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Wiring 120/240 and low voltage together through house

What's the story with running low voltage (thermostat, transformer, even outdoor reset sensors) alongside, or under, the same Romex staples with 120 and 240 volt circuits? 

Is that OK to do, or does space need to be maintained between the wires?  I see looking around it is not OK to mix low and high voltages within the same electrical junction box, but that it is OK to run within rated equipment.  But I also see conflicing information on running 120 volt Romex and thermostat cable together under the same Romex staples as the jackets are not both rated for the higher voltage. 

I'm asking because an electrician jammed everything in a run under the same staples, the quickest and easiest way to do it.  There's 120 Romex with thermostat cable in one run and 240 volt cable with ODR sensor wire bundled under common Romex staples in another.  When asked, the subcontractor electrician said it's fine and would be inspected, but it never has.

Another electrician doing some upgrading for us pointed it out and wants to redo it.


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am not an electrician.

    I also do not know the codes, though I have some idea what they must say. I used to design electronic equipment, though.

    So, ignoring the safety issues (that must not be ignored), I would advise not running thermostat or temperature sensor wires close to power wires unless you are using a very old boiler or furnace that does everything with relays and switches. Because electronic controls are more sensitive to noise and the noise in the power lines, especially ones where motors are switched on and off, can get into the thermostat and sensor wires and confuse the electronics. In extreme cases, it might even damage the controls.

    Perhaps some controls are designed to reject noise in the control circuits, but I would not count on it. They could use all DC in the control circuits and have filtering to reject the 60 cycle hum and all the spikes. But that would cost more and use more space, so I imagine it is not present.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited June 2012
    Not a good idea.

    THe sensor control cables running parallel to a major power line will pick up RFI and EMI (Radio frequency and electro magnetic interference) and the processors will freak out. It happens every time a high amp surge device turns on (refrigerator for example, or dryer, or whatever).

    If there is absolutely positively NO possible other route, a person could use shielded cable with a drain/ground, and ground all wire on one end, but even that is no guarantee that you will not have "issues". It is not even recommended that a conventional thermostat wire be run parallel to a high amp line voltage line.

    I've seen recommendations to keep 3 feet of separation between them.


    There was an error rendering this rich post.

  • zacmobile
    zacmobile Member Posts: 211
    I don't know the code either

    I am in Canada and all I know of the code I learned by getting my wrist slapped by the electrician everytime I violate it. I have learned that the typical brown thermostat wire's sheathing is only rated for 30V so you must keep it 50mm away from high voltage wires & not in the same box, that's why tekmar & other controls give you little dividers to keep things separated. I started using the red fire alarm cable for thermostats & control wiring, it's sheath is rated for 300V, makes things a little less complicated.
  • Steamhead
    Steamhead Member Posts: 16,944

    the inspector doesn't like the color ("Red is for fire alarms ONLY!!") We've seen that happen in Baltimore......
    All Steamed Up, Inc.
    Towson, MD, USA
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