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Just bought my first home and this is my first time posting, so please forgive me if my terminology is incorrect.

I currently have bedrooms upstairs each with 1 baseboard heater and thermostat for each. The circuit breaker is off and I've undone most of the wiring to it.

The thermostat is an analog, single pole line voltage.

I'd like to upgrade to a digital double pole.

It looks like there is 3 wires (really groups of wires, I don't know the term for this), possibly 10-3 or 12-3 coming into the junction box. I didn't specifically look that the gauge. It might actually be 10-2 or 12-2.

Do I simply need to connect 2 hots and 2 neutrals to the thermostat and I am set?

  • Can you provide photos of the insides of the boxes? Aug 6, 2016 at 20:21
  • The images I have are too big.
    – user_ABCD
    Aug 6, 2016 at 20:34
  • Can you get us another photo with the black bundle out of the way? Aug 6, 2016 at 21:13
  • Hopefully that picture is a little better, each yellow bundle has three wires. A bare ground, one white neutral, and one black hot. There are three yellow bundles coming to the junction box. The neutrals are all tied together with the wire nut, one hot leg from two separate yellow bundles go to the thermostat. And lastly one ground ties to the thermostat itself.
    – user_ABCD
    Aug 6, 2016 at 23:59
  • Is the breaker a single handle or double handle? From what we can see here it should be single handle or single pole. Depending on how your double pole new thermostats get power for their own electronics you may or may not be able to use them here as single pole devices.
    – Tyson
    Aug 7, 2016 at 0:31

3 Answers 3


OK, quick terminology issue: Single-pole and double-pole. The poles are channels, which could have any purpose. A single-pole switches one channel; a double-pole switches two. (ignore the "st"). (source)

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For a thermostat, one pole is sufficient to turn the heaters on and off. For the other pole, you'd simply bind the wires together - and I think that's what's been done with the white wires.

You say this powers 2 heaters, and that's the dead giveaway. The power supply would be one group of wires, and the outputs would be two groups. Now look at what's going on with that switch: you have one wire spliced into the red thermostat wire (that must be the power supply) and two wires spliced into the black wires (those must be the heaters).

Follow the one wire and it goes to the Romex on the right. That Romex goes to the power source, clearly, and its wires should be considered "LINE" (always-on). Which means the white wire in that bundle is the other pole.

The other Romex cables go to the heaters, and they are "LOAD" (switched).

This wire is /2 Romex since there's no red wire. (ground is not counted, so /2 means black and white). The yellow sheath suggests 12/ since some manufacturers recently adopted that as a color code. The markings on the sheath say for sure.

Are the white wires hot (240V) or neutral (120V)? We can't tell. It would be wired the same either way. 240V heaters don't need neutral, so they use 12/2 or 10/2 wire, and re-designate and supposedly, re-mark the white as another "hot". Somebody went to a lot of trouble to put red tape on the Romex cables... shrug. In the old days, marking wasn't required if the use was obvious.

So we must go down to the breaker panel. Looking at the layout, it should be obvious that there's a unit of "space". If the breaker takes 2 spaces, it is a 240V circuit. There won't be many of those.

Simply, turn off one at a time and see what it knocks out. Generally there is one thing on each 240V circuit (well, oven and stove may share a circuit). This is a good time to mark those breakers once you figure what they control. Not least it helps you eliminate; heaters are very annoying to test because they take a long time to make noticeable heat.

If it's a 240V breaker, obviously, these are 240V heaters.

Although the smart thermostat may not care if it's 120V or 240V. It needs to power itself, but it may be inherently multi-voltage. Many things are nowadays.

It goes without saying that you have to find the breaker in order to change the thermostat. If you don't realize that, you should not be doing electrical work.

  • The OP actually says there's one thermostat per heater - I think it's likely these are 120V heaters and the second wire on the thermostat is for other outlets. Dec 21, 2018 at 15:06
  • I unintentionally downvoted but will remove that if I can, I don't think I can unless this post is edited. Dec 21, 2018 at 15:11
  • @batsplatsterson You will be able to if the post is edited. Hold on... Yeah my ipad does that all the time... Dec 21, 2018 at 18:15

If I am reading this question correctly, there is just one heater per thermostat.

