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I have a situation (USA) where there's no practical alternative except run some low voltage wiring in the same conduit as 120/240V power.

The good news is, my situation calls for using entirely mains-type cable (THWN-2, 12/3 NM, etc.) for the low voltage stuff, and all the hardware will be enclosed in junction boxes listed for mains electrical service.

It's for relay circuits; the relay coil is tapped for 24V or 240V (I'm using the 24V tap) and the 120-24V transformer is obviously insulated for 120V (though it is not made to have 120V across its secondary).

What do I need to do in order for this to be allowable and safe?

  • Is the mains wiring associated with the low-voltage control wiring? – ThreePhaseEel Mar 7 '17 at 23:42
  • The transformer supplying the low-voltage DC comes out of the same panel as the mains wiring. The mains wiring goes to a subpanel. The low voltage operates relay coils. Those contacts switch loads originated from the subpanel. In other words the controls and the loads are sourced out of different panels, isolated by the relays. Given the application and distance it's not feasible to just carry the loads on the THWN-2/NM. – Harper Mar 7 '17 at 23:53
  • what prohibits having an extra branch circuit in the subpanel for the control transformer? – ThreePhaseEel Mar 7 '17 at 23:58
  • That's been twisting in my brain, source the 24V at the transformer. I think that would work. It would be inside grounded conduit, but it wouldn't be the subpanel's ground, it'd be the main panel's ground. – Harper Mar 8 '17 at 0:07
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    Maybe I am missing something here but with the wiring all thwn / nm the wire is fine in the same conduit. As far as relays within a controll enclosure this is very common not only with 120v but even 480. I am not sure I see any problem. – Ed Beal Mar 8 '17 at 0:27
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This is a case where what ordinarily would be a Class 2 control circuit is redesignated as a Class 1 control circuit, as per Exception 2 to 725.130(A):

Exception No.2: Class 2 and Class 3 circuits shall be permitted to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings required in 725.124 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Class 1 circuits.

Once that is out of the way, 725.46 applies (allowing the use of Chapter 3 wiring methods as you are doing), but 725.48(B)(1) also applies:

(1) In a Cable, Enclosure, or Raceway. Class 1 circuits and power-supply circuits shall be permitted to occupy the same cable, enclosure, or raceway only where the equipment powered is functionally associated.

Considering that you have relay control wiring being run with the circuits they are powering, I do not believe you will have any trouble meeting that requirement.

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