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I have some new AC ducts in our new house that will soon be concealed behind new drywall, but before the drywall goes up, I wanted to run some conduit from the upper floor down to the basement for cable and network. Unfortunately, in the space between the two ducts (where I pretty much would have to put the cables, there is, attached to the floor joists a run of knob and tube wiring. I tested it with a voltage tester and it doesn't appear to be hot, so I'm assuming it's the neutral line (I can see where it's partner live line is, however). It looks to be in pretty good condition with the insulation in tact, but I'm leery about trying to run flexible aluminium condition right next to it. I have limited space to work in and limit options on securing it, but obviously I don't want the potential for the conduit to end up live!

How worried should I be about this? If I secure the conduit away from the k&t, would I be okay? Or should I just forget the conduit altogether and just drop (insulated) cables down the hole and hope I don't need to change them or add more at a later date?

Matt Burland
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    Why not PVC? Will allow you to pull new runs easier in the future. – DMoore Aug 20 '14 at 15:06
  • @DMoore: Because, regretfully, that's what I bought. I may well go back and get plastic instead. – Matt Burland Aug 20 '14 at 15:16
  • At SOME point you really should consider replacing that k&t with NM-B. You know that, right? This might be a good time? – TDHofstetter Aug 20 '14 at 15:39
  • @TDHofstetter: I've heard conflicting things on that. But the truth is after dropping a lot of money on AC, I'm not over the moon about the idea of dropping even more on rewiring unless it really needs doing. – Matt Burland Aug 20 '14 at 15:41
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    **Knob and Tube wiring is a serious hazard to live safety.** Knob and tube wiring is not just an inconvenience like an old drafty window. It really needs doing. The AC won't keep you cool in the event if there's a fire, and if there is a fire, you and/or your loved ones may wind up dead. –  Aug 20 '14 at 16:21
  • @benrudgers: a little dramatic. See for example: http://diy.stackexchange.com/a/20279/24303 – Matt Burland Aug 20 '14 at 16:25
  • Empirical experience is why NEC 2011, 398.10 and 398.12 limit `Open wiring on Insulators`[aka "knob and tube"] to Industrial and Agricultural occupancies and prohibit installations concealed by the building structure. https://www.inkling.com/read/nfpa-national-electrical-code-handbook-2011/chapter-3/article-398-open-wiring-on The fact that you were thinking about how to install emt rather than how not to install it is why. If you want unvarnished opinions from Electricians, Engineers and Fire Service Professionals, consider asking about it on the Mike Hold forums: http://forums.mikeholt.com/ –  Aug 20 '14 at 16:51
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    @benrudgers the Mike Holt forum is for professionals, not DIYers. They likely will not be very helpful. – Tester101 Aug 20 '14 at 23:03
  • @Tester101 In this case putting the difference between professionals and non-professionals into stark relief is in itself helpful. Knob and tube ain't paint. –  Aug 20 '14 at 23:13
  • @benrudgers disagree, and so does NEC. What worries existed with K&T have been taken care of with AFCI, which takes care of arcs, and GFCI, which takes care of path faults. And the relaxed 2014 rules on ground retrofits fully modernize it. Scuttlebutt was it would start fires if packed with blown insulation, but studies have shown that's a lie. – Harper - Reinstate Monica Mar 07 '17 at 20:10
  • @Harper A knob and tube installation upgraded to AFCI smells like a corner case and does not sound like this case at all. Anyway, I'd be interested in the NEC code citation allowing concealed bare conductors in dwellings. I don't keep up with the NEC like I once did. –  Mar 07 '17 at 20:35

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You could use "smurf tube" (Electrical Nonmetallic Tubing) or rigid PVC to have a non-conductive conduit. Be sure to bond any metallic conduit that is separated by such non-metallic conduit, or just use nonmetallic the whole way with ground wires. I suspect you are not supposed to contact the K&T in any case and should be securing the wires or conduit to prevent such contact.

Ecnerwal
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I make the assumption that knob & tube insulation will fail. Maybe it will, maybe it won't, why take the chance. With stock K&T insulation failure is no problem: the ceramic tubes will outlast any plastic insulation anyway.

But adding a metal pipe in the same cavity? Why? Use plastic conduit.

See also https://diy.stackexchange.com/a/20279/5960

Bryce
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