Mystery of the Day

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Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
I had a service call to check some switches & lights in a bathroom. The bath has a 3G box with three SP switches. There are three lights, one over the tub, one over the vanity, and one over the stand-alone shower. The HO say/swear that each switch used to operate one light. I never saw this but pretty sure they were telling the truth.
So now, when you turn on either of the first two switches, both the tub & vanity light come on. The shower light still works as it should.

I took the plate off the switches and checked the wiring. All is correct in the box.
Hot comes in on a 12-2 NM cable
Each switch has a pigtail to the hot
Each switch has a 12-2 feed out to the lights
So (4) NM cables in the box total

I removed the feed wires off the two switches that were both working the same two lights. There is continuity between the two wires, thus why both lights work from either switch. I took the neutrals apart and checked them but found no continuity between them.

Further info: the house is a log house so I couldn't look in the ceiling or walls. Also the lights are can lights and I couldn't get to the wiring there either.

I have a theory (actually two) about what is causing this. But I wanted to hear other opinions to see if what I'm thinking is correct. Anyone care to comment on this?
 
12-2 to a single light? yikes.

disconnect feed ckt from switches.
take the bulbs out.
ohms test between each light hot in the 3G.
what do you get?
 
In addition to looking for continuity between the switched black wires, I'd undo the EGC and Neutral and check for continuity between the neutrals, and then the EGCs, of the pair of 12/2s that head off to the vanity and tub.

To my mind, the question is: just how many conductors the "pinch" or "nail" or "screw" is affecting.
 
If this is a real log house, sometimes the holes in the logs for the wiring becomes misaligned due to uneven shrinkage of the logs. That can cut cable, and also squish it together. If you don't find anything else that would cause your problem, you may want to ring out the cables in the logs.
 
Little Bill said:
Further info: the house is a log house so I couldn't look in the ceiling or walls.
Click to expand...


So you didn't bring your chain saw.:lol:

I would agree with Gold Digger. Nail or screw shorting out the two switch legs.

Need to know more about the lay out of the switch box and other bath fixtures to get any idea where they would have used a long screw.

Could try to use your no contact voltage tester to spot a energized screw.
 
K8MHZ said:
If this is a real log house, sometimes the holes in the logs for the wiring becomes misaligned due to uneven shrinkage of the logs. That can cut cable, and also squish it together.
Click to expand...

Could make it a little hard to get another cable in there, may need that chain saw after all. :happyyes:
 
My thoughts were either:
Nail or screw through both cables
Both cables pinched under a log

House settling would be why it just showed up after working for a few years.
 
al hildenbrand said:
In addition to looking for continuity between the switched black wires, I'd undo the EGC and Neutral and check for continuity between the neutrals, and then the EGCs, of the pair of 12/2s that head off to the vanity and tub.

To my mind, the question is: just how many conductors the "pinch" or "nail" or "screw" is affecting.
Click to expand...

I stated in the OP that I checked the neutrals and found no continuity with them. I didn't check the EGCs.
 
Little Bill said:
My thoughts were either:
Nail or screw through both cables
Both cables pinched under a log

House settling would be why it just showed up after working for a few years.
Click to expand...

If so, it's lucky that only the two switched conductors were affected. So far, anyway...
 
To test your theory swap the neutrals and hots on those two switches. (will also fix the problem temporarily) Might be better to have a screw in the neutrals than the hots.
 
GerryB said:
To test your theory swap the neutrals and hots on those two switches. (will also fix the problem temporarily) Might be better to have a screw in the neutrals than the hots.
Click to expand...

I'm not getting what you're saying here. The neutrals are all tied together in the box, and as I said, I checked the neutrals (after separating them) and there was no continuity. What I did do was leave the feed/switch leg off of one switch. I thought that it may at least tell me which wire was "robbing" from the other if one of the lights stopped working.

The HO is going to remove some wall boards upstairs above the bath to see if we can see the wiring from there. There is an empty space around a dormer behind the wall. All I know is I could see the wiring around the gap in the box and it went straight up which would be close to the area he is going to expose.
 
I would ask if any work has been recently done where a screw or nail could have penetrated through the cables. If the cables are run through bored holes and not stapled down, wiggling the cables may free up where the hots are touching.

if it was allowed by code, you could just switch the hots with the neutrals since the neutrals are connected anyway in the box, and the neutral wires do not appear compromised. It might be worth doing just for test purposes...
 
GerryB said:
To test your theory swap the neutrals and hots on those two switches. (will also fix the problem temporarily) Might be better to have a screw in the neutrals than the hots.
Click to expand...

I missed your post before I typed mine. You also have to swap them at the lights so the shell holders are not fed by the hots... of course, for test purposes only...
 
JFletcher said:
I missed your post before I typed mine. You also have to swap them at the lights so the shell holders are not fed by the hots... of course, for test purposes only...
Click to expand...
If it is truly a diagnostic test, no need to swap. But if it is an extended time :angel: run test to make sure there are no other problems, then I would tape the ends of the wires the right color and swap them at both ends. Be sure to tie the "neutrals" together so that the fault is not carrying normal current!
 
JFletcher said:
I would ask if any work has been recently done where a screw or nail could have penetrated through the cables. If the cables are run through bored holes and not stapled down, wiggling the cables may free up where the hots are touching.

if it was allowed by code, you could just switch the hots with the neutrals since the neutrals are connected anyway in the box, and the neutral wires do not appear compromised. It might be worth doing just for test purposes...
Click to expand...

Took me a minute but I think you are saying:
Take the neutral from the feed to light and put it on the load side of switch
Take the switch leg black and tie it to the neutral(s)
Do the same for the other light
 
Little Bill said:
Took me a minute but I think you are saying:
Take the neutral from the feed to light and put it on the load side of switch
Take the switch leg black and tie it to the neutral(s)
Do the same for the other light
Click to expand...

Yes. If the wire cannot pulled out and fixed/re-run properly, then swapping the wires would at least get the lights working independently again. the fault would be on the neutral side, not the hot. of course, it is not code compliant to remark that small of an ungrounded conductor as a neutral, but that is a technical violation and not a safety issue... the fault between ungrounded conductors maybe much more a safety issue
 
JFletcher said:
Yes. If the wire cannot pulled out and fixed/re-run properly, then swapping the wires would at least get the lights working independently again. the fault would be on the neutral side, not the hot. of course, it is not code compliant to remark that small of an ungrounded conductor as a neutral, but that is a technical violation and not a safety issue... the fault between ungrounded conductors maybe much more a safety issue
Click to expand...

No problem, I won't violate by remarking, I just won't remark!:D
 
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