This story shows how to install underfloor electric mats for radiant heat.
Connect the wires together according to the manufacturer's instructions. Splicing sleeves are a quick and easy method to make secure connections. Strip about 1/2 inch of insulation from the ends and slide the bare wire into a splicing sleeve. Squeeze the sleeve with wire crimpers.
Cut a 3-inch piece of electrician's tape and, starting at the insulation, wrap the tape tightly around the sleeve. Make sure to cover the joint from the insulation at one end of the sleeve to the other. Fasten the bare wires at the end mat to the circuit wires and tape these joints.
Before installing the junction box, push fish tape through the hole in the floor plate. If your helper can't grab the end of the tape after a couple of attempts, try hooking the tape with a bent coat hanger. Pull the tape through the hole with the wiring. Install insulation between the joists.