There are three cables in the box, the yellow jacket tells me 99% certain they are 12/2.

I think it's most likely that one cable is the feed / source, one is the heater, and one continues to other outlets - lights or receptacles.

So I'd figure these are 120V heaters; two blacks spliced to the black lead on the thermostat are the source and other outlets, and the black spliced to the red lead is the heater.

This is easy enough to confirm, if the breaker for the circuit supplying the heaters is a single pole breaker, it's a 120V circuit. If not, stop now, disregard the rest of this answer.

One remark: I would not use a two pole thermostat with a 120V heater. A two pole thermostat may make sense with a 240V heater, but I don't like the idea of switching the neutral on a 120V heater.

If however these are 120V heaters but you still want a two pole thermostat, to install a two pole thermostat, basically you just have to separate the white wires just as the black wires are separated. This is what I'd do:

  • Before taking anything apart, label the cable sheaths with a sharpie, A B and C, take a picture, and make a sketch what went where before you started. Let's say number the two on the left side of the box in the picture which are spliced to the black lead A and B, and the cable with the black wire spliced to the red lead C. This is super important if anything goes wrong.

  • Just because I have seen a lot of junky yellow wire nuts out there, buy some top quality wire nuts rated for three #12 solid wires. I like the tan Ideal Twisters. If you're not familiar with wire nut splices, do a little homework and practice on some scrap. Heaters are heavy loads and good splices are critical.

  • Verify which side is the source. Remove the thermostat leads from the splices and replace the wire nuts. Turn the breaker on temporarily, test with a non contact voltage tester, then turn the breaker back off. With the thermostat removed and breaker on the A and B black wires should be hot, the C black wire should be dead, and the white wires and grounds should be dead. If this is not the case, stop, put everything back as it was, and consider calling an electrician. If it is as expected, proceed - but don't forget to turn the breaker off first.

  • Review the instructions and labeling of the double pole thermostat. Identify the Line leads which go to the source, and the load leads which go to the heater - probably one black and one red line, one black and one red load.

  • Splice the A and B blacks to the line voltage black, the A and B whites to the line voltage red; splice the C black to the load black, the C white to the load red.

  • Close up the box and see if everything is working as expected, heaters, receptacles, lights, etc.


OP bought the wrong thermostat. You cannot substitute a double pole thermostat for an original single pole thermostat.The number of "poles" on the thermostat must match those of the breaker. One pole or space it takes up in the breaker panel is 120 volt, 2 poles is either 208 or 240 volt in the context of a wall heater. (There are, as an aside 30, 40 50 plus double pole breakers for other uses). So tell us how many poles and the amp rating of the heater breaker and go but the appropriate thermostat. From there we can help you with the wiring.

The wiring in the picture does not look like a pro did it. 1. Not enough twists in the wire. 2. Very long Romex sheathing extending into the box, which is a no-no. Both are potential fire hazards and need to be addressed, IMHO.

  • 1
    Can you explain why the 2 issues you've identified are issues? I've seen professional electricians not twist the wires at all except that which comes from twisting the wire nut on. And can you explain why having that much sheathing in the box is bad? Is it a code violation? Is it just bad workmanship? Telling a rookie that something is wrong doesn't help them much without explaining why it's wrong and how to fix it.
    – FreeMan
    Dec 24, 2020 at 23:40
  • A DP bimetal (electromechanic) thermostat is perfectly happy on a 120V system, simply connect the poles in series...(likewise, you can use a SP thermostat on 240V, just by having the other hot leg spliced through unswitched) Dec 25, 2020 at 2:34
  • I was called out for both during electrical inspections. I cannot cite chapter and verse of the code. The wires need to be twisted to ensure contact and that they won't easily come undone. I think that some inspectors look for the number of twists which may be an indicator that the job was done right. I was told once that there should be no more than a 1/4-inch sheathing extending into the box past the clamp. Just trying to share what I know to be helpful, not be a critic. It appears I accidentally commented on my own post and so this explanation got lost in what appeared to be a double post.
    – DAS
    Dec 27, 2020 at 4:13

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