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I have an old house with unswitched knob-and-tube running to a ceiling fixture. Previous owners ran surface conduit to a wall switch and to a second light fixture over the sink; none of this was grounded, needless to say. I recently had to replace part of the ceiling, so I took the opportunity to remove the surface conduit:

  • I Installed metal boxes with extensions for the original fixture and the one over the sink (I used expandable joist hangers to mount them)
  • Ran shielded metal cable above the ceiling to connect the fixtures, and down the wall to a newly-mounted metal switch box
  • Carefully routed the knob-and-tube wires through separate holes in the top of the box, secured them, flagged the neutral with white tape, then wired them to the fixtures and switch with wire nuts. (They originally just poked right through the ceiling and a surface plate)

I thought to route a three-wire lead up to the switch from the floor, but I still would have been left with a live knob-and-tube circuit in the ceiling. There are other fixtures upstream of the kitchen lights, so I couldn't just kill the whole circuit or easily terminate it upstream.

I'm pretty sure this is not to code, but it was the least disruptive method I could come up with. The wires are typically brittle, but they are essentially in the same position they have always been and connected the same way.

Here are my questions:

  • is this considered an extension of the circuit? I did not add fixtures or outlets. I only replaced wiring that was itself installed after the knob-and-tube.
  • What should I do with the ground wire? As it is, the wire sheaths, switch, and all three boxes are bonded together with the ground wires but don't connect to anything. Should I leave them unconnected, or otherwise signal that they aren't actually grounded? (This would be obvious in the main fixture box but not in the other fixture box or wall box.)

Thanks in advance.

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First, is actual neutral present in the switch box? That's a requirement now, so if it's a switch loop you'll need to re-run it with /3 cable.

Now the purpose of grounding is to

  • return natural electricity (lightning, ESD doorknob zaps) to its source (earth via the grounding electrode system), and
  • return artificial-electricity fault current to its source (via the equipotential bond to neutral back at the transformer).

You've created an island of ground. All it'll do is distribute ESD and fault currents to other places in the island. I talk about the badness of that here.

Easy to fix: Retrofit ground to connect your island to your house's grounding system. In 2014 they liberalized the rules to make it much easier. More on how to do that, here.

To answer your questions:

  • I agree, this is wire repair, not expansion of the circuit.
  • Grounds should be run home as I describe.

Also, whatever circuit(s) are still on knob & tube, slap an AFCI breaker on them. This will trip on any wire arcing, which is the usual problem people worry about with K&T.

  • Thanks. There is no neutral in the switch box. I can't see the logic in adding one, which would require two additional wires, which would connect to each other at both ends. – JoeCopley Sep 5 '18 at 16:45
  • @JoeCopley The logic is it's been code since 2008. Also b) better to do it now, before you mud up that wall, which is why I mentioned it. c) the #1 question we get on here is "I want to install a smart switch but there's no neutral". d) the #4 question we get on here is "I want to extend off this switch circuit but there's no neutral". You use /3 cable instead of /2. In the ceiling you connect the white wire to actual neutral; in the switchbox you cap it. Red is switched-hot and black is supply, those go to the switch. – Harper Sep 5 '18 at 17:01

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