Neutral wire going through a switch instead of hot

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  #1  
Old 07-18-10, 04:43 PM
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Neutral wire going through a switch instead of hot

I am replacing a light fixture in an old house that has relatively new electrical throughout but still has one remaining knob & tube lighting circuit serving the lights in several rooms.

I'm replacing one of those ceiling lights and noticed the neutral wire (instead of the hot wire) is running through the wall switch on it's way to the outlet box in this room. I determined this by using a voltmeter and putting the black probe into another circuit's grounded outlet and touching the red probe to each of the wires into the outlet box. One read zero, the other 120 volts. The one with zero volts is the one going through the switch. Am I correct then that the neutral is on the switch?

This is knob & tube wiring so both wires are black in color. The wires appear in good condition. Replacing the entire circuit, or even the wire to this one room's switch, would involve removing many walls/ceilings throughout the house which is obviously why no previous owner changed it.

My plan in replacing this light fixture is to tap into the K&T with a junction box and use NM cable for the run to this new light fixture.

The question I have is about the switched neutral. The obvious risk, I think, is that someone would turn the light switch off thinking there was then no energized wire at the light fixture. But, anyone changing a fixture should turn the breaker off anyway, not just a light switch.

If I label the outlet box, wires and panelbox circuit to identify the neutral is switched instead of the hot, is it ok to leave the neutral as the switched wire? Any other concerns I'm not thinking of?

Thanks for all the great advice from this forum.
 

Last edited by fromhollywood; 07-18-10 at 05:47 PM.
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  #2  
Old 07-18-10, 05:31 PM
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No neutral can be switched. It must be continuous and the hot switched. Hope it wasn't too simple an answer. You hit the nail on the head with your statement about the danger.
 
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Old 07-18-10, 05:33 PM
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It is certainly a code violation to switch the neutral wire. And it is a BAD IDEA!

I will let the pro electricians comment on "tapping into" K&t, because I am not sure you are allowed to do that. If you mess with this circuit, you may be required to bring the whole circuit up into the 21st century. That would be a GOOD IDEA.
 
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Old 07-18-10, 05:48 PM
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Was this the correct way to determine that the neutral is going through the switch?

I determined this by using a voltmeter and putting the black probe into another circuit's grounded outlet and touching the red probe to each of the wires into the outlet box. One read zero, the other 120 volts. The one with zero volts is the one going through the switch. Am I correct then that the neutral is on the switch?
 
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Old 07-18-10, 06:50 PM
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If you are 100% sure the black probe was touching an actual neutral. If your switch was messed up, I wouldn't rely on the other receptacles, etc. A tick tracer is helpful in determining the hot wire when they are separated from each other as your K&T is. Cheap, probably $7.
 
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Old 07-19-10, 12:55 PM
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What about reversing the wires at the panel?

If I determine that the 5 lights in this circuit all have switched neutrals, won't reversing the wires in the panel fix it?

In other words, I'll take that circuit's wire off the neutral bar and switch it with the one going to that breaker.

Won't that effectively make all the switches on that circuit then use the hot wire?
 
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Old 07-19-10, 02:08 PM
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It could. It could open another can of worms regarding receptacles, fixtures, and switch loops, so just be careful. Determine the hot wire first.
 
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Old 07-19-10, 02:29 PM
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I guess my idea of reversing the wires inside the panel won't work as one of the light switches on this circuit showed 120 volts at the switch which means it's switching the hot wire as it should be.
 
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Old 07-19-10, 05:13 PM
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IF the neutral is in fact switched, with the switch OFF you should get a 120 volt reading at one side of the single pole switch. Turn the switch on and check it again and you should get a very low voltage to ground reading on each side of the switch, near 0 volts. This would now be a neutral to ground reading. When you switch the hot wire, both screws on the switch will read 120 volts to ground when the switch is in the ON position.
 
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Old 07-19-10, 05:16 PM
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Originally Posted by fromhollywood View Post
I guess my idea of reversing the wires inside the panel won't work as one of the light switches on this circuit showed 120 volts at the switch which means it's switching the hot wire as it should be.
Not necessarily. That switch could also be switching the neutral and you could be reading 120 volts that is feeding through the fixture and lamp with the switch in the OFF position.
 
  #11  
Old 07-19-10, 08:41 PM
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Is this a 3-way? It may be a Carter 3-way. I've seen this twice in older homes. Look up Multiway Switching on Wikipedia.

