shared neutral wire

Old 12-30-02, 02:53 PM
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shared neutral wire

I am upgrading the electrical circuits in my home, and there are 2 older circuits that are wired in a way I have never seen before. A 3 conductor cable (similar to Romex, but with a cloth outer jacket instead of plastic) goes from the main panel to a sub panel. In the main panel the 3 wires, black, white and red, are connected as follows: white to the neutral bus bar, black to a 15 amp breaker, and red to a 15 amp breaker. In the sub panel, there are 2 BX cables, each with a black and a white wire. The 3 white wires are connected, and the red from one of the breakers is connected to the black wire of the first branch circuit, and the black from the other breaker is connected to the black to the second branch circuit. This means that the 2 branch circuits are sharing the same neutral wire. Is this a method that was once allowed? It seems unsafe.
I could run another 3 wire cable to the sub panel, and give each branch circuit its own neutral wire. I thought it might be a good idea to wrap the red wire of the existing cable with green tape at both ends and use it as a grounding wire between the main and sub panels. Is this the right solution?
Old 12-30-02, 05:05 PM
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You have described a multiwire circuit. They are legal as long as the circuit breakers are conencted to different hots in the main panel. If these two circuits serve a common receptacle, one circuit on the top and one on the bottom, then the two individual breakers need to have their handles tied together with an approved connector, so they trip together.
Old 12-30-02, 06:05 PM
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It sounds to me like a circiut that I once heard created a disaster. Remember that between the 2 hots, since they are different legs, you get 220V. If you should lose the neutral connection at the tie point but the two wires are together, you could possibly get 220v at your outlet which will destroy everything that is plugged into it. It sounds strange, but think about it. There would have to be something plugged into each outlet when it happened and it would series from hot-through the appliance-through the neutral-back out the other neutral-through the next appliance-to the next hot. It may not be exactly 220V but surely enough to do some dammage. I believe that HandyRon is correct that it was legal, but I have heard of people loosing equipment because of it. Now I admit that I'm not a registered Electrician and Ron may very well know something that I don't, but I would add the extra wire if it were me.
Old 12-30-02, 10:07 PM
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Thanks for your quick replies. I'm not sure if any of the receptacles on either of those 2 circuits have one half on one cicuit and one half on the other. I'll check that out tomorrow and proceed from there. I think to be on the safe side, I'll run another cable to the sub panel and have the circuits seperate from each other.
Old 12-31-02, 05:20 AM
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It is perfectly legal and still used. It is called a multiwre circuit. It main purpose is to save on wire. One 14/3 is cheaper than 2 14/2 wires. There is no reason for to run new wire.
The problems decribed above are possible but not likely if the wiring is connected properly. The box you describe as a sub panel is only junction box unless it has some breakers in it.
Old 12-31-02, 07:48 AM
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Thanks, I looked through one of my books on wiring and found this situation described as a 3 wire circuit. The purpose of using this method is to save on materials, as you said. And you are right, it is not a sub panel, just a very large junction box. Since it is only about 15 feet from the main panel to the junction box, I'm going to run a section of conduit and new wires to it and separate the 2 circuits, this should suffice until I can figure out a way to get new wires into the rooms that these circuits feed. I have to say that I am pleasantly surprised and delighted with how fast I got answers to my question on this forum. Thanks again to all of you who replied.

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