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thermostat wire

I can't find it in the code. But I have read it many times in the Carrier books minimum 18 gauge wire for thermostat and no splices for gas.


  • Boston_2
    Boston_2 Member Posts: 107
    22 guage thermostat wire

    I just installed a new VisionPro 8000 thermostat on a my boiler (24VAC). Is there any reason why It would be a problem to use 22 guage stranded wire for a thermostat wire? Could it cause a priblem?
  • Minimum wire

    size on heating and cooling applications is 18 gauge according to Natinal Electrical Code.

    22 Gauge has very high resistance per foot compared to 18 gauge.
  • GW
    GW Member Posts: 4,701
    did you go through the 10 minutes of programming?

    Are you sure you're wired up right?

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
  • GW
    GW Member Posts: 4,701
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
  • Boston_2
    Boston_2 Member Posts: 107
    the thermostat seems to work fine

    But I want to make sure that I don’t cause a fire or something. I will change it out tomorrow with 18 gauge, if there could be a problem.

    I had a RiteTemp thermostat(home depot brand) and the boiler would over shoot the target temp every time by several degrees. I was told by people on this site that most professionals use the 8000 for steam boilers.
    It seems to work really well now that I have it programmed correctly.
  • I love...

    when I see telephone wire used for t'stat wire. I've seen it a LOT. Amazing how some stuff still works no matter what you do to it. ;)
  • realolman
    realolman Member Posts: 513
    could you

    please cite a reference for the minimum size being 18... NEC Article #?

    I've been trying to find it. thanks
  • RealCasinoGuide
    RealCasinoGuide Member Posts: 15

    #18 AWG = 1.3ohms/100'

    #22 AWG = 3.3ohms/100'
  • Techman
    Techman Member Posts: 2,144

    Since real telephone wire comes in "pairs",what would be the result of useing both green wirers under the G screw and so forth. Two 22 ga.wirers is better than one. I also see telephone wire used a lot,and a lot of the time it is a real pain in the dupa to restring a new wire.
  • The 22 ga...

    will not necessarily cause any fire or safety issues but the added resistance to the circuit causes it to draw more amperage and if the run is long enough will cause problems with the functioning of the circuit. In other words in some cases it will suck up enough juice so that the relays or contactors it controls may overheat and fail prematurely, act intermittently (not work all the time) or not work at all. Generally if you run over 75 ft even with 18 ga this can occur and it is advised that any t'stat wire run over 75ft be done with 16 ga to avoid these problems. Don't know the length limit on 22 ga as I never use it but I'm sure it's way less than 75 ft. Given the resistance figures posted here my guess is 30 ft or so before issues arise. I do believe I have see 18 ga specified at LEAST in the installers manuals of most equipment although I can't say I've seen it in an electrical code book as I am not an electrician and don't own one.
  • yes it is, I don't normally replace it myself,

    unless the customer is willing to cover the expense of doing so or it is causing a problem. Rarely or never have I seen it doubled when I find it but that is a bandaid approach to resolving any issues it may be causing, provided enough wires are available to do so.If I'm not mistaken telephone wire is an even smaller diameter than 22 ga, it's like strands of hair.
  • Boston_2
    Boston_2 Member Posts: 107
    It's not telephone wire....

    It looks to be more like alarm wire or something else. All the telephone wire I have seen is sold core not stranded.

    The run is less then 20 feet and unfortunately I need to use three out of the four wires so I can’t double them up. (R) Power, (W) switched relay heat & (C.) common.
  • jp_2
    jp_2 Member Posts: 1,935
    got it backwards

    mpf, as resistance increases, current decreases. this would cause low voltage at the tstat. however, as rowland points out, the resistance between the 18-22 is very minor.
  • yes, you're absolutely right.

    oops... "draw" more current is not what I meant, I guess "use" or "use up" might have been more correct leading to less current/voltage available for the proper operation of the compnent it feeds. Nice snag! ;)

    When I said "suck up juice" I meant whatever is left at the other end might not be enough to do the job. Still, IMHO 18 ga is always the better way to go code or no code.
  • No,

    I didn't mean YOURS was telephone wire just that I've seen it many times in the past and it is even thinner than 22 ga but peiople use it. Chances are you're ok unless you find the contactors or relays are buzzing, chattering, won't pull in at all or only pull in sometimes. As far as whether it's up to code or not I'm afraid I can't answer that without consulting the book. I'm talking strictly about operation not legalities. I always use 18 ga so I have no code worries. Tim is most likely correct if he's saying it's code.
  • Rollie Peck_3
    Rollie Peck_3 Member Posts: 24
    Thermostat wire

    Under certain conditions, number 22 wire could be a hazard.

    If one side of the transformer is grounded and the hot side
    goes to the thermostat and the wire gets shorted to ground,
    the wire could overheat. If it passed through or under a
    combustible material, that material might catch fire.

    A class two transformer is designed in a special way to
    limit its output current to a value that will not overheat
    number 18 wire. Using smaller wire reduces or eliminates
    this safety feature. Using a non class two transformer does
    the same thing for even number 18 wire.

    Another trick is to always wire the hot side of the
    transformer to the relay or zone valve. Then if the wire to
    the thermostat gets shorted to ground, the relay or zone
    valve stays on all the time, but nothing overheats.

    Rollie Peck

This discussion has been closed.