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TL;DR: Can you terminate more than one common on the furnace C terminal?

Background:

I've linked a diagram of how my non-programmable thermostat is currently hooked up. I want to replace it with a Trane Z-Wave thermostat that requires constant power. There's already a blue wire in the 18/5 bundle, but it's disconnected on both ends. The C terminal on the furnace control board is already in use by the rooftop condenser. If possible, I'd like to connect the blue wire to the stat and terminate it on the same C terminal on the furnace that is already in use by the A/C.

Diagram

Some model numbers:

  • Furnace: Goodman GKS9 Gas Furnace with A/C (CAPF) - manual(PDF)
  • Old stat: Honeywell TH3210D1004 non-programmable
  • New stat: Trane TZEMT400BB3 Z-Wave
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2 Answers 2

up vote 3 down vote accepted

Yes you can terminate any number of connections on the C terminal. All of the connections will be in parallel. This is very common.

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1  
You are correct, however, you may not be able to terminate more than one wire under a single screw terminal. The terminal has to be listed and labeled to accept multiple terminations, otherwise you'll have to use a pigtail. –  Tester101 Mar 2 '13 at 14:28
    
@Tester101 I think you are referring to NEC, correct? Does this still apply to 24VAC circuits like what are found in your furnace? –  Steven Mar 2 '13 at 17:45
    
I'm not sure what codes HVAC techs follow, but I'm sure if they don't reference NEC directly they follow similar practices. I think NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems says something about wiring according to NFPA 70: National Electrical Code, And I would think NFPA 90B: Standard for the Installation of Warm Air Heating and Air-Conditioning Systems would say something similar. –  Tester101 Mar 2 '13 at 20:40
    
Technically speaking, if the transformer in your furnace is too small (eg: JUST big enough to power the control board, relays, and whatever else is already connected) then it's possible that the thermostat could push it over the edge, causing your furnace to not work and/or behave strangely. However, the amount of power we're talking about here is pretty small. Unless you're connecting a thermostat with a monster color touchscreen to a >=20-year-old furnace/air handler, chances are you won't have an issue. –  gregmac Nov 13 at 16:56

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The Common leg of the 24 volt power supply is where all the 24 volt circuits terminate to complete their circuit.

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