Installation reminders for the inslab cable
Always make sure to encapsulate the factory joint in thinset or floor leveler. It must never be inside the wall. Not doing this will result in irreparable damage to your floor heating system. Chisel out a channel deep enough in your sub-floor to accommodate the thickness of the joint.
If you absolutely need to extend the unheated lead that is connected to your thermostat, you will need to ensure that you have the proper gauge wire when splicing. A 20 amp circuit will use 12 gauge and a 15 amp circuit will use 14 gauge.
If you have more than one heating system, all the lead wires need to be connected in parallel to the thermostat. In this instance, it might be more convenient to run multiple lead wires to a junction box, and then a single wire (which has the appropriate rating) from the junction box to the thermostat.