mwbc residential

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joelj

Member
OK, I never install mwbc in residential applications but I see it frequently. I do mostly service and remodeling work and some of the homes in CA are wired almost exclusively with mwbc. I'm going to start an addition / remodel jub that has been going on for 3 years and I'll be cleaning up from some previous electricians. The last one installed 3 12/3 home runs to the new kitchen with the intent of using 1 for sm. app / disposal another for sm. app / dishwasher and the 3rd for refer / microwave. Since the permit is 3 yrs old, we're dealing with 2005 code. What is the real issue with using these existing home runs? I've never had a problem on small remodels using the original mwbc in kitchens. I see lots of posts where it's said to "stay away from mwbc in residential", Would it be worthwhile to pull these out and re-run. I think one of the reasons he used them is that the neutral bat is getting full in the panel, but I can deal with that issue.
 

joelj

Member
Box fill is no problem. Only the refer / micro goes to a sgl gang and it's 20 cu in. The other 2 go into a 2 and 3 gang box.
 

buzzbar

Senior Member
Location
Tacoma, WA
Occupation
Electrical Contractor
Instead of 'mwbc', please refer to it as the 'circuit that shall not be named'. It's better luck. ;)

Actually, the only real issue is that you can't use GFI or AFCI breakers. And if the (proper) two-pole breaker is turned off, BOTH circuits get turned off.
 

joelj

Member
If I'm working on a 3 year old permit, I should be working on 2005 code (CA just started using 2008 in 2011) and I shouldn't need to use handle ties or 2-pole breakers. None of the circuits will be arc fault and the sm app will use GFCI receptacles.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Okay you might not need to use 2 pole breakers however is good practice in a residential situation as it breaks both circuits.
Also you need to pigtail your nuetral as you cannot be able to remove a device that in turn breaks the nuetral connection. No 3 wires downstream of a GFCI either. Follow this and the DEvil will stay away:eek:
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
Okay you might not need to use 2 pole breakers however is good practice in a residential situation as it breaks both circuits.
Also you need to pigtail your nuetral as you cannot be able to remove a device that in turn breaks the nuetral connection. No 3 wires downstream of a GFCI either. Follow this and the DEvil will stay away:eek:
I don't know, maybe if you installed 200% neutrals. I mean what if the kids plugged a couple of cell phone chargers and a laptop into a MWBC. The *trippin harmonics could overheat the neutral and burn the house down. I believe that would please the Devil.



*trippin harmonics = psychedelic barbershop quartet. Another tool of the Devil
 

gndrod

Senior Member
Location
Ca and Wa
Scare tactic 101

Scare tactic 101

Since the permit is 3 yrs old, we're dealing with 2005 code. What is the real issue with using these existing home runs?
First up, the two year permit has probably expired and the renewal does not reinstate 2005 unless the AHJ makes the exception. Check to see if the permit has been extended.

Real issues, using 12-3 w/g and a GFCI receptacle in the first outlet can be a box fill issue as mentioned by George. Another would be if any Z wave or X10 multiplexed circuitry is going to be used...avoid MWBC. The client will thank you when they decide to tinker with using RF or carrier overriding controls later. (You might get future work when they call you back.)

If the permit has an issue usually with the Title 24 energy control requirements in lighting, I would be careful with MWBC's running dimmers also.

Modern technology is using many variations of solid state line controls that most electrician's can take advantage of in a pinch. I would not install MWBC's in this day and age because of the upgrading that may require future AFCI and GFCI protection circuits.

Cross-coupling can be a bear when not being able to determine what's bundled together in a finished wall and one heavy split phase load can affect the circuit imbalance of another protected circuit. I learned not to wire high-end homes with mwbc a long time ago. JMO
 

joelj

Member
The permit has been extended and box fill is not an issue. The 2 GFCI circuits are in a 2 gang and 3 gang box and the refer / micro is in a 20 cu in. sgl gang that doesn't require GFCI. I'm waiting for a compelling reason to replace because some of the home is now finished and I would need to make holes in the new drywall to re-run the circuits. Otherwise, I would just replace them. The original dw/disp circuit in the home was 10/3 AL and I am abandoning that. Everything else is copper. It's odd that it was they only piece of AL in the entire home as far as I can tell. Thanks to everyone for your input.
 

edward

Senior Member
If you know how the MWBC works and know your box fill calculations, then there is no problem using them. I use them all the time especially for kitchens.

I have heard contractors says MWBC do not work with GFCI and that is why they don't use them. Which is not true, GFCI work fine with MWBC. It depends how to use the MWBC.
 
T

T.M.Haja Sahib

Guest
GFCI work fine with MWBC. It depends how to use the MWBC.
Do you have in mind (I)any single GFCI at the start of MWBC controlling all the three wires to the receptacles?

OR

(II)Individual GFCI receptacles in the MWBC?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Sahib, let's not rehash all that.

Suffice it to say that since he said he is using GFCI receptacles, and the MWBC switches to 2-wire at the first receptacle, the GFCI will work fine.
 
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