12-3 Cable Puzzle

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  #1  
Old 12-20-14, 09:33 AM
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12-3 Cable Puzzle

I pulled an electrical outlet out of a box in my dining room to find that it was powered by the red wire of a 12-3 cable. That red hot wire is connected to a 20 amp circuit breaker (#24 in my service panel). Inside the outlet box that red wire is also connected to an outgoing 12-2 cable that powers the rest of the outlets in the dining room. The black hot wire of the 12-3 cable is connected to another 20 amp circuit breaker (#26) right below breaker #24. That black wire is connected (inside the outlet box) to yet another outgoing 12-2 cable. I haven’t yet discovered what that cable powers. All 3 ground wires from the 3 cables are connected. All 4 white common wires (from the 3 cables plus one from the outlet) are connected.

Here’s the puzzle. Both the red and black hot wires from the 12-3 cable read 120v with a tester. But if I trip the #24 breaker to the red hot wire it still read’s about 40v. If I reset that breaker but trip breaker #26 then the black wire read’s about 45v. If I trip both breakers then they both read about zero. Is someone able to explain what’s happening here? The house is only 13 years old.

Making it more confusing is that breakers 24 and 26 (at least) appear to have been mislabeled. Written on circuit 24 is “1st floor bath”, but it doesn’t do anything to any of the bath fixtures or outlets. Also written on that breaker is a circle with two parallel back slashes running through the center of it from top right to bottom left. Then there’s an “S” written next to that symbol.

Circuit 26 is labeled as “kitchen” with no symbols, but it doesn’t power anything in the kitchen. I haven’t actually figured out what that does supply power for.

The main reason I want to figure out these cables and the confusing voltage readings is because I want to run yet another cable out of that outlet box to supply power to an outlet and light I’m installing in a nearby cabinet.

Hope someone can help!
 
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Old 12-20-14, 09:50 AM
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The answer is probably at the other end of the unidentified "outgoing cable"
 
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Old 12-20-14, 09:54 AM
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As my diagram and text explain.... you have a multiwire branch circuit. Code now requires the circuits to be connected to a 2P circuit breaker so that both legs are off when being worked on.

The 40v you read was from the other leg in the cable still carrying 120v. It's an inductively picked up voltage.

Make sure with this type of circuit.... and especially since you are working at the point where the circuit splits.... the neutral connection is extremely important. Take you time and make sure it's solid.
 
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Old 12-20-14, 01:10 PM
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Thank you. But when I tested the voltage of the 12-3 hot wires they were entirely disconnected from the other two outgoing cables so I didn't see how those could've been contributing to the readings.
 
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Old 12-20-14, 01:21 PM
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Thank you very much for the diagram and your explanation! All of the neutrals are definitely connected via one wire nut, and I'll be sure it remains that way. So you're saying the 40v or 45v readings are just extraneous readings as a result of one wire being next to a hot wire in the same cable? Interesting. Having those 2 breakers connected is a good idea. I was actually working in that box for a time with one hot wire and didn't realize it. I've since marked the wires for clarity. What's the name of the piece that I can buy to connect both switches?

So was a 12-3 cable used just as an efficient convenience in this case, instead of running two 12-2 cables to the same area of the house? I'm a novice, but I've never seen that before. I've seen other applications, but not that one.
 
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Old 12-20-14, 02:11 PM
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So was a 12-3 cable used just as an efficient convenience in this case, instead of running two 12-2 cables to the same area of the house?
That's it..... exactly.

For some breakers you can buy snap on handle ties. You could also install a two pole breaker in place of both of those singles and keep them for spares.
 
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Old 12-20-14, 07:23 PM
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Alright, I appreciate the help!
 
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