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Old 05-05-2011, 10:12 AM   #1
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


Hi, my house is wired with MWBC's almost exclusively. At first I was not very comfortable with them, but now I'm a convert. Using double pole breakers, wire ties to group associated wires, and splicing into the neutral vs. daisy chaining, seems pretty safe. Further I'll be protecting each circuit with a GFCI outlet (yes I can, and will: possible if done properly).

I'm wondering if it's possible to go further with this. Say I were to have 3 "pairs" of MWBC, that is 6 poles of breakerage across three double pole breakers.

And for the sake of discussion, we'll say all the breakers are 15A. That's 45 amps each side, 45 amps through neutral worst case.

What, then, prevents me from using a single neutral capable of 45A? Say gauge 6.

In a discussion of the old fashioned 2 hots plus ground sockets, I read that one of the reasons this practice is considered passable is that the heavier gauges typically used in these circuits are less likely to break or come loose.

It stands to reason that an oversized neutral would also be less likely to break or come loose, which is in fact a major concern for MWBC.

Soooo.... what section of code would prevent me from doing this?

Thanks in advance!

-Jeff

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Old 05-05-2011, 01:23 PM   #2
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


I haven't researched the code to answer that question.

But consider this: Suppose you need to do some service or work on that fat neutral. It would be too easy to forget which or how many breaker pairs control the hot wires for that neutral, forget to turn one breaker pair off, and disconnect the neutral while related hot wires were still live causing all kinds of voltage problems (and a spark at the point of disconnect).

Whereas with one neutral for each hot wire of a regular circuit or for each pair of hot wires on a 120/240 volt multiwire branch circuit, you flip off one breaker (or breaker pair) and that neutral is dead.

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Old 05-05-2011, 01:27 PM   #3
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


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Originally Posted by mysterylectric View Post
What, then, prevents me from using a single neutral capable of 45A? Say gauge 6.
There are several problems with your idea.
1) Wire nut size
2) Box fill
3) Cabling - the neutral would have to be in the same raceway or cable. So far I haven't see 12/2 +6 w/G on the market.....
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Old 05-05-2011, 01:34 PM   #4
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


I think you might be required to handle tie all six breakers IF this was allowed.
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Old 05-05-2011, 04:38 PM   #5
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


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....Soooo.... what section of code would prevent me from doing this?...
Try looking at section 200.4 in the 2011 Code:

Quote:
Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in the Code.
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Old 05-05-2011, 04:56 PM   #6
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


Yep, 200.4 says it all...
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Old 05-05-2011, 08:16 PM   #7
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


you are trying to create something like a super neutral. But in fact by using one neutral for more than 1 mwbc you will probably end up cooking the neutral and burn your house down. This is why code was created. Ever wonder how codes were created???? Its because of incidents that have occurred over the years.
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Old 05-05-2011, 10:16 PM   #8
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Multi Wire Branch Circuit MWBC w/4X or 6X hots or more, 1X oversized neutral


With typical single phase residential service, a mwbc may have only have to hot conductors to 1 neutral. It's not uncommon in 3 phase installations for all three to share one neutral. But no more than one conductor of each phase can share a neutral. That being said the biggest problem with mwbc circuits is the possibility of a neutral opening up and creating a high voltage situation between the two hots. this has the potential to destroy any appliance and especially electronic devices by sending a 240 volt surge through the equipment.

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