I am installing a 220V Hydrosil baseboard heater. It came with no thermostat, so I bought one locally that is rated for 220V. I am not an electrician. I am using 12/3 wire as my power source and the htr uses only 8.6 amps so they say. I have a Red, Blk, Wht and Ground wire coming in. The heater has a Red and Blk wire. The thermostat has a Red (line) wire and a Blk (load) wire. What wires do hook up to the thermostat and what wires do I connect to the heater?

If you haven't bought the wire yet, skip the 12-3. 12-2 with ground will do. You use 3 conductor for things like dryers and stoves that have 120 volt components. Start at one pole of a 2 pole circuit breaker. Colors don't matter, but say start with the black wire. Connect one end of the black wire to one pole of the breaker. If using 12-2, connect the white to the other. Run it to the heater. Connect the 2 black wires. Run another length of cable from the heater to the thermostat. Connect that white wire to the white wire from the breaker and to red wire of the thermostat. Connect the black wire of the thermostat to the black wire. At the heater, connect that black wire to the red wire. Mark the ends of all the white wires with black tape or paint. Connect the bare wire to where all the other bare wires are connected in the breaker box. At the thermostat ahd heater, connect them to any green wire or screw, and to each other. It is very important to connect it to the metal housing of the heater. If you have already bought 12-3, and can't take it back, there are far worse mistakes you can make working with electricity. You will survive. Just use the red wire where I said white, and curl up the ends of the white wire, maybe capping it with a wire nut.

Inside the breaker box isn't a good place for DIY. Even with the main breaker shut off, the wires feeding it are still hot, very hot. It is amperage that kills, and they are sized to carry 100-200 amps without heating up. Same with the buss bars down the center with it on.

Sit down and draw up your circuit. You want a loop running from one pole of the breaker through the switch inside the thermostat to the heating element in the heater, and back to the other pole of the breaker. You want perhaps a distorted ''Y'' running from the ground bar of the box to both the thermostat housing, if metal, and the housing of the heater. You can also run the cable directly to the thermostat, and on to the heater. Connect the black wires from the cables. Connect the white wire that goes to the heater to the 2 reds. Connect the one from the box to the black at the heater.