Part of my house has the old knob and tube wiring, so any chance that I have to replace it I do. There’s very little of it left. The other day I went to replace an old light switch in the basement with a occupancy sensor. The only other thing on this circuit breaker is my freezer. Replacing this light switch should have been a simple enough job, but replacing this light switch has turned into a can of worms nightmare.
The old light switch must have been around 40 years old, and did not have a spot for a ground wire. The new occupancy sensor would need a ground wire so I ran one from the main panel to where the light switch/occupancy sensor would be. So far so good.
At the downstairs light switch the old knob and tube was run through conduit from the ceiling to the light switch box. I had to remove the conduit to run the ground wire through the conduit to the switch box. By the time I had the conduit replaced with the new ground wire, it was getting late and dark outside so I figured I’d clean up and finish installing the occupancy sensor the next day. I reinstalled the old light switch, taped up the ground wire and tucked it aside, and turned the power back on. Everything seemed normal but then around 10 minutes later the circuit breaker surprisingly shut off. I left it off until I could look at everything closer the next morning.
When I looked at it the next morning I found bad insulation on the old knob and tube, which is normal for wire this old when it's moved around, so I figured it was a good time to replace the wire with some new 12/2. I installed a junction box near the top of the conduit because I would not be able to replace the old wiring going to the light fixture (it was run on top of the heater duct).
I replaced the old knob and tube from the basement light switch with new wire run inside the conduit, and was ready to start wiring everything back up when to my surprise I found that the hot wire (knob and tube from upstairs, acting as a feed wire to this basement light switch) was also shorted between the heater duct and the floorboard. When they originally wired this house they ran the wires between the floorboards and the heater ducts. It would be a nightmare to try and fix the short, and besides, it was another chance to replace some more knob and tube with some new wire.
Just for reference (as far as I know) this branch of the circuit runs from the main fuse box to a bedroom light on the main floor, and from there it goes to the bathroom light and then to a hallway light, and finally back downstairs to the light switch that I was replacing.
I would only need around 6 feet of wire from the upstairs hallway light switch to the junction box downstairs (luckily the upstairs hallway light switch is right above the basement light switch). I removed the old knob and tube and ran new 12/2 from the upstairs hallway light switch to the junction box downstairs.
So basically all the old knob and tube from the hallway light switch to the junction box, and from the junction box to the basement light switch was now all new wire. The only knob and tube that was left untouched was in the bedroom light and the bathroom light (I’ll replace those later) and from the junction box to the light fixture. All the wiring was pretty straight forward. I connected everything together, flipped on the circuit breaker, and heard the freezer kick back on.
The lightbulbs in the bedroom and the bathroom were removed when I was tracing down the short in the old knob and tube, but I figured that with those lightbulbs removed that it shouldn’t matter for the basement light, and the basement light should still work on its own. To my surprise neither the basement light or the hallway light would work. I quickly shut the breaker off and looked over everything again.
All the connections looked good, so I replaced the lightbulbs in the bedroom and the bathroom and flipped the circuit breaker on again. This time the basement light and the hallway light still didn’t work, but the bathroom light did work. Here’s where it starts to get weird. The bedroom light uses two bulbs, one was on, the other was off, and the bedroom light dimmer would not dim. I shut the circuit breaker back off because nothing was making any sense.
All I did was replace one old knob and tube wire from the hallway switch to the basement light switch. Why would all of these bizarre things be happening now? I suppose it could be from a bad dimmer switch, but wouldn’t that just effect the one light? There is no neutral wire in the light switch boxes, just black hot wires, so how can I check for power at the switch with my meter? What is the best way to trace down this bizarre problem?