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Old 07-07-2011, 10:24 PM   #16
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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.

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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Old 07-07-2011, 10:37 PM   #17
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MWBC's for Dedicated Circuits?


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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Old 07-07-2011, 10:38 PM   #18
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MWBC's for Dedicated Circuits?


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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Old 07-07-2011, 10:48 PM   #19
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by mysterylectric View Post
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.
since 2008 code.just not well enforced.
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Old 07-08-2011, 06:33 AM   #20
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by oleguy74 View Post
since 2008 code.just not well enforced.
Please stop spewing incorrect information.
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Old 07-08-2011, 06:36 AM   #21
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by mysterylectric View Post
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.
Wrong impression. NEC 2011 Article 240.15(B)(1).
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Old 07-08-2011, 08:42 AM   #22
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by brric View Post
Please stop spewing incorrect information.
not spewing incorrect info.do you say 2008 code don't say that?
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Old 07-08-2011, 08:44 AM   #23
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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
talk about spewing incorrect info...
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Old 07-08-2011, 08:51 AM   #24
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by oleguy74 View Post
talk about spewing incorrect info...
You are totally wrong. 2008 NEC does NOT say that. Article 240.15(B)(1).
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Old 07-08-2011, 09:37 AM   #25
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MWBC's for Dedicated Circuits?


2008NEC does NOT require handle ties, except when MWBC serves two receptacles on the same yoke. 2011NEC requires handle ties, but they may be on individual breakers (not a common trip DP), although a common trip DP is OK.
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Old 07-08-2011, 09:53 AM   #26
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MWBC's for Dedicated Circuits?


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Originally Posted by HouseHelper View Post
2008NEC does NOT require handle ties, except when MWBC serves two receptacles on the same yoke. 2011NEC requires handle ties, but they may be on individual breakers (not a common trip DP), although a common trip DP is OK.
How do you comply with 210.4(B)?
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Old 07-08-2011, 10:42 AM   #27
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MWBC's for Dedicated Circuits?


Here is 210.4(B) from the 2011.

(B) Disconnecting Means. Each multiwire branch circuit
shall be provided with a means that will simultaneously
disconnect all ungrounded conductors at the point where
the branch circuit originates.
Informational Note: See 240.15(B) for use of single pole
circuit breakers as the disconnect means required by this
section.

Here is 240.15(B)

(B) Circuit Breaker as Overcurrent Device. Circuit
breakers shall open all ungrounded conductors of the circuit
ARTICLE 240—OVERCURRENT PROTECTION 240.15
2011 Edition NATIONAL ELECTRICAL CODE Copyright NFPA 70–79
both manually and automatically unless otherwise permitted
in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).
(1) Multiwire Branch Circuit. Individual single-pole circuit
breakers, with identified handle ties, shall be permitted
as the protection for each ungrounded conductor of multiwire
branch circuits that serve only single-phase line-to neutral
loads.
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Last edited by Jim Port; 07-08-2011 at 10:45 AM.
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Old 07-09-2011, 12:27 AM   #28
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MWBC's for Dedicated Circuits?


Excellent! Thanks Jim for posting code.

So... everyone's right? Good thing I went with common trip across the kitchen counter as there are 240V appliances there.

I think in the future the NEC will require common trip breakers to help ensure proper phasing of the MWBC circuit. One of the recognized hazards of MWBCs is that if both hots end up on the same phase the shared neutral can be subjected to 2X the expected current. I saw a LOT of that when I worked in the trade here in SF. Granted, often there was no useful way to tie handles on some of the older panels ie: FPE 100A so there was no visual/physical clue that an MWBC might be involved... and requiring common trip or ties in future won't help those older installations much.

For the disposal/dishwasher circuit I'll give it a bit more thought. I'm running out of my 8 spaces: 6 (3 double pole) for main counter, one small counter, fridge, disposal,hood makes 10. I'm leaning toward using a tandem or quad breaker with ties, if I can get the ties, and just running two traditional circuits to a box under the sink. I might put outlets in, might hardwire. But in any case a handle tie would be slick: if the disposal pops the breaker it won't interrupt the wash cycle, but if you need to work on that box you have to cut both feeds.

Last edited by mysterylectric; 07-09-2011 at 03:30 AM.
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Old 07-09-2011, 12:57 AM   #29
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MWBC's for Dedicated Circuits?


Oh, what about AllanJ's comment:

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

Anyone know what he might be talking about?

Allan?

Why can't a GFCI outlet be used to protect downstream outlets if it's associated with an MWBC?

Or am I missing your point?

Oh, regarding "points" my point as the OP was to ask about possible restrictions on MWBCs associated with circuits required to be dedicated. Not quite sure about the bottom line there. While we're at it is there a list of locations or uses where MWBC's are prohibited (I'll probably flip around in the NEC myself for that info, soon as I find it again).

I'm thinking there's a good chance that if the furnace/outlet combo is prohibited, it would be prohibited in plumbing or building code... maybe not NEC.

Thanks for all the answers so far guys! I dunno how often this comes up but we should pin it up as a classic.

-Jeff
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Old 07-09-2011, 07:44 AM   #30
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MWBC's for Dedicated Circuits?


Quote:
Originally Posted by mysterylectric View Post
Oh, what about AllanJ's comment:

>Specifically the continuation of protection from the load terminals of a GFCI receptacle to additional receptacles cannot be
>part of the MWBC any more.
A separate neutral (connected to the GFCI silver load terminal) must accompany the GFCI load hot and this disqualifies the continuation portion from being part of a MWBC. THis also prevents using the 3 conductor continuing MWBC cable from carrying both the protected continuation and any other part of the MWBC to the next outlet box.

I'm not sure of any prohibitions of use of MWBC's, just easily overlooked complications such as the preceding.

Note that a typical 120/240 volt feed to a subpanel is a MWBC.

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Last edited by AllanJ; 07-09-2011 at 07:54 AM.
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