Knob and tube wiring question.

[rogsmart]

Hi Folks,
I've got a lady friend who wants me to install a nice little ornate hanging ceiling lamp and a dimmer switch for same. When I was checking out the wiring I still got a buzz on my non contact voltage tester after I flipped off the light switch. A quick trip into the attic revealed that the original electrician had run the hot wire direct to the ceiling fixture and installed the wall switch on the neutral/return wire. (I've read that it wasn't all that uncommon for knob and tube light and switch runs to be done this way back in the glorious days when knob and tube was the latest technology.)
So I've got a bad feeling about installing a dimmer switch after the load. It just doesn't seem like a very good idea to me. Or that it would even work. However, I need to be able to explain it to my friend using something a little more logical than, "I have a bad feeling about this." A little help here guys?
Much appreciated.

 

[Tscott]

It will work, all you are doing is using the dimmer to slow current through the light fixture. As long as the dimmer is in the only path for current flow, then it will work just fine.

You may also want to have your friend get that wiring checked out. That stuff can cause fires if it's old and brittle.

Tom

 

[Friartuck]

The simple and realistic answer: Remove the Knob and Tube Wiring! This is likley to be a violation against the homeowners insurance policy where the policy will be void if certain conditions exist. Knob & Tube is one of them. A neighbor recently purchased a 1915 era farm house and the insurance inspector based on a hunch given the vintage of the house found one existing circuit that was spliced into the romex going to the panel. The Mortgage Company gave him 24 hours to correct the problem when the insurance company inform them the policy was about to be cancelled. As for the dimmer, now that you're replacing that wiring circuit, can either before or after the load.

 

[sgrady11]

It's possible its not knob-and-tube, but wired like it was.

When I bought my house, I had noticed that a few circuits had the switch on the neutral. (the house was rewired, probably by previous owner, there are remnants of the 'knobs' and some detrius they left behind in the attic)

I talked to several 'seasoned' electricians who told me that the neutral was switched on knob-and-tube installs. So, I'm assuming what happened is the guy replaced everything leaving the circuits wired the way they were.

But yeah, you gotta go up there and see what type of wiring is going on.

 

[rogsmart]

Thanks guys. I appreciate the advice. Yeah the knob and tube is scary stuff. The wire insulation is old and fried. It falls right off when you handle the wiring. I'll definitely pass that recommendation along.

 

[VHF]

If I had any live knob & tube -- or early vintage NM -- I would protect it with an arc-fault breaker.

(I think the NEC requirement for arc-fault breakers in new houses with modern wiring is silly, but in an old house with potential wiring problems it could prevent a fire.)