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MWBC solutions

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thammel

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Oct 3, 2005
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Location
Maryland
My house built in 2000, had a bunch of MWBC circuits. I don't like them....trying to add combination CAFCI/GFCI breakers (Square D QO) is a problem. So I've been finding the boxes where the 3 wire nm cable goes, running new 2 wire nm to that box and essentially getting rid of the MWBC. But in doing this, and subsequently installing the combo breakers, in a couple of spots, this doesn't seem to work out. What I mean is that the breaker immediately trips. A regular non arc fault/ground fault breaker works fine. What would cause such a breaker to trip immediately? I'm guessing a ground fault somewhere and not an arc fault. I check all the receptacles with my GB receptacle tester and all test ok. Would GFCI receptacles somewhere in the circuit cause a problem? I am at a loss as to what's going on here. I greatly appreciate all and any suggestions!


Thanks,
Tom
 
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T

thammel

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Maryland
Adding them to enhance the electrical safety in my house. Will check the test function. But when I add it, it immediately trips. I will see if the test function tells me anything.
 
A

AntonLargiader

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Nov 20, 2016
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Charlottesville, VA
Just for clarification, you are running TWO new NM-b cables to the separation point and then each side of the old MWBC is now a separate circuit? You said "running new 2-wire" but that's not specific enough for me. :)

As I'm sure you know, any neutral sharing between circuits will trip a single-pole GFCI.
 
K

kbuhagiar

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Joined
Dec 27, 2005
Messages
1,759
Location
Escondido, CA
thammel said:
My house built in 2000, had a bunch of MWBC circuits. I don't like them....trying to add combination CAFCI/GFCI breakers (Square D QO) is a problem. So I've been finding the boxes where the 3 wire nm cable goes, running new 2 wire nm to that box and essentially getting rid of the MWBC. But in doing this, and subsequently installing the combo breakers, in a couple of spots, this doesn't seem to work out. What I mean is that the breaker immediately trips. A regular non arc fault/ground fault breaker works fine. What would cause such a breaker to trip immediately? I'm guessing a ground fault somewhere and not an arc fault. I check all the receptacles with my GB receptacle tester and all test ok. Would GFCI receptacles somewhere in the circuit cause a problem? I am at a loss as to what's going on here. I greatly appreciate all and any suggestions!


Thanks,
Tom
Click to expand...
Is there anything inherently wrong with your existing multi-wire branch circuits?
What is it you don't like about them?
 
E

exranger06

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CT
kbuhagiar said:
Is there anything inherently wrong with your existing multi-wire branch circuits?
What is it you don't like about them?
Click to expand...
I'm assuming the only thing he doesn't like about them is they're preventing him from adding AFCI/GFCI combo breakers. He needs 2-pole breakers that have AFCI AND GFCI built-in, and such a thing may not even exist. I know that no such thing exists for Eaton CH panels, for example (You can get single pole AFCI/GFCI, or 2-pole with JUST AFCI or JUST GFCI, but not 2-pole with both). Not sure about Square D.
 
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u2slow

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Nov 20, 2011
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BC
Updated electrical (mismatching the age) on a home without a permit history is a red-flag for future buyers.

What was legit and approved when built continues to be legit (aside from a few exceptions where equipment has been been deemed hazardous after the fact.)

Fyi, every 120/240 breaker panel is a MWBC in itself. It's the nature of electrical distribution.
 
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dave*99

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May 5, 2009
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Coastal NJ
I usually see folks cursing and removing AFCI breakers. Not adding them voluntarily.

I'm in a newly built house that has many AFCI breakers (2014 Code) We have been here 18 months and not had a single nuisance trip. BUT the house was wired by experienced electricians that deal with AFCI circuits every day. There are many many branch circuits which should make troubleshooting a bit easier should problems arise.

Your 2000 house was probably not wired that way....... Just sayin.
 
