Lighting circuit with switched neutral?

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
Helped a little bit today during a re-wire and came across a scenario involving a switch loop for an outside light. Just one single nm cable bringing power down to the switch than the switch leg back out. I assumed anyhow.

BUT while testing for voltage I realized I only got voltage on the cable when the light was turned OFF. I’m assuming someone accidentally switch the neutral instead of the ungrounded conductor.

Would That make sense being that while the switch is on all is well it being a neutral conductor carrying current, but than when it is opened there is no voltage drop due to no load.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
Where and how were you measuring voltage?
Well at first just with a ticker, than I brought out a multimeter and had 120v from from one side of the switch to EGC. It was all tapped up and never read from the other side of the switch I was getting ready to leave so didn’t dive in to much. But that’s the only thing I could thing of was it opening the neutral because that’s the only time I had voltage present was when the light was turned off.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
If you were trying to measure to each side of the switch, you would read voltage with the switch off because you would have a potential difference between the two points. You would show no voltage with the switch on as there would be no potential difference. Does the light work?
You need to measure from a known good ground or neutral. Like taking an extension cord and plugging it in to a known good receptacle. Take one lead and insert it in the ground or neutral of the cord and the other lead to each of the conductors on the switch. With the switch on, you should read voltage on either side of the switch, with it off, you would read voltage to only one side.
If you are truly dealing with a switched neutral you would get no or little reading using the extension cord' neutral or ground slot.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
If you were trying to measure to each side of the switch, you would read voltage with the switch off because you would have a potential difference between the two points. You would show no voltage with the switch on as there would be no potential difference. Does the light work?
You need to measure from a known good ground or neutral. Like taking an extension cord and plugging it in to a known good receptacle. Take one lead and insert it in the ground or neutral of the cord and the other lead to each of the conductors on the switch. With the switch on, you should read voltage on either side of the switch, with it off, you would read voltage to only one side.
If you are truly dealing with a switched neutral you would get no or little reading using the extension cord' neutral or ground slot.
The light does work. The ground I was using was good from an outlet I just installed because the switch loop is old and has no EGC. I know what you mean about it off you will have 120v from line to load side of the switch and 0volts across terminal when’s it on because they are both energized 120v. What I don’t understand at all is why with the switch turned on I got no measurement (0volts) from terminal to the known good EGC. (don’t know wether it was line or load).

Than when I turn switch off ( light off as welI) I have a reading with the ticker as well as 120volts from the switch to the known EGC.

switch on- from terminal to EGC (0volts)
Switch off- from terminal to EGC (120volts)
 
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Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Try the neutral from the outlet you installed. If you are dealing with a switched neutral, you should get no reading on either side of the switch. A tick/noncontact tester shouldn't pick up the neutral at all. Well it may if you also are very close to the hot.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
Try the neutral from the outlet you installed. If you are dealing with a switched neutral, you should get no reading on either side of the switch. A tick/noncontact tester shouldn't pick up the neutral at all. Well it may if you also are very close to the hot.
If I end up going back I will. But if it was a switch opening the neutral while In the off position than wouldn’t the one side of the switch have a 120volts to ground.Than while on it would be 0volts to ground.

Because if it were a correctly wired switch loop a non contact ticker should buss continuously on the cable. Wether switch is on or off right. And in my case it was only detecting voltage when the switch and light were off.
 

markebenson

Senior Member
Location
fl
This is not relevent but the subject reminds me of the old ferris wheels and skating rinks in the 80s with hundreds of light bulbs where the hots and neutrals were sequentually switched to make cool patterns.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
If you create an open neutral, it can most certainly beep on the lead coming through the load
9C632BFF-C6D0-4BEB-AC8A-0217939BE783.jpeg
Just imagine the switch on the neutral rather than the ungrounded conductor. It appears to me while the switch is closed there will be no voltage. When is opened it will be the same as a “loaded” neutral if anyone has ever gotten into that before they understood how a series circuit works like I have before.
 
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James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
View attachment 2555544
Just imagine the switch on the neutral rather than the ungrounded conductor. It appears to me while the switch is closed there will be no voltage. When is opened it will be the same as a “loaded” neutral if anyone has ever gotten into that before they understood how a series circuit works like I have before.
Exactly right.

The house I live in has switched neutrals. And in the bathroom, I rebuilt a flush mounted fixture with an LED kit.

When the switch is off, the light is on, very dim.
There's voltage coming across, and it will measure at the switch in the off position. I haven't put a meter on it, but it's plain to see the effect
 

Attachments

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Exactly right.

The house I live in has switched neutrals. And in the bathroom, I rebuilt a flush mounted fixture with an LED kit.

When the switch is off, the light is on, very dim.
There's voltage coming across, and it will measure at the switch in the off position. I haven't put a meter on it, but it's plain to see the effect
I have never dealt with “on purpose” switched neutrals..
Wouldn’t it be an easy fix though just to change it to hot switched by changing the loop connections at the light and switch?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Switched neutrals were not uncommon in knob-and-tube wiring. I don't know whether it was intentional or just a side-effect of re-feeding existing wiring during service upgrading.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I have never dealt with “on purpose” switched neutrals..
Wouldn’t it be an easy fix though just to change it to hot switched by changing the loop connections at the light and switch?
Think K&T. Sometimes there may be an old Romex two wire here and but I wouldn’t count on it being that simple. You most likely have only the neutral at the switch. Then add in the 3 way variants.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Think K&T. Sometimes there may be an old Romex two wire here and but I wouldn’t count on it being that simple. You most likely have only the neutral at the switch. Then add in the 3 way variants.
Ahh.. OK... got it now.
My K&T experience is looking at it, and one job cutting it all out on a whole house remodel. Never adding to it or working on it..
 
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