I sometimes get confused about understanding exactly how to determine a class 1,2, and 3 circuit. I also get confused to know exactly when can and cannot mix line voltage with low voltage. What classification does the temperature sensor of a floor heat stat come under? The wires are 20 gauge and have very thin insulation. The stat company says I can have the stat sensor wire in the same box as the line voltage. Other companies say do not mix the wires in the same box (FlexTherm for instance). Some companies say it is OK to mix the sensor with the element leads in the box but prohibit the mix in the same conduit (but they don't say why...like say false reading problems...they hint to it being a NEC driven). I would like to be assured that it meets code to have the sensor wires in the same box. It is uncomfortable for me to mix these wires in the same box because the sensor wires are so small which such thin insulation (seems hazardous). I have been told that basically if the voltage is line voltage is 300V or less and the insulation of the Low Volt wire is 300V, that you can mix line voltage with low voltage in conduit and box. For many years I was under the impression that the low voltage wire had to match the line voltage wire, so 600 volt insulation was the way to go. I thank you in advance for clearing me up!