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Old 09-10-2011, 09:43 PM   #1
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appropriate multi-wire branch circuit conditions


I'm wondering if there is a right and wrong time to use multi-wire branch circuits.

For example, the electrician who re-wired my panel and my kitchen renovation put four of the houses original circuits in MWBC's--so I have two 14/3's leaving the box, each goes to its own J-box from there two original house circuits from each continues. In the new kitchen wiring he used a MWBC for my disposer and dishwasher.

I understand the reasoning for using it--you have only one cable to pull, saves labor (especially old work) and saves money because using 50' of 14/3 is cheaper than 100' of 14/2. It may also help with box fill requirements in some cases...

My root question though is whether there are any times where it's NOT appropriate to use one, or if it's always OK to use when it makes sense (regardless of the type of circuit).

Here's a specific set of examples that pertains to me--
I have a dual sump pump setup, it's a double-sized sump with two separate pumps. I don't actually get that much water lately so they never run, and when I do get water their floats are set differently so only one will ever run. However, I want both pumps on their own circuit so that a breaker or GFCI trip will not kill both (GFCI are required in basements here). So, what I was thinking was that I could run a piece of 14/3 over to a 4" box and install two GFCI receptacles, one using black hot and the other using red hot.

Basically the same question, but for my "workshop"... currently it gets along just fine with a single 15A circuit, I haven't blown it yet... but I have a single 32W CFL and use only one saw/tool at a time... Today I installed additional shop lights (now up to 225W w/ all lights on), I'm installing a panasonic bath vent to provide some forced ventilation*, and I'm going to be using my shop vac w/ dust deputy whenever I run my table saw (previously I was just letting the dust fly then trying to clean afterward, that has been a losing battle). So what I'm considering for the workshop is two new 20A circuits just for tools. My current 15A will be lighting and a couple of GP outlets at the work bench for light duty items like chargers and the radio. To run the two 20A I was considering a 12/3 run from the panel. Unlike the sump pump scenario where it's unlikely both circuits would be loaded at the same time, in this case I specifically would be loading both circuits--granted probably not up to 20A since I have nothing that will even pop the 13A CB in a power strip at the moment--but is this a case where a MWBC would be frowned on?

One other Q--I've heard people say that a breaker tie should be on the breakers in a MWBC, but in my panel neither breaker pair has one. Should there be one to be code compliant?

Thanks for any advice, tips, info...

*about the Panasonic fan... I bought a 50CFM stand alone fan for my bath remodel but we ended up using the 80CFM Panasonic light/fan combo instead... it was too late to return the 50CFM, so I've been trying to come up with ideas for it... my idea here is sort of like a "whole house fan", but the idea being that it just generates air movement when I have the window open in the shop (it's a basement work shop with a single 20"x26" "hopper" style window)

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Old 09-10-2011, 09:50 PM   #2
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appropriate multi-wire branch circuit conditions


You have a good understanding of a MWBC.

Your plan sounds good for your shop. You will need to add GFI protection for the receptacles. A handle tie or a two pole breaker will be needed. This is a recent requirement.

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Old 09-10-2011, 09:58 PM   #3
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appropriate multi-wire branch circuit conditions


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You have a good understanding of a MWBC.

Your plan sounds good for your shop. You will need to add GFI protection for the receptacles. A handle tie or a two pole breaker will be needed. This is a recent requirement.
Thanks for the quick reply...

FYI, I'm planning a GFCI receptacle at the start of each circuit so that should cover me. To be honest the workshop is only ~20x10, so at this point I'm thinking even two duplex per circuit would almost be over kill.

re: the handle tie... recent as in NEC 2008, or recent as in some addendum to it? The real question--was it required in NEC 2005?

I'm trying to understand why my licensed electrician did NOT use handle ties or two pole breakers for the two MWBC's he created. He's a pro, I've personally known him over 10 years and he's a Master in the biz for over 25 years. This work was done as part of a permit, so it is going to be inspected at some point (when I call them for the final ).

I'm basically wondering, will I be flagged for lack of the two pole... I'd almost rather just grab a couple of them from Home Depot and put them in as a "better safe than sorry" measure before the inspector comes in...
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Old 09-10-2011, 10:03 PM   #4
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appropriate multi-wire branch circuit conditions


I can't be sure, but in 05 handle ties were only required when the device was on the same yolk.
In other words, you break the tab from the hot side and have 2 single receptacles on the same yolk.

MWBC do not get along well with arcfaults, so that is a case for not using them.
Other than that, I see no reason not to use them.
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Old 09-10-2011, 10:06 PM   #5
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appropriate multi-wire branch circuit conditions


Quote:
Originally Posted by bubbler View Post
Thanks for the quick reply...

FYI, I'm planning a GFCI receptacle at the start of each circuit so that should cover me. To be honest the workshop is only ~20x10, so at this point I'm thinking even two duplex per circuit would almost be over kill.

re: the handle tie... recent as in NEC 2008, or recent as in some addendum to it? The real question--was it required in NEC 2005?

I'm trying to understand why my licensed electrician did NOT use handle ties or two pole breakers for the two MWBC's he created. He's a pro, I've personally known him over 10 years and he's a Master in the biz for over 25 years. This work was done as part of a permit, so it is going to be inspected at some point (when I call them for the final ).

