Run single strand of K&T wire through conduit under floor?

Question

I have a contractor putting a pocket door in a wall in our 1925 house. He removed plaster from one side of the wall and exposed a single strand of Knob and Tube wiring right where the door needs to go.

This conductor was feeding a 3-way-switched ceiling light, carrying current routed through our 2nd-story floor, up the wall, through our attic to the ceiling box (where it was simply hanging through a knock-out, no extra insulation, grommet, or clamp). It seems to be always hot, and was attached to the hot wire of our light fixture.

I needed to allow the contractor to keep working, so shut off the circuit (which also feeds several of our original first-floor lights), and then carefully pried off all knobs and pulled the wire -- maybe 25 feet -- through our attic, down the wall, and into the floor cavity (I cut an access through exposed subfloor I'll eventually be covering). I snipped, capped, and taped the end, then rolled it up neatly inside the joist bay while I figured out how to proceed.

I now know I can use the last stud bay on the other side of this wall -- the one that's getting the new door -- to re-route. Originally, I thought about possibly reusing all the knobs and tubes I saved to keep this wire taught and held on insulators just up into the stud bay, where I'd put in a box and splice to Romex to feed back into the attic. But I think it will be difficult to reattach knobs without seriously opening up the floor, and I know I can't make a junction inside the joist bay (or without an accessible box, in general).

I started to think it might be safest, actually, to get non-metallic conduit and run the wire through that all the way from the tube in the floor joist where it's originating to a tube I've put in the new hole I've drilled in the base plate of the stud cavity. I thought this would provide support and protection of this one wire -- the only one in the whole space -- over about 5 feet, and that I could use conduit clamps to keep the entire run safely centered in the joist bay.

But I can't find any reference for whether there would be any safety concerns doing this. I can't imagine it's less safe than trying to reinstall it with old knobs, but just not sure. Wondering if anyone has any thoughts on this.

Thanks

Answer 1

I can't say it is official code. But my hunch is that even if code may technically allow reinstalling knob & tube in existing circuits, it is not a good idea. Certainly if anything goes wrong, your insurance company will not be happy. But it is in conduit? I would not trust non-metallic conduit to provide much in the way of fire protection. In metal conduit you would have a better chance of that. There have been plenty of reports over the years of insurance companies not wanting to insure a house with knob & tube at all because of safety concerns, though as I understand it good quality knob & tube installations left untouched are generally safe. But this isn't "untouched".

Since you are willing to consider conduit and you have already at least temporarily removed much of the circuit anyway, I recommend replacing any parts of the circuit that are either damaged or have already been removed with a modern wiring method. That means either:

• Non-metallic cable (a.k.a., Romex), assuming that is permitted in your jurisdiction. That is generally going to be the easiest and least expensive method.
• Conduit with modern wires (THHN, THWN, etc.) with all wires (hot, neutral, ground if not metal conduit) together.

Both of these methods meet modern standards and use materials available at any hardware store, big box store or electrical supply house. Both of them also supply a proper ground wire, though that is only of use if you can get ground all the way back to the panel.

If you have a circuit with multiple sections and there are sections that you have not manipulated at all (i.e., between junction boxes every knob/tube/wire is the original installation) then you can leave those sections as is and only replace sections that have been worked on.

You may also want to consider arc-fault (AFCI) protection if there are concerns about old wires that are not being replaced. You generally only have to add AFCI when adding a new circuit, not fixing an old circuit, but this is one place where it may be a good idea unless you are replacing the entire circuit with NM cable and/or conduit/new wires.

Answer 2

You might find some inspiration in NEC 394.19(B):

Where space is too limited to provide these minimum clearances, such as at meters, panelboards, outlets, and switch points, the individual conductors shall be enclosed in flexible nonmetallic tubing, which shall be continuous in length between the last support and the enclosure or terminal point.