Z

Zeke

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Aug 13, 2009
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Location
Long Beach CA, the sewer by the sea.
All neutrals have to be paired with the circuit they are on when installing AFCI's. You cannot cross connect them. I know, they all go to the same place at one point, but that point has to be after the pigtail from the breaker is tied to the neutral bus. AMHIK.
 
OP
T

thammel

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Oct 3, 2005
Messages
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Location
Maryland
Wow - this generated a lot of replies! I do know how an MWBC is wired. And Square D QO type breakers offer a combination AFCI/GFCI breaker. This will not work with an MWBC circuit. I am simply interested (since it is current code and adds another level of safety) in replacing some of my standard breakers with the combination breakers. And when I started doing this I learned about MWBC circuits. So what I've done is located the box where the 3 wire+ ground nm goes and feeds the red and black circuits. Then I take an appropriately sized nm 2+ ground and feed it to that box (from the main or subpanel). I then use the box as a junction box and effectively get rid of the MWBC circuit. Then I can add a combo breaker to either or both of the circuits.

I first heard about arc fault protection when the new garage was built in 2008. Arc fault breakers were used in that subpanel. Got me thinking.

Anyway, you guys really did help me! I wasn't aware of the timesaver diagnostic built into the QO combo breakers. And I did solve my problem today with the few spots that the breakers would trip immediately. The problem was that that these breakers were in a subpanel. And I'd fed the neutrals from the main. This is a no-no. When I kept all the neutrals, etc. in the subpanel, everything worked fine!

Long story on the sub - it's a subpanel being fed by either my main or my standby generator. The control is an ASCO switch. The system works great! I like this more than the limited number of spots with most standby generators in that I can have any size subpanel I want.

Anyway, guys - thanks for your help! Problem solved!!
 
W

wyliesdiesels

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Joined
Aug 14, 2012
Messages
20,074
Location
Modesto, CA
thammel said:
Wow - this generated a lot of replies! I do know how an MWBC is wired. And Square D QO type breakers offer a combination AFCI/GFCI breaker. This will not work with an MWBC circuit. I am simply interested (since it is current code and adds another level of safety) in replacing some of my standard breakers with the combination breakers. And when I started doing this I learned about MWBC circuits. So what I've done is located the box where the 3 wire+ ground nm goes and feeds the red and black circuits. Then I take an appropriately sized nm 2+ ground and feed it to that box (from the main or subpanel). I then use the box as a junction box and effectively get rid of the MWBC circuit. Then I can add a combo breaker to either or both of the circuits.

I first heard about arc fault protection when the new garage was built in 2008. Arc fault breakers were used in that subpanel. Got me thinking.

Anyway, you guys really did help me! I wasn't aware of the timesaver diagnostic built into the QO combo breakers. And I did solve my problem today with the few spots that the breakers would trip immediately. The problem was that that these breakers were in a subpanel. And I'd fed the neutrals from the main. This is a no-no. When I kept all the neutrals, etc. in the subpanel, everything worked fine!

Long story on the sub - it's a subpanel being fed by either my main or my standby generator. The control is an ASCO switch. The system works great! I like this more than the limited number of spots with most standby generators in that I can have any size subpanel I want.

Anyway, guys - thanks for your help! Problem solved!!
Click to expand...
theres no evidence that AFCIs do that
 
M

mcbane

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Jul 23, 2017
Messages
794
Location
California
IMHO, having AFCIs just increases the chance that an outlet wont work reliably. So you need to make more use of extension cords, and extension cords do not enhance safety.
 
P

PCustoms

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Jul 23, 2011
Messages
23,513
Location
VT
AntonLargiader said:
This is almost funny. The OP's workmanship habits are such that he runs the hots and neutrals to different panels and all you care about is that he wants to use those evil AFCI breakers.
Click to expand...
I'm actually scratching my head right now trying to figure out how the hots came out of the sub panel, and the neutrals came out of the main...
 
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