 12, 2015 at 14:35
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    No issues, but have you had it inspected? An inspector might have an issue with it.
    – ShemSeger
    Jun 12, 2019 at 3:47
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The 24V system in a HVAC typically has a fuse or breaker for either 3 amps or 5 amps. Sometimes it is a fuse on the control board and sometimes it is a breaker built into the transformer.

Typically, the current is about 0.5 amp - 1.5 amp in the 24V system when the AC or heat are running. The 24V control board usually shows the expected current flow.

At 1 amp in a 24V system with the thermostat 50 feet from the HVAC control board (100 feet total -- 50 feet of red or yellow plus 50 feet of white make the circuit ) there is a 1.5 volt drop through the resistance of 100 feet of 18 gauge wire. If you drop that wire down to 24 gauge, then there is 5.3 volt drop at 1 amp across the same distance.

That's in a normally operating system. If the system has a problem and runs 3 amps it might not trip the breaker or fuse, so there would be no indication of a problem. At 3 amps in that same 24V system, there would be 16 volts dropped due to resistance in that 24 gauge wire, but less than 4 volts dropped due to resistance in 18 gauge wire. Remember that the only thing limiting current in this system is either a 3 or 5 amp fuse or breaker, so any system problem causing current just below those limits might go on indefinitely without any indication.

You're taking a heck of a chance of a fire by using 24 gauge wire in this application. Don't do it.

enter image description here

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    Are you sure? HVAC thermostats are generally designed to drive a simple transformer, and 24VAC transformers generally take less than a few hundred mA to trip (the first one I saw used 40mA). 1A seems egregious; 3A seems like a fantasy.
    – Daniel Griscom
    May 16, 2016 at 17:36
  • I have checked plenty of HVAC units out, and typically there is 0.5A to 1A running through the 24V system when it is engaged.
    – user54015
    May 16, 2016 at 17:42
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    Just for fun, I measured the current through one of my thermostats in my house (gas modcon boiler). 100mA. I'm done here.
    – Daniel Griscom
    May 16, 2016 at 22:03
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    Thank you for the insight and discussion. FWIW: I didn't see anything in the comments here that felt condescending. Just an outsider's point of view who posted the original question. And we're now pushing 4+ years later with the Ethernet cabling controlling the T-Stat/HVAC system without any issues (AFAIK!). Mark, I hope you continue to post here as you obviously have some passion about these types of issues. Cheers!
    – Metro Smurf
    May 16, 2016 at 23:18
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    Please keep comments respectful, and if there is a need for an extended conversation consider moving to a chat. Thank you. [multiple comments purged]
    – BMitch ♦
    May 17, 2016 at 1:55
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My 24 VAC transformer is rated at 40 VA. So that would translate to a rating of 1.67 Amps on the 24 VAC side.

Cat-6 uses 24-gauge wire which according to this site:

https://www.powerstream.com/Wire_Size.htm

rates 24-gauge at 3.5 amps for chassis wiring and 0.577 amps for power transmission.

So a short run of CAT-6 might be OK, but I'd tend to think of wall wiring as more in the power transmission category and go with 18 gauge.

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  • Yeah, the voltage drop will kill you anyway...
    – ThreePhaseEel
    Mar 30, 2018 at 18:23
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Household stats are 24v. 450ma max because the transformer can only output that much, so difference in stats won't matter.

POE is upwards of 50 volts and each strand of cat5 is rated up to 60v and 630ma. That's at least 10% wiggle in amperage and obviously more on the voltage.

The wire is capable. If it's plenum, then it's fire rating is also sufficient.

But it should only be solid core (riser cable), not stranded.

At the same time, 50' of stat wire is like $20...

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  • No, you'll run into way too much voltage-drop with Ethernet cable (it's 22AWG)
    – ThreePhaseEel
    Oct 7, 2018 at 1:53

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