Does NEC specifically prohibit NM cable in an old K&T switch box?

I need to do the calculations, but I think I have room as I'm just terminating a 12/2 cable to a receptacle (K&T removed from box).

My question relates more to the clamps. The old clamps are designed for the circular profile of the K&T insulation. Not sure if this will cause cable damage or cause other problems with cable. Also, since it is K&T - no grounding screw present. My plan was to use a grounding screw on the existing unused threaded hole for a second set of internal clamps

Thanks in advance.

  • Were the old clamps fitted into standard KOs, or are they integral with the box? Does the box have unused KOs still for that matter? – ThreePhaseEel Nov 15 '18 at 23:49
  • KOs are knob and tube style, and intergral to the box. – Uncle Woody Nov 16 '18 at 0:43
  • Is replacing the box an option? – ThreePhaseEel Nov 16 '18 at 1:24
  • I could put a remodel box in, but was hoping to avoid a lot of plaster repair. The existing box is rock solid so it would be great to use if possible. Was just hung up on the legality of using the existing clamps. Also, see edit regarding grounding screw. – Uncle Woody Nov 16 '18 at 3:50
  • Can you get access to the outside of the box? – ThreePhaseEel Nov 16 '18 at 3:56

The NM has to be secured near the box, or by a NM clamp in the box. When you're fishing NM, you can't secure it near the box, so you really have to secure it at the box. I think your best bet is to install a new old-work box.

  • Thanks for the response. I assume that you're inferring that it's unacceptable to use the old K&T clamps on romex? – Uncle Woody Nov 17 '18 at 1:17
  • @UncleWoody - strictly speaking, you have to use the box according to it's listing and I don't think you could say the box's clamps are listed for NM. I'm not saying it's a huge hazard but I don't think it's 100% up to snuff. – batsplatsterson Nov 17 '18 at 10:38
  • 1
    Thanks, I just ended up using an old work box 1 stud bay over. Easy enough to do. – Uncle Woody Nov 18 '18 at 20:26

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