floor heat stat...mix line voltage with low voltage

[Kris E Hansen]

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!

 

[Dennis Alwon]

The low voltage wire has an insulation rating either equal to or greater than the largest voltage contain in the box then in many cases you can mix them together. So you can have 300v insulation on the low voltage cable and 600v insulation on the higher voltage as long as the voltage on the high voltage wire is not greater than 300v.

Here is the article but take note of the informational note

Informational Note No. 1: See 725.136(A) for Class 2 and
Class 3 circuit conductors.

(C) Conductors of Different Systems.
(1) 1000 Volts, Nominal, or Less. Conductors of ac and dc
circuits, rated 1000 volts, nominal, or less, shall be permitted
to occupy the same equipment wiring enclosure, cable,
or raceway. All conductors shall have an insulation rating
equal to at least the maximum circuit voltage applied to any
conductor within the enclosure, cable, or raceway.
Secondary wiring to electric-discharge lamps of 1000
volts or less, if insulated for the secondary voltage involved,
shall be permitted to occupy the same luminaire,
sign, or outline lighting enclosure as the branch-circuit conductors.

 

[hbiss]

I can understand your confusion. Without seeing actual installation instructions I think you would be correct that with 20ga wire the sensor is low voltage. Assuming that I have to believe that it would be CL2.

First, always follow the manufacturers installation instructions. If they say you can run the probe wire into the box with the thermostat and line voltage I would take that as being compliant. There might be a requirement to maintain the jacket to as close as possible to the termination.

Second, if the instructions do not allow the probe wire in the box, you can splice onto lengths of THHN or TFFN outside the box. This would satisfy the requirement for all conductors within the box to have the same insulation rating.

-Hal

 

[JFletcher]

fwiw, the last flooring thermostat we put in had the line voltage in the box, and the low voltage/sensor wire attached to the very bottom of the thermostat so that the wire to the floor was in front of the attachment plate and not in the box - the drywall was notched ever so slightly to achieve this. Not sure if novel or hack, but there would have been no way to use a divider in the box to separate. 725.136(D)(1) (2008 NEC) would seem to allow the sensor wire in the box provided you can maintain 1/4" spacing from the line voltage conductors.