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Old 12-04-2010, 05:48 PM   #1
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12/3 mwbc


I am troubleshooting a potential problem on MWBC. The circuit consists of 12/3 Romax (red, black , white, ground) connected to a split outlet. Each hot conduct was connected to separate 20 A circuit breakers while the white and ground are connected to the neutral bar. When the black wire circuit breaker is turned on and red wire circuit breaker off, my pen voltage indicator detected voltage on the red conductor. I measured the voltage on the red line with a meter and found 40 volts and 120 volts on the black wire. I disconnected the wires from the outlet and this had no effect. I have also hooked up a separate 12/3 wire in the same matter and found the same results. IS THIS NORMAL?


I realize the latest code calls for a 2 pole breaker in this application

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Old 12-04-2010, 05:51 PM   #2
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12/3 mwbc


what kind of meter? An inexpensive digital meter by chance?

It is likely you are reading what a lot of guys call ghost or phantom voltage. It is induced voltage from the wire running next to it.

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Old 12-04-2010, 05:53 PM   #3
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12/3 mwbc


Plug a load into the other circuit, such as an incandescent lamp. Your phantom voltage will disappear.
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Old 12-04-2010, 06:26 PM   #4
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Thanks for your thoughts. Meter is fluke 87
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Old 12-04-2010, 07:48 PM   #5
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12/3 mwbc


I confirmed that the voltage on the red line went ~0 once a small load was added to red circuit.
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