I'm basically wondering, will I be flagged for lack of the two pole... I'd almost rather just grab a couple of them from Home Depot and put them in as a "better safe than sorry" measure before the inspector comes in...
2005 did not require handle ties/2 pole breakers on MWBC unless two circuits were on 1 yoke, MWBC or not. 2008 requires all MWBCs to be handle tied/multi pole breakers.
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Old 09-10-2011, 10:07 PM   #6
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appropriate multi-wire branch circuit conditions


Quote:
Originally Posted by bubbler View Post
Here's a specific set of examples that pertains to me--
I have a dual sump pump setup, it's a double-sized sump with two separate pumps. I don't actually get that much water lately so they never run, and when I do get water their floats are set differently so only one will ever run. However, I want both pumps on their own circuit so that a breaker or GFCI trip will not kill both (GFCI are required in basements here). So, what I was thinking was that I could run a piece of 14/3 over to a 4" box and install two GFCI receptacles, one using black hot and the other using red hot.
The problem with a MWBC for your sumps is that it will require a 2-pole breaker (or handle tie). If one trips the beaker the other will be shut off too. As others said, the requirement for MWBC's changed in 2008, now requiring all of the 'hot' wires to be disconnected at the same time. Your existing MWBC's you mentioned that don't have handle ties were probably installed before the requirement. For your sumps, I would consider running 2 independent xx/2 cables to their own GFI's.
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Old 09-10-2011, 10:20 PM   #7
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appropriate multi-wire branch circuit conditions


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Your existing MWBC's you mentioned that don't have handle ties were probably installed before the requirement. For your sumps, I would consider running 2 independent xx/2 cables to their own GFI's.
The electrical work was done in late 2010 and early 2011... IIRC Mass. was late in adopting 2008, so _maybe_ we're still on 2005 and that is why it's OK... or maybe my friend is getting old

For the dishwasher/pig I don't care, I'll throw a double pole in... for the other two circuits they represent the entire first floor lighting and outlets (except kitchen and bath which are all new re-wired, and living room light which is new). We have yet to trip a circuit breaker at all, but if we need the handle tie that would mean we'd lose pretty much all the lights on the the first floor at once. Fortunately it's an easy fix, the J-boxes are within a couple of feet of the load center, they represent splice points for the old wiring linked to the new. To de-MWBC I believe I could simply run a new 14/2 from the panel to the J-box and abandon the red conductor in the 14/3.

For my sump pump and work shop the requirement for the double throw makes me not want to do a MWBC, after all what's the point of the dual pumps if a wiring fault in one ends up killing the power to the other one... so I'll just plan to run two pieces of romex for each...
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Old 09-10-2011, 10:43 PM   #8
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appropriate multi-wire branch circuit conditions


The sumps I would run 2 independent circuits.
The shop…it’s your call. Running a new MWBC for receptacles shouldn’t pose any problems, since you can keep your lights on the current circuit. Kicker is if you don’t use a GFCI breaker, once you get to your first GFI receptacle, you have to split the neutral to the next GFI receptacle. Then from both GFI recepts, independently you’ll be using 2 wire (12/2 with ground) to keep the sub-circuits separate, so the GFI’s will work properly. So the only real savings is in the 12/3 cable from the shop to the panel.
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Old 09-10-2011, 10:48 PM   #9
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appropriate multi-wire branch circuit conditions


Should have asked before, your shop is in the basement or attached garage? If it's in a detached structure, then things are different, meaning you can't have more than one circuit supplying it.
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Old 09-10-2011, 10:52 PM   #10
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appropriate multi-wire branch circuit conditions


MA adopts on the first of the year, you are already on 2011.

Ma has amendments. Can't find official copy right now

http://www.mass.gov/Eeops/docs/dfs/o...red/527012.pdf
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Old 09-10-2011, 10:58 PM   #11
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appropriate multi-wire branch circuit conditions


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Should have asked before, your shop is in the basement or attached garage? If it's in a detached structure, then things are different, meaning you can't have more than one circuit supplying it.
Shop is basement. We're talking ~30' max, so I'll just run the two cables in both cases... while I'm in there I'm very tempted to de-MWBC my original house circuits as well.

I should mention, when I took the cover off the load center I found a box cutter, several sections of romex sheathing, and the screw on top from a tube of AL/CU goop at the bottom of the box... ... I'm thinking more and more that having the MWBC w/o the tie may have been an oversight by my friend and his team. Fortunately the electrical inspector looks like Burgess Meredith from Grumpy Old Men, so _hopefully_ my troubles will not include failing inspection (his rough inspection took 4 minutes and he didn't get within 10' of any wiring or boxes )
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Old 09-10-2011, 11:05 PM   #12
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appropriate multi-wire branch circuit conditions


LMAO

I love inspector like that
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Old 09-10-2011, 11:14 PM   #13
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appropriate multi-wire branch circuit conditions


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LMAO

I love inspector like that
Yeah,

Pro: rubber stamped inspection approval...

Con: another government official not doing their job, and in this case a job that could prevent fire or even death if done properly and a problem is found...

Thank you everyone for your help on this!

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