Multiway switching - Wikipedia, the free encyclopedia
 
  #12  
Old 07-19-10, 11:14 PM
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Would the volt readings at the switch be as you say CasualJoe even though I'm touching the black probe of my clamp meter to a ground on another circuit? This circuit in question is knob & tube so there is no ground. I ran a 3-prong extension cord from a different, newer circuit (with a ground) nearby and used the ground hole on that cord while testing the old K&T circuit.

I had thought finding 120 volts anywhere on a switch meant that it was the hot wire at the switch.

It's not a 3-way switch ArgMeMatey.

Thanks!
 
  #13  
Old 07-20-10, 01:53 AM
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Originally Posted by 594tough View Post
That would be a GOOD IDEA.
And also very costly $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ wise. For K&T can you just remove the wires at the circuit breaker or fuse box then just do a fishing job with the new wires without ripping walls.
 
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Old 07-20-10, 04:51 PM
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Originally Posted by fromhollywood View Post
Would the volt readings at the switch be as you say CasualJoe even though I'm touching the black probe of my clamp meter to a ground on another circuit? This circuit in question is knob & tube so there is no ground. I ran a 3-prong extension cord from a different, newer circuit (with a ground) nearby and used the ground hole on that cord while testing the old K&T circuit.

I had thought finding 120 volts anywhere on a switch meant that it was the hot wire at the switch.

It's not a 3-way switch ArgMeMatey.

Thanks!
For test purposes, it makes no difference where you find a ground to test voltage. If you have even a little understanding of how electricity works, think about this carefully. Assume that your polarity is reversed and that the hot wires in the switch box are tied together instead of the neutral wires. Now, assume the two neutral wires are attached to the two screw terminals on the single pole switch. Now, with the switch in OFF position, won't the 120 volts flow through the light fixture and lamp (bulb) all the time? The lamp won't light up till the switch is turned ON, completing the neutral path back to the panel. Before you turn the switch ON, the 120 volts flows through the light fixture and lamp and back to the switch on ONE screw terminal. You can test the one screw terminal with your meter with one probe with the other probe to your ground on the extension cord. When you turn the switch ON and the lamp lights up, test that same screw terminal again with one probe with the other probe to ground and you will no longer see 120 volts. This means the neutral conductor is being switched.

Now, if the hot wire is switched, with the switch ON and the lamp lit full bright, you can test each screw terminal with your meter to ground and both screws will show a reading of 120 volts. This is the way it should be and will only allow 120 volts to flow through the fixture and lamp when the switch is turned ON.
 
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Old 07-20-10, 08:04 PM
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Also test with the lightbulb unscrewed this will tell you if the 120v is backfeeding thru the bulb or if your neutral is actually what is switched
 
  #16  
Old 07-21-10, 02:28 PM
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Thanks very much for the helpful suggestions, especially CasualJoe----you should write electrical books as you make things very clear.

I didn't realize that voltage could flow through a fixture and light bulb without the bulb being lit. I'm still a bit fuzzy on that concept. I guess that makes sense as you need a continuous circuit to get electricity to do it's job.

After reading your postings many times to get it into my brain, I retested two of the five switches on this old circuit and clearly have one that is switching the neutral (the one where I am replacing the fixture) and another that is switching the hot.

So, I can't use my idea of just switching hot & neutral back at the panelbox to get all the switches working with the hot wires.

I figured a way to use a 14-3 cable to get the hot to the switch on this one part of this K&T circuit so that should at least allow me to change the light fixture in this one room and know that I'm now switching the hot.

Thanks for the help.
 
  #17  
Old 07-23-10, 09:27 AM
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Originally Posted by fromhollywood View Post
I didn't realize that voltage could flow through a fixture and light bulb without the bulb being lit. I'm still a bit fuzzy on that concept. I guess that makes sense as you need a continuous circuit to get electricity to do it's job.
Even if it's not a 3-way switched light, you might want to look at the diagrams on the wikipedia page to get a general idea of how and why this was done.

I never had K & T. My old 1920s cloth and rubber BX was in good shape inside the walls, but at the devices it was flaking apart. We "made safe" some sections by disconnecting them but ultimately rewired the whole house. Most people won't do that due to cost and inconvenience, but I sleep better now.
 
  #18  
Old 02-12-11, 02:52 PM
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3 Ways

We used to call them _astard three ways...

In the knob and tube days they used the neutral for one of the travelers for the 3 way switches. Usually I would get a call when someone would replace their 3 way switches and they would start popping the breaker. As long as the insulation is still in good shape where it enters the boxes the installation is safe. They are separated with one wire on each side of the studs which makes distance your safegard, 120 volts is not going to jump 16".

Best of luck...
 
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