Upgrading thermostat: 2 baseboard heaters (6 wires)

Question

I'm upgrading a thermostat, and the installation isn't quite as simple as I'd hoped. In this COVID era, I think I'd like to avoid having an electrician inside my home...
Some facts:

the circuit breaker box has 2 switches that are "tied" together. I think this means 240V.
the existing thermostat controls 2 baseboard heaters (one is longer than the other by ~30%)
inside the gang box, there are 3 cables, each with white, black, and ground wires.
the 3 ground wires are all twisted together
one of the cables coming in to the gang box (the middle one) looks different, it has yellow casing, and its wires are smaller gauge

The thermostat I'm replacing has 4 inputs: T1 and T2 (termed Black/Line) and L1 and L2 (termed Red/Load). I attached pictures of the existing thermostat (I haven't changed anything), but tracing the lines is a bit hard so here I'm writing it out too. These are currently connected by:

L1/Black/Line is connected to the top and middle blacks
L2/Black/Line is connected to the top and middle whites
T1/Red/Load is connected to the bottom black
T2/Red/Load is connected to the bottom white

So which one is the supply? The middle one perhaps because it's conspicuously different? The bottom one because it's connected differently?
The installation instructions for my new thermostat say:

Note: The supply and load wires are not interchangeable. If you are unsure which of the two wires is the supply or load wire, simply try one configuration.

Does that advice still apply when there are 3? I suppose I could pick 2 cables to figure it out and then add the 3rd.
I'm reluctant to turn the breaker back on and test voltages, but I do have a voltmeter and I think I could be careful enough to use it (but which pairs should I check?)
EDIT: I mixed up my L and T, now fixed. That is, it looks like two line wires are combined to create 240v and somehow one load wire is used for both heaters? Not sure if that makes sense or if I'm still mixing something up.

Harper - Reinstate Monica · Accepted Answer

When you see 2 wires connected to supply/LINE side, it simply means that one of them is supply from the panel, and the other is supply going onward to somewhere else.  They are both supply, and it doesn't matter which is which.
You need to connect them both to supply/LINE, or to be more precise, connect them to each other, and then, connect a pigtail to your device's Supply input.

Ecnerwal · Answer

Supply == Line == T1, T2 old stat (if you typed correctly), L1, L2 New stat you show a picture of.
Load == Load == L1, L2 old stat (if you typed correctly), T1, T2 New stat
If there's nothing else on this circuit, line is the single wire, load is the two wires, one to each heater.
If there are other things on the circuit, could be different - line could be going on to somewhere else, and the single cable might have a junction to feed the second heater not in this box.

Ted Mittelstaedt · Answer

I can't tell how the existing stat is wired nor whether you updated your description but assuming this is correct:
L1/Black/Line is connected to the bottom black
L2/Black/Line is connected to the bottom white
T1/Red/Load is connected to the top and middle blacks
T2/Red/Load is connected to the top and middle whites
Then it SOUNDS like the bottom black and white are the supply line and the top and middle are the 2 heaters.  If you disconnect everything and measure resistance between the 3 pairs of wires you would get infinite on the supply, (assuming the breaker is off) and a low resistance on the 2 loads.  It may be your old stat was wired "backwards"
But - something about this looks suspicious.  If it was me I would leave it hanging out and power it up then use my meter to measure voltages on the stat wiring in the wirenuts when the heaters were on and off.  Also I would look for the power rating plate for each heater it should be in the front at the bottom and visible, you may have to get on the floor and look to find it.  It's possible for a very short baseboard heater to be 120v although unusual.  But most important is that the combined power rating of both baseboard heaters is under that 16A rating on the new stat.
Take some voltage readings and let us know what you find.  Be careful working with live wiring, wear electricians gloves.

Upgrading thermostat: 2 baseboard heaters (6 wires)

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