I have a Goodman Janitrol furnace and I am needing to add a C (common) wire, for the newer thermostats. I would like to avoid the substituting G for C, and just run an additional wire.

I have read several post that suggest to use one of the Blue wires coming from the furnace transformer. I have attached pictures for reference. In picture 1, you can see that I have two blue wires and one green wire (which I'm assuming is still part of the blue family), one red wire, one white wire, and one black wire. One of the blue wires runs from the transformer to the switch (which says ON), the other blue wire goes to the board, and the green wire goes to a ground bolt.

Which of these wires would it be best to connect the C wire to? If I connect it, is there any potential electrical issues that could occur? Lastly, if I connect a C wire to one of the blue wires (or green), can I then just run that wire straight to the thermostat and plug it into the C-labeled connection?

Picture 1 enter image description here

This is where one of the blue wires goes. All the wires run from point A to point B, and they have not been split. enter image description here

Any help would be much appreciated, if you have done this successfully please let me know how you did it.

Would I be able to connect the Common wire from the ground circled in the picture below?enter image description here

Thank you

  • could you confirm what you ended up doing?
    – Insearch
    Commented Nov 29, 2018 at 13:51

1 Answer 1


You are looking in the right place

The cluster of blue and green wires at the transformer is the correct place to tap off of using a piggyback tab (quick disconnect) terminal (Amazon link provided as an example only, they are fairly widely available through electronics parts suppliers). Make sure to crimp it to the new wire using an appropriate crimp tool though (pliers will produce bad results).

  • Thanks for the information. Let me run through this so I make sure I got this correct. So I will take what will be my C-wire running from my thermostat and install the piggyback (quick disconnect) tab to the end of the wire located in the furnace. Once I got that crimped on, will that fit next to the existing wire in the slot on the transformer?
    – Willy D
    Commented Jun 12, 2017 at 14:29
  • Would I be able to connect to the spot circled on the last picture added to my original post? It is the ground wire coming off of the transformer. Is it possible to crimp a round connection on the end of the C-wire, loosen the bolt, slip it on with the green wire, and tighten it back up then run that wire to my thermostat and connect it in the C connection port?
    – Willy D
    Commented Jun 12, 2017 at 20:34
  • I'd rather use the piggyback terminal -- it will go on in place one of the existing wires on the slot on the transformer, and then the existing wire will plug into it. That way, it all comes apart a fair bit more neatly. Commented Jun 12, 2017 at 22:09

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