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To revisit our occasional questions on sneaking a low voltage cable into an AC mains conduit...

Suppose you have a fairly large electrical conduit, say, 1-1/2". You also have some sort of continuous flexible electrical conduit, like smurf tube, where this conduit itself could be fished into the larger conduit.

Can you use the large conduit to contain both of these:

  • loose low voltage cabling
  • a continuous flexible conduit appropriate for Class I wiring, and inside that, AC mains wires type THWN.

Is that legal? Good answers will call out Code cites if applicable.

I am not planning to self-answer this.

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  • 1
    One way to look at this: Would it be any different from a wall cavity? "An empty space that can hold conduit and other stuff" Jul 28, 2020 at 3:00
  • Are these LV cables communications cables (Ethernet/phone or coax) or control/signaling cables (Class 2)? Jul 28, 2020 at 4:59
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    The small conduit would be the same as using a separator in gutter. You would not need to use cable rated for mains voltage as the smaller conduit isolates the low voltage cable.
    – Ed Beal
    Jul 28, 2020 at 5:11
  • @ThreePhaseEel let's assume ethernet/phone/coax. Jul 28, 2020 at 6:01
  • What about the opposite situation - running the mains voltage over THWN in the large conduit and using the smaller smurf tube for the LV wiring?
    – FreeMan
    Jul 28, 2020 at 11:35

2 Answers 2

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Filling uncharted waters

While the use of innerducts (conduits inside conduit, basically) is not completely unheardof (NEC 800.110(A)(3) permits purpose-built innerducts for communications work, which is where they are usually found, and the Code is silent otherwise about "nesting" Chapter 3 conduit types within each other), the problem with any approach that mixes innerducts (whether Chapter 3 or purpose-built) and wires or cables in the same conduit is that there is no clear way I have found to proceed with evaluating such configurations against the fill rules, especially when it comes to the fill of the outerduct, as the 40% fill rule doesn't apply to comms cables.

Unsupported

Furthermore, the conduit support rules never envision Chapter 3 conduits being used in such a manner; for flexible conduits, one could argue that the outerduct forms a "concealed space within a building" that the innerduct is then fished through, as permitted by various exceptions in the NEC, although the wording is not consistent between said exceptions, so the validity of this argument depends on the precise type of conduit being used as the innerduct.

Matryoshka conduit bodies

Another issue you'd run into with this approach is that you'd need to "nest" conduit bodies within each other wherever you had to use a conduit body for a direction change. This is a problem because it's not at all clear how one'd support the inner conduit body, as required by NEC 314.23, under the circumstances involved, especially since none of the conduits connected to the inner body can support it in that case. (Conduit bodies normally fall under the Exception to 314.23(E) when it comes to support.)

Terminal woes

The final issue comes when you go to terminate this assembly. If there are no mains wires directly in the outerduct, one could use a bell end fitting for the termination if the outerduct is PVC and the innerducts are suitable for in-wall usage, but this doesn't work if you have to terminate into a box with dividers inside it. In that circumstance, you have to field-punch KOs into the dividers and run the innerduct into a fitting mounted into the divider, which is not something Code particularly envisions, either. (With a metal outerduct, one would use a threaded insulating bushing instead of the bell end fitting.)

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    +1 for use of "Matryoshka" and spelling it correctly! :)
    – FreeMan
    Jul 29, 2020 at 13:23
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I think that in the case of putting a smaller conduit into a larger one for a straight shot, the larger one could be considered a "chase", basically a duct. But once you introduce another current carrying conductor, you add too many other issues like fill, transitions, etc.

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