In my experience, the biggest advantage is that you only have to drill and fish one 12/3 or 14/3 NM wire, but you get two circuits on the other end. For example, I ran 12/3 for one MWBC up to my attic and then broke it out with a junction box into new receptacle circuits for two different rooms. I ran another 12/3 MWBC up to the attic and then broke it out into a new GFCI for the bathroom, and power for a bathroom heater.

I don't think the priority for an MWBC to reduce load on the neutral wire or for any other electrical load advantage.

I am not a code expert, but I think an MWBC is legal as long as its used for two 15 amp or 20 amp circuits, and you use the right gauge cable, and a double breaker that is tied together. On the other end where you break it out into two circuits you also need to use an appropriately sized junction box for the number of wire connections.

Bruce

The main advantage for me in wiring my shop was the ability to put 240 and 120 V outlets in every location with minimal wire. Also it makes it easy to have two circuits at locations where there is the potential for two large draws at the same time, either by splitting a duplex receptacle or installing a pair of them.

• Caveats: don't put opposite phases on the same duplex receptacle.

I'm not sure I understand this either - why can't you do this?

Thank you Ron and Bruce. I read many code-relate sites regarding this, and the possibility of someone later screwing it up was mentioned. I agree with Ron that this shouldn't bother me, for the reasons he stated. Another thing I saw was the need for a tied-together breaker pair and, for me, that's a deal killer. If I'm going to run a multiware branch circuit, it would go to one room where part is for lighting and part is for utility receptacles. If I cause the breaker to trip, I don't want my lights killed also!

In a related question, I prefer to keep my lighting circuits and receptacles on separate circuits. I haven't been 100%, because there are a couple of locations where I wanted a receptacle but the closest box I could tap into was an unswitched light or fan.

Ron, on an earlier post, you mentioned that a bedroom must have an AFCI breaker. Is that for lighting, receptacles, or both? And, that is the situation I have, so maybe I should just run two independent circuits with AFCI breaker(s) on one or both of them.

Dennis: I don't know why, but that's another no-no I read in my internet browsing. Ron?

No idea why he'd suggest you can't put both phases in one box (how would you wire a 240 v outlet otherwise?) Anyway multiple inspectors haven't even raised an eyebrow over doing it.

You can put both sides of the feed on the same yoke as long as the breaker is handle-tied (or a 240V). Later versions of the code require such ties on all MWBC, earlier versions only if they feed the same yoke.

It's actually the common way to wire kitchens up north of the border.

I used MWBCs in my son's kitchen in Framingham MA because there wasn't a lot of counter /wall space and places where he could well have wanted to put multiple high draw appliances (toaster, tea kettle, waffle iron, etc) Splitting the outlets allowed us to provide capacity and flexibility without taking up a lot of space. Inspector was completely happy with it -- he asked what I was doing, I think to tell whether or not I had a clue. Apparently I passed.

So, based on this discussion, I decided to run a 2 Wire Branch Circuit pair to two parts of my basement, one of which will be a bedroom. I've run all the wires and was on the verge of installing the AFCI breaker for the bedroom part, when I read the following in the instructions: "4. Connect panel neutral (pigtail wire) to load center neutral bar. 6. Connect load power and load neutral." And, under testing (after pressing white button) "Trips instantly: ....shared neutral". So I guess I can't use the combined load neutral for the MWBC, unless there is another type of breaker I could use (mine is a Square D QO120CAFIC). Could I use the MWBC if I have two AFCI breakers and use the shared neutral for both? Otherwise, I will have to pull another white neutral through all the conduits. Right?

You can't use single pole AFCI breakers on a MWBC.

Do double pole AFCI breakers exist? If they do, then perhaps that's my best bet. It would be easier than running another neutral wire!

I just did a web search for "double pole AFCI breaker" and found several listed, included one for the Square D. https://www.lowes.com/pd/Square-D-QO-20-Amp-2-Pole-Combination-Arc-Fault-Circuit-Breaker/50311201

Bruce

So now my choice is to get one of those double pole breakers ($99 at Lowes), or run a second neutral wire and use my one single pole AFCI breaker for the bedroom and a regular breaker for the other circuit. HOWEVER, in researching this, I discovered that the code now requires AFCI breakers for ALL residential structures. Looking at my breaker box, the builders installed 5 AFCI breakers, for the master bedroom outlets, the art studio outlets and fan, the living room sconces, and two living room outlet circuits. There's a total of 39 circuits on this box! I think I will go for the $99 breaker and use my existing single pole AFCI breaker for my workshop, which still hasn't been wired yet.

I guess the thing I don't understand is why you are installing a MWBC?

It all depends on which verion of the code you're on. The Pre-14 ones only required AFCI on circuits serving bedroom outlets. The later codes require it just about in any living space.

Progress update: I went to Lowes (which is an hour's drive away, I live in rural northern NY) and they didn't have the two-pole AFCI breaker in stock. I had already checked with the local electric supply house and they carried them (at $169 each, vs $99 for Lowes), but they didn't have them in stock either. So I finished wiring using regular breakers, with the plans to order the 2-pole one later. I then wired my third circuit, which is for lighting in my TV room, which is also in my basement, and decided to put in the one AFCI breaker. It works fine, but I discovered a problem: there is a short pigtail which connects to the neutral bus, which is on the left, near the bottom of my second square D load center. Since this load center is almost empty, it was no problem, but clearly 3/4 of the spaces won't allow that neutral connection.

Now, to get the 2-pole AFCI breaker, as well as another single pole one for my future workshop connection. I Googled 2-pole AFCI breakers, and noticed that there are several kinds, including a Siemans type QAF 20 AMP Double-Pole Combination AFCI Circuit Breaker (from Home Depot), which is actually a CAFCI breaker. But HD's website doesn't say whether it's for a Square D box, and I don't want to order it without seeing it.

Regarding my load center - I can't find a part number on it. When I asked the salesman about the breakers, he asked "Is it Homeline or Square D?", that's why I'm trying to find the part number. The cover panel is QOC42UF, but when removing the cover, I can't find the number for the box. But, when I Google 42 place, 200A panels, none look like mine (with the neutral bar at the bottom left). The single pole, CAFCI breaker I have (QO120CAFIC) fits, and the word Homeline doesn't appear anywhere on the breaker's tag. There is the message "For use only on Square D QO load centers".