We've replaced our oil boiler heating system with electric heat pump, and need to add some baseboard heating. Each area will have it's own thermostat. We are using aube TH106 thermostats... "2-wire non-polarized" programmables. The 2 short wires that extend out of the thermostat are black, and multistrand silver wire (aluminum?). We have to run the wires from the heater placement to the place where the thermostat will be mounted on the wall. What gauge wire do we use?

Thanks in advance.

The same gauge that is being used to supply voltage to the heaters.

14 gauge for 15 amp circuit
12 gauge for a 20 amp circuit
10 for a 30 amp circuit

I have no idea what size circuits the heaters require to start with. The thermostat brand and model number means nothing when sizing these circuits unless you overload the thermostat and you will find that out soon enough. Everything has to be matched size wise for the heaters to work correctly. I suggest reading the instructions that came with the heaters to figure out the correct wire size for the heaters and thermostats.

Ok, I'm busted, I'm not an electrician. I am only running the wires, a true electrician will do the connections.

We ran 12-3 w/ground and the heaters are each on their own 20 amp circuit breaker, with a max amp of 8.

I have one wire run from circuit breaker to position for heater.

Now I have to run another 12-3 to the thermostat, right?

Do I have to I have to run a second 12-3 from heater to circuit breaker and to the thermostat, or does the "3" wires in the 12-3 make the circuit?

This is my first foray into electrical projects, so please be kind.

Now I have to run another 12-3 to the thermostat, right?

Yes if you are using metal boxes and or if the thermostat itself requires a ground you need to use 3 wire. Now there is a easier way to do this BUT I am not going to confuse the issue.

Do I have to I have to run a second 12-3 from heater to circuit breaker and to the thermostat, or does the "3" wires in the 12-3 make the circuit?

One circuit feed will do for both the thermostat and the heater. The thermostat will basically be a on off switch to control the heater. Usually there is enough space to use the baseboard heater itself as a junction box where it will all come together. OR you could pull the conductors to the thermostat wall box first then make a drop to the baseboard heater. Continue the ground path from main breaker panel to wall box to baseboard heaters or visa versa. All metal areas that need to be grounded have to be grounded correctly. Most if not all of this grounding should be on the manufactures paperwork. They want to make sure the grounding is done so no one gets the shock of there life.