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In 2001, some electricians added two circuits to my house in California. I took a close look at their wiring and had some questions.

At the Service Panel, there are two 20-amp circuit breakers on the left side of the panel. Both breakers are connected to the same 12/3 Romex cable, with the red wire attached to one breaker and a black wire attached to a second breaker. The two breakers are not tied together.

The 12/3 cable feeds into the a junction box in the crawlspace where it connects to two 12/2 Romex cables. The red wire on the 12/3 cable attaches to a black wire on one 12/2 cable which provides power to my living room, and the black wire is connected to the black wire on a second 12/2 cable which provides power to a bedroom. Thus, the two 12/2 share a neutral.

This wiring configuration is confusing. Is this a "Shared neutral" situation? I am pretty sure that this was inspected in 2001, but is this configuration allowed by modern code? Was this allowed in 2001? What are the hazards of this sort of configuration? Does this present a hazard to the receptacles in the living room and bedroom?

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3 Answers

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Typically the connection of 12/3 to two side by side breakers (black on one, red on other) is used to run a 220v line. A 220v line has two hots - each carrying 110v - pulled from adjacent breakers on the panel. In such cases, the single white line is the 220v Neutral.

Speaking electrically : What this guy did was use 12/3 to carry 220v to a junction box where he divided it back up into 110 - which should be fine because in reality the 12/3 is two parallel 110v lines, with a single neutral. The single neutral isn't really a problem here - ultimately all lines share a single neutral back at the breaker box. In your case, the shared neutral stops at a junction box - and the lines to the outlets are on their own neutral lines.

Are there any hazards to the receptacles? No, as long as they're wired properly its no different than connecting the neutrals back at the neutral bus bar.

Speaking in terms of CODE and what's allowable - I honestly don't know if it's legal or not, but I cannot think of a reason why it wouldn't be. The junction boxes are accessible, and using 12/3 to carry 220v is legit, and each outlet is wired to 110 properly.

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I'm not thinking too clearly right now, but if I were to turn off the living room breaker, to work on it, could the neutral somehow become charged via the bedroom? I'd like to think the easiest path would be to the neutral bar of the box, and everything would be alright. –  Chris Cudmore Aug 27 '12 at 18:38
I may have to delete this answer - let me think a minute... –  The Evil Greebo Aug 27 '12 at 18:39
No, I've diagrammed it out to be sure. I'm sure it's no different than wiring both neutrals to the common bar in the box. It's just a question of where the neutrals come together - either in one junction box or in another, they ultimately all still come together somewhere. –  The Evil Greebo Aug 27 '12 at 18:49
Yeah, I came to that conclusion as well. I'm going to leave the comment, as it illustrates a thought process. –  Chris Cudmore Aug 27 '12 at 19:08
Its different if that neutral somehow gets disconnected from the common bar. Then you have >120V on both outlets. Or both sill powered (vs. ground only), despite one of the breakers off. –  derobert Aug 30 '12 at 17:43
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A "multi-wire branch circuit" typically has the breakers joined so that if someone turns off one breaker to work on the circuit, they don't miss the second circuit going to the same box. Since you branch off into a 12/2 for most of the circuit, I'm not sure if the code would allow separate breakers (hopefully Tester101 will chime in). But at the very least, I'd put a label on the crawlspace junction box that says "warning: multiple circuits".

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Thanks for highlighting one of the hazards. Some electricians recommend against multiwire branch circuits because they can lead to confusion. Labels can help prevent confusion although labels might fall off sometime during the 40-70 year lifespan of this circuit. The two breakers must be tied together to ensure that power is disabled to both circuits at the same time, so it is safe to open the box. This configuration will sometimes power a single split-wired duplex receptacle under the kitchen sink for the disposal & dishwasher so shutting off one circuit would leave the second outlet powered. –  Stefan Lasiewski Aug 29 '12 at 18:34
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When a circuit is complete, the neutral line carries current.

If you were to fully load one of the circuits (originally fed by the black wire) with 20 amps, and, at the same time, you were to load the other circuit (originally fed by the red) with 20 amps, you could have 40 amps returning on the single neutral (at least from the crawlspace junction box back to the panel) under certain circumstances.

While there is a good deal of over-engineering built into the code, you could be creating a risk of overloading that neutral line (basically an overheating risk, but perhaps a voltage drop issue as well) under certain circumstances. If you did, the breaker only senses the load on the hot leg, and that overload would not trip the 20+ draw traveling over the neutral.

SUPPLEMENT: [Please see the comments below. Also modified answer above, shown by italics, in light of comments.] As indicated by Tester101 and Matthew PK, if the two breakers are adjacent, they are on different legs of the panel and therefor should not cause an overload on the neutral. If they were on NON-adjacent slots (e.g., if they were on breaker slots 1 and 5), there could be a problem. Breakers can be shifted to different slots (or wires shifted), but only do so if you are well acquanted with electric work, and be certain to turn off the main breaker (often a large pull switch) before handling any individual circuit breakers.

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I had not considered this, but you're right - 12/3 is not rated for 40 amps, it's rated for 20 amps. The line from the junction box to the breaker box should be 10-3. Of course if you're only powering two electrical boxes - one each per line - there's very little real risk of loading up 40 amps on the line, but of course it is possible. –  The Evil Greebo Aug 27 '12 at 19:27
It sounds like the two hots are on different legs (which is how it should be done), so the neutral will only ever carry the unbalanced load between the two circuits. If you have 20A draw on black, and 20A draw on red, the neutral will have 0A. –  Tester101 Aug 27 '12 at 19:32
Most devices on these circuits are things like a clock radio and a fan; or a mid-sized television + audio equipment. Less then 1000W maximum. –  Stefan Lasiewski Aug 27 '12 at 19:36
@bib no, alternate busses in the panel are on different legs. This is how single breakers (on one side of the panel) send 220v. Therefore in this case the two hot conductors in the same run should be connected to two adjacent bus prongs. –  Matthew Aug 27 '12 at 19:57
@bib Most panels are set up where every other pair is on a different leg. You'll notice one side will be odd numbers and the other even, so you'll have 1-2=A, 3-4=B,5-6=A,7-8=B... So if you connect a double pole breaker at 3 and 5, you'll have Leg B on 3 and Leg A on 5. Next time you're at HomeDepot, open up a service panel package and look how the terminals on the legs are configured. –  Tester101 Aug 27 '12 at 20:09
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