thermostat and doorbell wiring


Old 04-18-07, 10:06 PM
ddr is offline
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thermostat and doorbell wiring

Iím looking at changing some thermostat and door bell wiring and wanted to get some information/advice. Iíll give the thermostat and doorbell info and then ask the questions.

The thermostat is a simple heat-only Honeywell. The house (built in Ď52) was wired with standard 14/2 BX running between the thermostat and furnace; I donít know if the original setup used line voltage, but the current furnace has a 24V system. There are 2 18 AWG wires running through conduit from the furnace to a junction box (along with the 120V lines) where the #18 is spliced to the old BX, which runs from the basement, through the first floor wall, into a crawl space and on to the thermostat. Iím looking to change out the BX as the sheathing is dried out and crumbling and Iím doing work in the area anyway.

The doorbell system still has the original wiring which is a twisted pair of 18 AWG wires running through walls and crawl spaces to a 10V transformer attached to the breaker panel. Iíve noticed some nicks in the wiresí sheathing and the doorbell buttons have been changed enough over 55 years that thereís no slack left, so I figured I might as well change that wire too.

I saw some 18/2 CL2 sheathed thermostat/bell wire and was thinking of using that for both jobs.

So, the questions:

1. What does the CL2 stand for?

2. Since the the BX is being pulled out, is it okay to fish the 18/2 through the existing holes and leave it loose in the wall?

3. When the new wire enters the junction box through the 1/2Ē knockout, can a just put a protective bushing in the knockout and leave the wire loose or should I try to clamp it with an NM fitting?

4. It might - emphasis on ďmightĒ - help me with fishing the wire to the thermostat to pull the old 14 AWG wire out of its cable jacket and use the old jacket as flex conduit for the new wire. Is this legal?

5. The doorbell wire is run through small holes in some areas and I donít think the new sheathed wire will fit. Since there are some areas where the wires run through studs, I wonít be able to widen those holes. Would it be legal to remove the two wires from the outer sheathing and run them as individual wires where needed?

6. If I do the above, would I have to twist the wires (if they arenít already)?

7. I havenít seen any unsheathed thermostat or bell wire thicker than 20 AWG in the big box stores. Should I just go with the sheathed 18/2 CL2 for everything or just the thermostat and get unsheathed twisted pair #18 from a supply place for the doorbell?

Sorry for so many questions, just wanted to get it all sorted out. Thanks in advance for any help.

Last edited by ddr; 04-18-07 at 10:11 PM. Reason: corrected mistake
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Old 04-19-07, 12:45 AM
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1. CL2 is not an NEC descriptor for insulation so I assume it means the wire is suitable only for Class 2 use. Class 2 is a special category of circuits using low voltages and limited currents such as thermostats and doorbells.

2. Yes.

3. No. The thermostat wiring may NOT be in the same conduit as the Class1 power wiring. Your present arrangement with the #18 running in the conduit with the power wiring is not code-compliant. Obviously the original thermostat and control was line-voltage and when it was changed the thermostat wiring should have been run outside of the power conduit. The thermostat wire may be strapped to the outside of the power conduit. This is one of the few times that strapping a wire to a conduit is acceptable.

4. No. The BX is a cable assembly and you may not remove the factory installed wires and substitute your own. However, if what you have is not BX but instead flexible metallic conduit (which I seriously doubt) then it would be acceptable to remove and replace the wires.

5. Probably not since the cable is "listed" (by UL) with the jacket and not as unjacketed wire.

6. See answer #5

7. Either way is acceptable.

Don't apologize for asking questions, better to ask than to be unsafe.
Old 04-19-07, 12:38 PM
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Thanks for the info furd.

Okay, so when the furnace was put in some 10 years ago, they wanted to save a few feet of conduit and an extra 15 minutes or so and ran the low voltage wire in the same conduit with the 120.

If I were to try and fix this, can the 120V and 24V wires share junction boxes but be run in separate conduit or do they need to be separate all the way? (I can keep them separate from the thermostat end, but the guy mounted a junction box to the side of the furnace and all wires go through the back of the box and into the furnace and I'd rather not mess around with drilling holes and mounting another box.)

What is the reasoning behind this rule and would it be dangerous to leave it as is? The whole furnace system only pulls 1 amp when running.

If I ran 14 AWG all the way to the J-box on the furnace would it be okay to use the same conduit then, or is it a low voltage issue and not a wire gauge issue?

Thanks again.
Old 04-19-07, 12:43 PM
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You do not need a junction box on the furnace or conduit for the low voltage wires. They cannot be in the same conduit, they cannot be in a junction box together.
Old 04-19-07, 03:04 PM
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"What is the reasoning behind this rule and would it be dangerous to leave it as is? The whole furnace system only pulls 1 amp when running."

The ammount of current that the furnace uses when running has nothing to do with the amount of current available if the wires somehow were shorted together.

If the line voltage wires shorted to the low voltage wires, then a bell wire would be run arround your house carrying 120volts. The t-stat would probably give out, and if it did not go up in flames immediatly, it could short partly closed causing enough amps to fllow to start a fire but not enough to trip the breaker. (meaning amps on the bell wire.)

As said already, you can tie wrap a tx wire to a conduit as long as the conduit you are tieing to is for the same system. So the transformer wire can be tied up to the conduit feeding the hvac system.
Old 04-20-07, 07:05 AM
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One thing I've seen done before, and it seems like a nice easy solution is to drill a 7/8" hole and mount a NM cable clamp (the same you would use to clamp a 14/2 or whatever cable) in the hole. Then run the thermostat wire out that hole.

The cable clamp will protect the wire from being nicked against the sharp sheet metal, and the clamp will also keep it from being accidentally pulled. Plus, it's real easy.

(but I may be wrong with the 7/8" measurement of the cable clamp... just check that before you drill)
Old 04-20-07, 09:40 AM
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jwhite/racraft : Thanks for the replies/info. In hindsight the reasoning for the code is obvious; I should have realized it myself.

Zorfdt: Thanks for the advice. Upon closer inspection of the furnace I found a small (maybe 1/4Ē - 3/8Ē) hole with a plastic grommet near the area the guy mounted the junction box. This is most likely where the chucklehead was supposed to have run the thermostat wires through in the first place.
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