leaking voltage on mwbc

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GerryB

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I ran a 12-3 to the exercise room in this new house I am working in. At first my guy put in 2 sp breakers as we didn't have a 2-pole. Before I changed it yesterday I wanted to flip each individual breaker for labeling purposes. Turned one on, ok, hit the other, dead short. I figured I must have spliced the red to the black somewhere, and that was the case. Fixed it, but in checking the power with my fluke with one breaker on I was reading 44 volts on the one that was off, in the room and at the panel. The other leg was reading 36 volts with one breaker on, the other off. Turn both on and everything is perfect, also with the 2-pole I put in. Usally when I have run into this it's a neutral problem, but this has me confused. Unoccupied house, no load on the circuit. Any answers?
 

GoldDigger

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Location
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Retired PV System Designer
I ran a 12-3 to the exercise room in this new house I am working in. At first my guy put in 2 sp breakers as we didn't have a 2-pole. Before I changed it yesterday I wanted to flip each individual breaker for labeling purposes. Turned one on, ok, hit the other, dead short. I figured I must have spliced the red to the black somewhere, and that was the case. Fixed it, but in checking the power with my fluke with one breaker on I was reading 44 volts on the one that was off, in the room and at the panel. The other leg was reading 36 volts with one breaker on, the other off. Turn both on and everything is perfect, also with the 2-pole I put in. Usally when I have run into this it's a neutral problem, but this has me confused. Unoccupied house, no load on the circuit. Any answers?

If your Fluke is a high impedance meter, then with the breaker open and no loads connected to the circuit that is off, that voltage could be just capacitive coupling between the red and black wires running next to each other for a long distance.
You will measure the same voltage at either end of the open wire.

Try plugging a small (20 watt) incandescent lamp into the circuit and turning it on. The ~40 volts should go immediately to zero, or within measurement error of zero.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
I ran a 12-3 to the exercise room in this new house I am working in. At first my guy put in 2 sp breakers as we didn't have a 2-pole. Before I changed it yesterday I wanted to flip each individual breaker for labeling purposes. Turned one on, ok, hit the other, dead short. I figured I must have spliced the red to the black somewhere, and that was the case. Fixed it, but in checking the power with my fluke with one breaker on I was reading 44 volts on the one that was off, in the room and at the panel. The other leg was reading 36 volts with one breaker on, the other off. Turn both on and everything is perfect, also with the 2-pole I put in. Usally when I have run into this it's a neutral problem, but this has me confused. Unoccupied house, no load on the circuit. Any answers?
It sounds like you're using a digital multimeter with a high impedance input that is capable of measuring induced extremely low power voltages. Doing the same voltage tests, as you described, with a meter that has the old d'Arsonvol movement, or a plain old wiggy solenoid voltage tester, with their corresponding low impedance inputs, will not show the "phantom" voltages.
 

GerryB

Senior Member
It sounds like you're using a digital multimeter with a high impedance input that is capable of measuring induced extremely low power voltages. Doing the same voltage tests, as you described, with a meter that has the old d'Arsonvol movement, or a plain old wiggy solenoid voltage tester, with their corresponding low impedance inputs, will not show the "phantom" voltages.

Thanks to you and Golddigger. So It is induced voltage from running next to each other. This would be the same reason the little tic testers don't work well on 3-wire circuits?
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
. . . the little tic testers don't work well on 3-wire circuits?
I'm not sure what you mean by "tic tester".

Think about the high vs. low impedance this way: The moving needle d'Arsonvol or the solenoid testers require enough power to operate that they effectively short out the induced voltage on a multiwire circuit before they can be deflected to a "reading". The induced voltage has so little power in it that there is no safety issue with the short. However, a digital multimeter has such a high input impedance that it doesn't draw enough current to "short out" the induced voltage so the electronics of the meter, using the meter's internal battery, put up a numeric display of the "voltage".
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
I'm not sure what you mean by "tic tester".

Think about the high vs. low impedance this way: The moving needle d'Arsonvol or the solenoid testers require enough power to operate that they effectively short out the induced voltage on a multiwire circuit before they can be deflected to a "reading". The induced voltage has so little power in it that there is no safety issue with the short. However, a digital multimeter has such a high input impedance that it doesn't draw enough current to "short out" the induced voltage so the electronics of the meter, using the meter's internal battery, put up a numeric display of the "voltage".

Another term for non contact tester.:)
 
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