Outlet stops working when light is on

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Old 11-09-18, 03:15 AM
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Outlet stops working when light is on

This is the weirdest electrical issue I've ever seen.

My Mom recently noticed that her bathroom outlet wasn't working*. I plugged a night light into it to test it, and discovered that the outlet works when the overhead bathroom light is off. It stops working again when the light is turned on at the switch.

But here's the really freaky part: the overhead light is on a different line than the outlet. When I flip the breaker for the overhead light, the outlet works fine. When I flip the breaker for the line that the outlet is on, the overhead light works fine.

But when both circuits are live, the outlet stops working when the overhead light is on. No circuit breaker trips, the outlet just stops working until I turn the overhead light off again. Apparently it's been like this for years - nobody's touched the electrical in there for at least 20 years.

How could this be possible? I can't get my head around it. All I can think is that somewhere in the house, the hot wire of one circuit is connected to the neutral wire of the other, so they're phase cancelling when the two lines are both active...but that's just a wild guess.

We're in New Orleans. People find all kinds of bizarre ways to screw up electrical circuits down here, but this is a new one on me.

Any help would be most appreciated - if anyone's seen anything like this before I'd love to hear about it.

* she had a regular grounded, non-GFI outlet when I discovered this weird problem, so I replaced it with a GFI outlet but that didn't change the weird behavior that I just described.
 
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Old 11-09-18, 04:24 AM
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Tell us about all the wires at the receptacle, swotch, and light.
 
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Old 11-09-18, 05:02 AM
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Okay:

The outlet box only has one Romex cable going into it, which is properly wired to the outlet. I don't know where the other end of this cable connects though, so it's possible that the hot wire and the neutral wire are switched before they get to the outlet.

The light switch has three Romex cables going into it:
1.) The line power (which has an odd 2-3VAC current when that circuit breaker is off, for an unknown reason)
2.) The line to the overhead light
3.) A second load out line which feeds a bunch of other outlets in the adjoining room.

The black power line wire goes to the switch, which has two terminals:
- the black load out is coupled to the same terminal as the black line in so the outlets in the adjoining room are always on.
- the black load wire to the overhead light is on the other side of the switch.

The three white neutral/common lines are all joined and capped.

The three ground wires are all joined together and capped.
 
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Old 11-09-18, 05:55 AM
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has an odd 2-3VAC current when that circuit breaker is off, for an unknown reason)
Bet yiur using a digital multimeter. That's just ghost (induced) voltage. What is the wiring at the light?
 
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Old 11-09-18, 06:37 AM
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Bet yiur using a digital multimeter. That's just ghost (induced) voltage.
Yep - I'm using a Fluke multimeter. So I guess the line is running close to a different line someplace in the walls, interesting.

What is the wiring at the light?
I don't know - The fixture is old and mounted to the ceiling without any obvious way to take it off to look, so I didn't. But I ran a resistance test with the light bulb taken out and that opened the circuit, and then I tested the resistance with the bulb in and it read a few Ohms, so I figured everything was fine.
 
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Old 11-09-18, 11:07 AM
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When common devices are seemingly controlled by two breakers...... it usually involves a MWBC (multiwire branch circuit). That's where two circuits share one common neutral.

Are the two breakers controlling these items one above the other in the panel ?
Can you open the panel to see what color wire is on each breaker ?
 
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Old 11-09-18, 12:54 PM
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Originally Posted by PJmax View Post
When common devices are seemingly controlled by two breakers...... it usually involves a MWBC (multiwire branch circuit). That's where two circuits share one common neutral.

Are the two breakers controlling these items one above the other in the panel ?
Can you open the panel to see what color wire is on each breaker ?
I'll have to make a trip across town to open up the panel and see what color the wires are, but I do recall the placement of the two circuit breakers: the breaker that supplies the outlet is at the very bottom on the right side, and the breaker than supplies the light is the second one from the bottom on the left side.
 
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Old 11-09-18, 02:10 PM
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OK. That means they are on opposite legs of the 240v service. They should be one above the other and have the handles tied together if they are a MWBC, We'll wait for your wiring confirmation.
 
 

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