I need to install a GFCI receptacle in my unfinished basement for the new power-vented hot water heater that I had installed. The plumber said I could use the existing furnace circuit. I mapped out the circuit but am unsure as to how I should wire this up. Can someone take a look at the attached drawing and tell me how the new receptacle should be wired? Ideally, the switch would in the middle of one of the two 12/2 runs at the top of the diagram, because this is what I have easiest access to. (It would be much more difficult for me to mess around with the junction box because it is in a cramped location. Thanks.

Note that the switch at the top right is the "Emergency furnace shut off" switch in the kitchen, and the switch on the bottom is the one that is attached to the side of the furnace.

Furnace circuit wiring diagram


1 Answer 1


You cannot connect there. Those wires are connected to the black and red wires of the feed from the breakers. This is probably a MWBC and these two wires have 240v across them!

You need to connect to the white wire from the breaker panel, as well as either the black or red. This only goes into the junction box and then out to the furnace. Suggest that you add another run from the junction box.

  • Okay, thanks, that makes sense. Would you mind giving me a little more detail. I have to get this completed today for the plumbing inspection tomorrow morning, so I don't have a lot of time to figure it out on my own. Should I basically add another knockout to the j-box, run 12/2 from there to the GFCI? And the connection would be black from the GFCI to black (or red) from the 12/3 (that runs to the panel) and white from the GFCI to the white from the 12/3? (Basically adding a third wire to two of the existing wire nut connections.) Thanks.
    – jevron1984
    Jul 24, 2016 at 18:21
  • @johnnyb1970 That looks correct. You may need to replace the wire nuts with larger ones to fit the extra wire. Also, no ones talked about the ground. Do the same thing for the green/bare wires.
    – DoxyLover
    Jul 25, 2016 at 0:09

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