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Old 02-02-2012, 11:03 AM   #1
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trying to figure out Knob & Tube wiring on old light fixture


I'm living in an old building built in 1926. It's got knob & tube wiring in it and I want to replace two light fixtures in a couple of hallways. The new fixtures are very simple, one 60W bulb each, very easy to do. Except...

When I took off the old light fixture, there are two sets of wires coming into it (of course, two hot and two neutral wires). The two neutral wires (white) were connected to the light fixture and the two hot wires (black) were capped. I just tested out all four wires with my voltage detector to make sure of this.

I'm not an electrical expert of course but I wanted to ask the following questions:

1) How can two neutrals power a light and why the heck are both hot wires capped (by an old ceramic cap nevertheless)?

2). Do I power off my place and hook up a hot and a neutral to the new light fixture (and individually cap the remaining hot and neutral wire separately)? If so, do I use the hot and neutral from the same set of wires or one from each (as both neutrals are from both sets of wires)?

3). Do I hook up both the hot and neutral wires to the new light fixture?

I'm attaching a picture of this as well.

TIA,
Dale




Last edited by dalethompson; 02-02-2012 at 11:30 AM. Reason: adding picture
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Old 02-02-2012, 11:38 AM   #2
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trying to figure out Knob & Tube wiring on old light fixture


Test the 2 whites with switch in on position one should have power that is the switch leg other is neutral leave the other two black wires capped.

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Old 02-02-2012, 02:04 PM   #3
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trying to figure out Knob & Tube wiring on old light fixture


Are you sure that is not BX? Looks like it to me. What you have is a switch leg most likely.
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Old 02-02-2012, 02:06 PM   #4
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trying to figure out Knob & Tube wiring on old light fixture


I did test out all four wires... both neutrals (white) have no live wire, the hot (black) do.

Could this wiring be connected to another switch? I do notice all the other switches in my place only have one neutral and one hot wire except...


My bedroom switch is located about 5 feet away front the hall switch (for the hall light) and it has the following wiring setup:

- 3 white neutral wires all capped together
- two black hot and one red wire capped together
- one black hot wire capped (was for the light switch) and a neutral wire spliced off the white wires was also attached to light switch.

I think I need an electrician to figure this out. All the wires in the bedroom switch except for the single black one are not hot, not sure how this works now.

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Old 02-02-2012, 02:07 PM   #5
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trying to figure out Knob & Tube wiring on old light fixture


sorry, what is BX?

I think I'm in over my head on this one, I have no idea how this works now
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Old 02-02-2012, 03:10 PM   #6
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trying to figure out Knob & Tube wiring on old light fixture


BX is a cable assembly with a metallic spiral wrap to protect the conductors that are inside.
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Old 02-02-2012, 03:15 PM   #7
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trying to figure out Knob & Tube wiring on old light fixture


Ahh, ok, gotcha.

No, the wires are def cloth covered, no metal at all for them.
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Old 02-02-2012, 03:17 PM   #8
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You would see the sheath outside the box, not on the conductors themselves.
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Old 02-02-2012, 03:22 PM   #9
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trying to figure out Knob & Tube wiring on old light fixture


I did see inside our one wall some of the wiring but I don't think it was metallic at all, I thought it was cloth covered all the way around it.
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Old 02-02-2012, 04:29 PM   #10
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trying to figure out Knob & Tube wiring on old light fixture


Quote:
Originally Posted by Jim Port View Post
BX is a cable assembly with a metallic spiral wrap to protect the conductors that are inside.
Jim, NM came about in 1930. It is also a possibility that this place may not have been wired until that time. I can date my home, due to we bought from the original homeowner, and there was a sticker on the old octopus. I found two different decades of NM, along with original BX/Greenfield in my home, with the BX being original.

As for OP, if you apply power to that circuit, with a no touch, while wire nuts or electrical tape is protecting the bare ends, do you get any voltage on the wires? If you have a helper, you can use a regular tester that will light when you have voltage, but need to find the circuit at what light switch.

Old homes at the most, depending on the size may have originally had four circuits, then during the years may have been split out, but lighting would still be on a original circuit. At that age, it is better to just start pulling new NM, since the wiring you have is pretty much at the end of its age, at being 86 years old.
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Old 02-02-2012, 04:54 PM   #11
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trying to figure out Knob & Tube wiring on old light fixture


Remember, Just because the wires are white doesn't mean they both act as a 'neutral'.

Sounds like switch loop.
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Old 02-02-2012, 06:06 PM   #12
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trying to figure out Knob & Tube wiring on old light fixture


That has to be the case in terms of a switch loop because there's no way this thing is working with two neutrals. Again, I'm not an electrician but I don't understand how this person wired it.

I tested out the wires for sure in the fixture. The white wires are def not hot, according to my voltage meter, but the black ones are. I just need to have someone take a look at this, I'm pretty confused (doesn't take much though when it comes to electrical things lol) but I'd rather err on the side of caution for something as important as this.

Thanks for everyone's help, I'll update this when I find out what the problem is (when the electrician comes by).

-Dale
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Old 02-02-2012, 06:16 PM   #13
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trying to figure out Knob & Tube wiring on old light fixture


Again, you have a switch loop. Unless the white wires are connected, you are not going to get any voltage through them.
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Old 02-02-2012, 06:29 PM   #14
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trying to figure out Knob & Tube wiring on old light fixture


In other words, in your scenario, one white wire will never test 'hot' with a meter. It is the grounded conductor (neutral). The other white wire should test 'hot' when the switch is ON. If neither white wire is hot with an ON switch, yet the light works, then you have gremlins. Gremlins are seldom resolved on this site.
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Old 02-02-2012, 07:04 PM   #15
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trying to figure out Knob & Tube wiring on old light fixture


Thanks for the help guys, I'll test it out in the morning again with the switch on.

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