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Discussion Starter · #1 ·
Hi, my house has a lot of MWBCs and I've learned to embrace them.

Curious to know if MWBCs are prohibited when associated with dedicated circuits required under 2011 NEC.

Keep in mind you need a 2 pole, common trip breaker for MWBCs as of late.

On the one hand I think it's common to use an MWBC to a well pump/tank pump setup. That's a fine idea, you wouldn't want one pump to start while you're working on the other, anyway.

On the other hand it might not be the best idea for an attic furnace, where a dedicated circuit for the heater and a separate dedicated circuit for an outlet nearby is called for. If the heater is tripping the breaker, you might need a trouble light or power tool to fix it. There are probably much better examples of places where it wouldn't be smart.

Specifically I'm wondering about dishwasher and garbage disposal.

Whatchyathink?

-Jeff
 

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One important item to consider is whether or not AFCI protection is required. Two pole AFCI breakers are not common.
 

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I can't speak to MWBCs in the 2011 NEC, I am sure that someone much smarter than me will be able to help you with that info.

I can speak to a MWBC on a garbage disposer and a dishwasher as that is what I did in my house. Many people are opposed to any MWBCs anyplace, I think that they are a great way to save a few bucks on wire and in this application it makes for a very nice clean installation if you use a duplex plug with the tab removed. Just my 2 cents

Another great application fr MWBCs is the outlets above the counter in the kitchen, again just my 2 cents.
 

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You run into some complications with GFCI protection on multiwire branch circuits also.

Specifically the continuation of protection from the load terminals of a GFCI receptacle to additional receptacles cannot be part of the MWBC any more.

Now (both sides of) the MWBC can continue from the line side of a GFCI receptacle with additional GFCI units used downstream where needed.

Or a double pole GFCI breaker can be put in the panel to protect the whole MWBC.
 

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Hi, my house has a lot of MWBCs and I've learned to embrace them.

Curious to know if MWBCs are prohibited when associated with dedicated circuits required under 2011 NEC.

Keep in mind you need a 2 pole, common trip breaker for MWBCs as of late.

On the one hand I think it's common to use an MWBC to a well pump/tank pump setup. That's a fine idea, you wouldn't want one pump to start while you're working on the other, anyway.

On the other hand it might not be the best idea for an attic furnace, where a dedicated circuit for the heater and a separate dedicated circuit for an outlet nearby is called for. If the heater is tripping the breaker, you might need a trouble light or power tool to fix it. There are probably much better examples of places where it wouldn't be smart.

Specifically I'm wondering about dishwasher and garbage disposal.

Whatchyathink?

-Jeff
2 pole common trip circuit breakers are not required for MWBC's
 

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You run into some complications with GFCI protection on multiwire branch circuits also.

Specifically the continuation of protection from the load terminals of a GFCI receptacle to additional receptacles cannot be part of the MWBC any more.

Now (both sides of) the MWBC can continue from the line side of a GFCI receptacle with additional GFCI units used downstream where needed.

Or a double pole GFCI breaker can be put in the panel to protect the whole MWBC.
There is absolutely no problem with GFCI's on MWBC's if one knows what one is doing.
 

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2008..210.4(b).multi wire branch ckts,shall have a disconecting means that will simontainously open all ungrounded (hot)conductors at the point of origination.
That doesn't mean common trip, just tied handles.
 

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that what common trip is...
No it isn't. Single pole breakers with handle ties are acceptable.
Either way it will work.

Two single pole with handle tie or a two pole with common tripping device ditto with three pole verison.

Basically it almost the excat the same in our French codés.

Merci,
Marc
 

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On dedicated circuits that don't require AFCI or GFCI, I usually run a MWBC or two single circuits and break the tabs. I figure, if I need a 15 or 20 amp outlet now, maybe later I'll need more power.
 

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Discussion Starter · #16 ·
>One important item to consider is whether or not AFCI protection is required. Two pole AFCI breakers are not common.

Good point, that's something I'll add to my list of questions for the department. I'll ask about a new circuit (ie: kitchen) as well as the legacy MWBC's going throughout the house. For the latter I'd like to use the existing thickwall but I'll be pulling mostly (if not all) new wire and in this way adding circuits. The thickwall is only 1/2" so losing MWBC would be a real blow. Better make sure what I propose is grandfathered in.

>Specifically the continuation of protection from the load terminals of a GFCI receptacle to additional receptacles cannot >be part of the MWBC any more.

Uh oh. Is this a recent change? What's the rationale?

I know you can't use a pair of GFCI receptacles to feed an MWBC for some of the same reasons you can't use a pair of single pole GFCI breakers: once the neutral currents are summed they can't be teased out to balance the individual hot currents. One or both pops immediately under load.

But you're saying that if I might otherwise put a string of (kitchen counter) outlets on the load side connectors of a GFCI outlet, I can't feed that circuit (at the GFCI line terminals) with an MWBC? I don't get it. Of course, I think I can see how you can't go through a GFCI and then expect to tie the load-side neutrals back into the original MWBC neutrals: that would seem absurd. But once you've picked off a phase, I don't see how that "feed" is different that any other branch circuit feed, as from a panel.

I am aware of the additional safety concerns around MWBCs and will accept for the sake of argument that under certain conditions GFCI's might not work as expected, but I don't see why 1 GFCI + N load side receptacles is more hazardous or troublesome than N + 1 GFCI outlets.

>Or a double pole GFCI breaker can be put in the panel to protect the whole MWBC.

Yep, though you must ensure it has a neutral sense lug. Not all do.

I'm hoping a 50A "whole kitchen" double-pole GFCI will cover me without nuisance tripping. The fridge will be on a separate circut straight back to the service panel.

>Another great application fr MWBCs is the outlets above the counter in the kitchen, again just my 2 cents.

Oh yeah... I've gone all-countertop with my kitchen appliances this year. All stainless/industrial theme. I've got four 1500W appliances and a bunch of little ones, so I really need 5 circuits. For aesthetic reasons (some cool corrugated SS tubing material that slips over 1/2" IMC) I need to use 1/2" IMC. Couldn't do it without MWBC's, and got a 6th circuit with one more wire. Saves copper, energy, and you win big on derating: instead of 4 wires (two circuits), you derate based on 2: NEC recognizes that even though three wires are carrying current, some of them can cancel such that the total heat generated is never more than that of a pair of traditional (single phase) conductors (ie: 120V hot-neutral or 240V hot-hot).
 

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The intent is that both circuits will be powered down when one side or the other is shut off.
Pullouts on fuse panels achieve this also!
 

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Discussion Starter · #18 ·
I got the impression 2011 NEC finally does require common trip. If not, great: that opens up choices in panels and breaker selection in my project and further lowers cost.
 
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