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For example, I have a 12/3 cable in which the red and black wires are controlled via separate breakers. The 12/3 drops into a single-gang outlet, out of which runs a 12/2 to another room. Is there a word/term for this (joined circuit, sistered circuit)?

I'm looking for a polite term ;)

(EDIT: Changed /4, /3 to /3, 2 - all cables mentioned contain grounds)

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When you say "12/4 cable" and "12/3" do you mean 12/3 with ground, and 12/2 with ground? –  Tester101 Nov 7 '11 at 19:51
    
@Tester101 - Yes, exactly. –  overslacked Nov 7 '11 at 20:12

2 Answers 2

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From Wikipedia

Split phase shared neutral (North American Wiring)

In split phase house wiring, for example, a duplex receptacle in a kitchen is typically connected with a cable that has three conductors, in addition to ground. The three conductors are usually colored red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from a ganged breaker, i.e. a breaker in which the handles are tied together for a common trip, so that if one kitchen appliance malfunctions and pops the breaker, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit.

Notes:

These types of circuits must use a double pole circuit breaker

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While the NEC recognizes this configuration, there is open debate as to weather or not this is a safe practice. For more information see Understanding the Dangers of Multiwire Branch Circuits

EDIT

Multiwire circuits for 3 and 4-way switches

Switch legs, also called travelers must now be in a different color to be easily identified. A few years ago the white wire could be used as a traveler but code changed that to lessen the confusion of identifying the traveler. Now instead of running a piece of 14/2W/GRD or 12/2W/GRD for 3-way you have to use 14/3W/GRD or 12/3W/GRD. Same goes for 4 Way's also.

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This sounds like what I'm looking for, I think. A double pole breaker is not used however. This is original wiring (1967). I don't know whether this was acceptable then but hopefully the terms will make it easier to search. Thanks! –  overslacked Nov 7 '11 at 23:05
    
I've seen this used to go from a new breaker box, to where the fuse box used to be. –  Brad Gilbert Nov 8 '11 at 6:26
    
@overslacked this wiring technique goes back a long way, and was originally used to reduce the amount of wire needed (lowering installation cost). –  Tester101 Nov 8 '11 at 21:23
    
@lqlarry, that edit probably makes more sense as a comment. –  BMitch Nov 27 '11 at 19:51

It sounds like a poor man's 220. Are the breakers next to each other vertically in the box? If so, then black is carrying 110 from one line, and red is carrying 110 from a second line.

Strictly speaking those lines should be fed into a single double throw breaker which covers two posts in the box.

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It's not ever used as 220 - just two 110s that happen to be in the same sheath most of the way.... –  overslacked Nov 7 '11 at 23:00

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