Trying to install a new thermostat to electric baseboard heating. Heating is 240 V, new thermostat is Honeywell RLV3150 Current dial thermostat is hooked up, and the junction box has 3 wires going to it, so I'm stuck with 3 whites, 3 blacks, and 3 grounds. New thermostat wiring diagram for 4wire install shows all whites joined with nut, one thermo wire to power, and other thermo wire to load. Confused as to why the jbox has 3 wires, and how to proceed. Haven't done anything yet, but would like advice before wiring something wrong and causing an electrical fire. Thank you! current jbox wiring

  • 1
    How many baseboard heaters are controlled by this thermostat? Commented Oct 9, 2022 at 23:20
  • is that the old or new thermostat you are showing
    – Traveler
    Commented Oct 9, 2022 at 23:56
  • This is the old stat pictured. Only 1 heater controlled by this stat Commented Oct 10, 2022 at 0:00

1 Answer 1


Confused as to why the jbox has 3 wires

Either there's more than one heater/thermostat on this circuit, and constant-hot is going somewhere else to power another thermostat (or other 240V load, but not likely), or there's more than one heater cabled directly to this thermostat.

Since there's no obvious line/load marking on the 'stat, you'd need to check with a meter or voltage detector to determine which is the case here, unless there's a non-obvious line/load marking you can see that does not show in the picture.

As for connecting it to the 6 wires you have, you need to figure out which is which so that you can keep the correct set paired when connecting to the thermostat - since it's single-side-switching (as opposed to the current stat switching both lines) you get the wiring they show, but either two wires connect to the line side, or two wires connect to the load side, rather than one as shown.

I was dubious that your box was large enough for 3 12AWG cables and a device. 6 wires, 1 for all the grounds, 2 for the device makes 9, X 2.25" for 12AWG makes 20.25 cubic inches required. You say 20.3 below, so that's fine.

  • As far as I can tell, this thermo only controls 1 baseboard, but there is another thermostat for the other room. But they are on the same breaker/circuit, and the 2nd thermostat on the floor has 2 wires to the jbox. Presumably they are 1 to power and one to baseboard Commented Oct 9, 2022 at 23:53
  • Jbox is 20.3 cu in Commented Oct 10, 2022 at 0:02
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    Good, you have enough space. Then the doubled wires (connecting to reds on old 'stat)are evidently feeding the other 'stat on the circuit, and go on the line side of the new 'stat (& all whites together.)
    – Ecnerwal
    Commented Oct 10, 2022 at 2:22

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