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My new house is done being built and I am going to come in and install my new gas range and dishwasher, both 120V 15A appliances.

I got thoroughly confused looking at the breaker box because gas range and dishwasher were labeled on different circuits but at the breaker the electrician installed a double pole breaker. I traced the two ungrounded conductors to two separate wires coming out of the box, telling me it is really two separate circuits(EDIT: On second thought I did see a red wire in the one end so I must not have traced that right. It probably is going to one 14/3 cable now that I think about it).

Clearly if the gas range or the dishwasher were to trip the breaker then it immediately trip the other circuit as well. I am curious why it was done this way.

Is this a new NEC code that I am unaware of? Could this be a regional code of some kind? Are there any safety concerns for why the electrician decided to do this over using two separate 15A breakers?

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Do the circuits share a neutral? When you say "I traced the two ungrounded conductors to two separate wires coming out of the box" did you mean "two separate cables"? Are you sure they are labeled properly? –  Tester101 Dec 13 '12 at 20:00
    
@Tester101 I am sure they are labeled properly, when I swich the breaker off I lose power to both appliances and only those appliances. I am editing to provide the additional detail. –  maple_shaft Dec 13 '12 at 20:04
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If the circuits are sharing a neutral, it's called a Multiwire Branch Circuit. If this is the case, National Electrical Code (NEC) requires the circuit to be wired this way. There are already a few questions on the site that discuss these types of circuits, so this should probably be closed as a duplicate. –  Tester101 Dec 13 '12 at 20:24
    
Well you are the moderator so thats your call ;-) –  maple_shaft Dec 13 '12 at 20:27
    
maple_shaft, if you confirm there is a shared neutral, then you have a MBC and there's nothing new about it. If it is not a shared neutral, then it would be curious. Can you ask the electrician? –  Philip Ngai Dec 13 '12 at 23:17

2 Answers 2

up vote 2 down vote accepted

I'm guessing the circuits are sharing a neutral, and so are considered a multiwire branch circuit.

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

In the 2008 version of the National Electrical Code (NEC), they introduced sub section (b) to section 210.4. This sub section covers the disconnecting means of a multiwire branch circuit, and requires a simultaneous disconnect of all ungrounded conductors.

NEC 2008
210.4 Multiwire Branch Circuits.
(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

This subsection can be satisfied by using a multipole breaker, which provides both simultaneous trip and simultaneous disconnect. The subsection can also be satisfied by simply attaching handle ties between multiple single pole breakers, which would provide simultaneous disconnect (and may or may not provide simultaneous trip).

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It should also be noted that the shared neutral is used in order to reduce the amount of wire used and thus reduce costs. Without the shared neutral, six conductors would be run to the pair of appliances. With a shared neutral, only four are. The grounds can easily be shared since they normally carry no current. The neutrals can be combined without increasing the wire size because the two hot wires are 180 degrees out of phase with each other, causing the neutral to carry no current when both appliances are on. –  Pigrew Dec 19 '12 at 15:21
    
Well, strictly speaking, we're talking sharing a neutral between 2 120V AC, 3-wire "Edison" branch circuits derived from the opposite poles of a single phase step-down transformer on the pole outside your house. The neutral can be shared because it ties back to the center of the transformer coil, while the hot wires go back to the two poles of the transformer coil and are indeed 180 deg out of phase with each other. Connect the two hot leads and you get 240V. Connect either one to the neutral and you get 120V (half the transformer windings, half the voltage, essentially). So... (next comment) –  Craig Nov 24 at 3:34
    
...the out of phase poles cancel each other out. The neutral doesn't carry zero current. It carries the imbalance between the loads on the two shared circuits. Imagine the dishwasher pulling 6A, and the gas range pulling 3A. The neutral will be carrying 3A. 3A (the range) cannot totally cancel out 6A (the dishwasher). In fact, since the dishwasher is probably always going to be pulling more current than the range (motor and heating element, versus, what, a clock and a pilot light?), the neutral will always have some current on it if either or both appliances are on. –  Craig Nov 24 at 3:35

Am I totally off base in suggesting a terminology issue? a pair of breakers tied together is called a "tie-bar breaker" not a "double pole breaker." This is not to say that the breaker in question is not connected to both poles(Hot A and Hot B). Historically, in CA, dishwasher and disposal were installed on a tie-bar breaker, separate from all other kitchen circuits.

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