I am attempting to wire a new double pole four wire 240 volt thermostat to replace the single pole thermostat that is pictured. Coming into the junction box on the right are two Komax lines. On the left side of the box one line is going out. You can see two black wires connected (lower red wire nut) to the black wire of the old thermostat. Also on the right side are several white wires (neutrals) connected by a red wire nut.

The red wire coming off of the thermostat is connected to the single black wire.

Attached is also a wiring diagram. I guess my main question is how do I determine which of the wires is coming in from the panel and which is going to the heater? Any suggestions or thoughts are greatly appreciated..... enter image description here circuit box
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  • Are there other heaters and/or other thermostats on this circuit? Mar 16, 2021 at 23:59

3 Answers 3


You have EITHER... supply power being split here to power other thermostats... OR... this thermostat controlling two heaters. Don't know, don't care if the thermostat doesn't.

Let's convert this thing to 4 wires.

A "Cable" is several "wires" inside a sheath, which you seem to grasp. We're going to label the cables A1, A2 and B (not really, we can't reach them, but pretend). Turn the power off.

If you look at the wire-nut with 3 black wires, it includes these 3:

  • Black "A" pigtail to the thermostat
  • cable A1 black
  • cable A2 black

So, I want you to follow A1 and A2 blacks back to their cables. Those will be black A1 and A2. Follow those back to their respective cables, so you can identify White A1 and White A2.

Get a new white pigtail wire, and wire these exactly like the blacks. Add pigtail White A, which splices to White A1 and White A2 with a red or tan wire nut.

The remaining black and white from the wall are Black B and White B.

Now, if you ignore the pigtails, you have 4 wires: White A, Black A, White B and Black B.

Now you can hook up the thermostat as instructed.

By the way, wire-nut-wise, those Scotchlok wire nuts are garbage. 3M is a great company but these were a miss... and they're not reusable, and the products on the market today are vastly superior. If you're not touching them, you can leave them alone. But if you have to disturb one, throw it in the trash and replace it with an Ideal brand nut.


Double pole heaters do not have a neutral.

The white wire would be a hot also. Normally the red wire goes to the load so the black connecting to the black that would be the hot side.

The white that is in the same cable as the black /black connection would be the other hot.

I can’t tell if it matters on the drawing for load and line most do want the line on a specific side like the original stat did.


Generally you use a non contact volt detector - they are less than $20.

Turn off the breakers, use non contact to make sure no power, remove wire nuts.

Separate wires to each side put nuts on each single wire, turn breaker back on, use non contact volt detector to determine which wires are live.

I like this one - AAA batteries, small like a pen.

  • 2
    Before disconnecting wiring: Be sure to label each wire so you can put it all back together exactly as you took it apart! Sure, the point is to install the new switch, but this site is littered with "I took everything apart and now I don't know how to put it all back together" questions - be sure not to add a new one!
    – FreeMan
    Mar 17, 2021 at 11:20

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