Old House Wiring: Knob and Tube

By: Mark Clement , Contributing Writer
In: Old Houses, Old House Construction, Home Improvement Tips, Old House Musings, Old House History

If you have an old house, you know this refers to the wiring that’s probably coursing current to your lights and plugs.

The knobs are ceramic clips (ceramic–imagine the expense installing those today!) that secure and protect the fabric-jacketed wire parallel to the joist and/or serve as a pivot point to change the wire’s direction. The tubes are ceramic sleeves that protect the wire as they pass perpendicular through a joist or stud.

Old House Wiring: Issues with Knob and Tub

The good news is that knob and tube is intrinsically safe because the hot and neutral lines are so far apart from one another that there is very little chance of them arcing. However, it is not grounded–required by modern convention–and often can’t serve the loads of with modern electrical demands (think hair dryer, lights, coffee maker, bathroom fan and microwave all running on the same circuit at the same time…no dice.) So if you’re planning a full-room upgrade, plan on running a new circuit with modern electrical wire and devices back to the breaker panel.

And, as my electrician says, you want an old sparky working on old wires. Reason is that unlike modern electrical plans, knob and tube wires run from the panel all the way through the house before the circuit returns back to the panel. Nowadays it happens within the same wire. See, the guys drilling the holes with a brace-and-bit pommel jammed in their chest wanted to drill as few holes as possible so they ran the wire as efficiently through the house–leaving about one outlet per room to frustrate us modern old house homeowners! And while it works, the upshot is if you cut the wire, say to re-wire your bathroom, you cut off whatever is down stream. Also, while it is modern convention to “switch” the hot wire only, back in the day when wiring was new, it was often easier to switch the neutral wire. It works but it makes trouble-shooting as much art as science when trying to figure out what feeds what then goes where.

Old House Wiring: Use the Right Tools

If you plan to get tricky, as I’ve seen done many times, and add modern vinyl jacketed cable (Romex) to existing knob and tube, well, you can’t do that. And that’s really not safe. That’s also why I carry a wire tester with me on old house remodels. I like to be know right away if some buried cable is hot before I grab for it.

The good news is that you can probably fish wire through cavities in your home. The bad news is that you may have to punch holes in walls and ceilings on rooms other than the ones you’re working on to get the wire from point A to point B.

It’s a small price to pay for the convenience, safety and peace of mind doing it right–even if you had an old guy like my electrician doing it (you’re welcome Bob!)

Share/Save/Bookmark

Post a Comment

Enter the text shown above

  1. 4 Responses  to “Old House Wiring: Knob and Tube”

  2. Aug 29, 2011
    It's also a really nice screwdriver. Even though the lion's share of what I do is in the carpentry category, I find that I always need to have a good screwdriver for something--with multiple bits like this one has on board--Now stop touching hot wires!! :)
  3. Jeremy
    Aug 29, 2011
    I have that same wire tester. It is very handy and also a pretty nifty piece of technology. I've had some bad experiences touching some wires and unexpectedly you get perked up real quick with the heat.
  4. Aug 29, 2011
    John--Sounds like you are on the right track. Nice hustle!
  5. Aug 29, 2011
    All of the second floor fixtures in my house are on the same knob-and-tube circuit. And some where in a wall (I know approximately where, but not exactly), that knob-and-tube is spliced to BX, with the BX cable going back to the service panel. Needless to say, I keep this circuit de-energized all the time! And it will soon be relegated to history when I upgrade everything to 200A service and NM. Thanks for this article, though. I enjoyed reading it.