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Old 07-07-2011, 07:38 PM   #1
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MWBC's for Dedicated Circuits?


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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Old 07-07-2011, 08:01 PM   #2
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MWBC's for Dedicated Circuits?


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

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Old 07-07-2011, 08:03 PM   #3
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MWBC's for Dedicated Circuits?


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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Old 07-07-2011, 08:09 PM   #4
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MWBC's for Dedicated Circuits?


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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Last edited by AllanJ; 07-07-2011 at 08:11 PM.
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Old 07-07-2011, 09:09 PM   #5
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by mysterylectric View Post
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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Old 07-07-2011, 09:11 PM   #6
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by AllanJ View Post
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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Old 07-07-2011, 09:17 PM   #7
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MWBC's for Dedicated Circuits?


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2 pole common trip circuit breakers are not required for MWBC's
They are in Ontario, Canada.
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Old 07-07-2011, 09:21 PM   #8
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by Wildie View Post
They are in Ontario, Canada.
cool......
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Old 07-07-2011, 09:22 PM   #9
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MWBC's for Dedicated Circuits?


Wildie is the an Ontario rule, if you go by the CEC you don't always need a two pole breaker.
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Old 07-07-2011, 09:42 PM   #10
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by brric View Post
2 pole common trip circuit breakers are not required for MWBC's
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.
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Old 07-07-2011, 09:45 PM   #11
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MWBC's for Dedicated Circuits?


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Originally Posted by oleguy74 View Post
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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Old 07-07-2011, 09:46 PM   #12
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MWBC's for Dedicated Circuits?


Quote:
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That doesn't mean common trip, just tied handles.
that what common trip is...
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Old 07-07-2011, 09:51 PM   #13
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by oleguy74 View Post
that what common trip is...
No it isn't. Single pole breakers with handle ties are acceptable.
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Old 07-07-2011, 10:01 PM   #14
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by oleguy74 View Post
that what common trip is...
Quote:
Originally Posted by brric View Post
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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Old 07-07-2011, 10:06 PM   #15
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MWBC's for Dedicated Circuits?


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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