Adjacent outlets on two different phases??

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  #1  
Old 07-01-11, 09:48 PM
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Adjacent outlets on two different phases??

In my garage, a 2 phase Rommex wire (Black,Red,White,Ground) comes, presumably from the attic, and feeds two separate outlets in a two gang outlet. So the white wire is shared, while the black phase feeds one outlet and the red phase feeds the other outlet. I’ve never seen this anywhere else in my house so I’m wondering is this ok? That is, to feed two adjacent outlets with different phases sharing the white neutral?
 
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  #2  
Old 07-02-11, 12:32 AM
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Oui it is called MWBC ., Multi Wire Branch Circuit.

It pretty common used in MWBC in both single and three phase fashon.

Really there is no true 2 phase at all { expect very few old industrial or commercal location may have it } it more properly term used in North Américiane side is split phase so therefore from black to white is 120 volts while white to red is 120 volts also but when you get black to red then it become 240 volts.

In your appaction some case someone may have actual former 240 volt circuit which it still a MWBC due someone may have a 240 volt equiment before and put in MWBC split receptale arrangement as you describing it and by per NEC code you can do this is either run indepent GFCI recepteals however once you run downstream you can NOT share the netural from either circuit otherwise the GFCI receptale will trip or other legit option is put in 2 pole GFCI breaker then it will work like normal MWBC fashon but have GFCI protection there.

Just give you a head up the two pole GFCI breakers are not cheap they run about 60 Euros or more.

Hope that help you on this one.

Merci.
Marc
 
  #3  
Old 07-02-11, 10:39 AM
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Thank you french277V. You actually also answered the next question I had about GCFIs relating to this “split phase MWBC” situation, as it is apparently called.

See the problem is that this MWBC wire comes down from the attic (I assume) feeds this first 2 gang BUT THEN CONTINUES (inside the wall) to feed additional 2 gang situations on other walls of my garage - and all these downstream 2 gangs are wired to the MWBC wire exactly the same (that is one separate phase feeds each receptacle, the white neutral is shared)

So I had been going through all the circuits in my house and replacing the first receptacle in the chain with CGFI for better protection. But now, for this particular circuit seems it is NOT POSSIBLE to do that because if I replaced the first 2 gang in the chain with CGFIs the white wire would be common downstream and I imagine that would cause the CGFIs to trip on unbalanced current whenever anything was plugged in the downstream 2 gangs (I have not tried it but reading how CGFIs operate I suspected there may be a problem).

So from the insigntful advice that you posted regarding CGFIs in this MWBC situation seems like I cannot have CGFI protection on this circuit unless I put a two pole CGFI.

So is there a 2 pole CGFI/2 receptacle combination or would this have to be a separate breaker/switch I have to cut into the wall to install? (and an expensive one seems like)

Thanks.
 
  #4  
Old 07-02-11, 11:50 AM
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So is there a 2 pole CGFI/2 receptacle combination or would this have to be a separate breaker/switch I have to cut into the wall to install? (and an expensive one seems like)
I see two options if you want all the receptacles GFI protected. The first and easiest option, but probably most expensive, is to install a 2 pole GFI breaker in the panel like French suggested. The second option would be to install two GFI receptacles at each 2 gang box, wired to line terminals ONLY. You would have to pigtail conductors and NOT wire the remaining circuit to the load terminals on the GFIs. If you have very many of these 2 gang boxes you could quickly spend more on GFI receptacles than the 2 pole GFI breaker would cost, but I like the convenience of having the GFI test/reset buttons handy at the receptacle location.
 

Last edited by ray2047; 07-02-11 at 05:03 PM. Reason: Change load to line.
  #5  
Old 07-02-11, 04:37 PM
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wired to load terminals ONLY
I hope you mean the line treminals.
 
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Old 07-02-11, 05:05 PM
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Typo corrected.

..........
 
  #7  
Old 07-02-11, 05:27 PM
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Thanks CasualJoe for summarizing.
I prefer the central CGFI breaker solution because the MWBC wire goes through some exposed metal conduit and metal boxes in the garage (some of the wire is in the wall some in exposed conduit that I can tell). I don’t know how well my house is grounded, which is one of the reasons I wanted the CGFI in the first place. So if I just intall a number of CGFI’s (I would need 6) the conduit and other metal work would not be CGFI protected.
 
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Old 07-02-11, 05:34 PM
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$60 for the gfci receptacles, $ 80-170 for a gfci breaker depending on the company.

the conduit and other metal work would not be CGFI protected.
If it is properly grounded, It doesn't need gfi protection.
 
  #9  
Old 07-02-11, 06:02 PM
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?%#@ another complicating factor I just discovered is that this MWBC wire feeds the two outside outlets in my back yard (seems like there was an old Jacuzzi area there which previous owners took with them). Apparently the MWBC travels to the outdoor outlets through underground PVC conduit. I do not really need those outdoor outlets so I could disconnect them really but if there is a way to preserve them I would rather keep them. Perhaps irrational fear but I would like to have CGFI protection on a wire that travels underground.

I get the feeling that these MWBCs have significant shortcomings. I wish whoever wired this would have just run two separate circuits after the first outlet in the chain.
 
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Old 07-02-11, 07:24 PM
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The proper initials are GFCI for Ground Fault Circuit Interruption.

There is NOTHING inherently wrong or dangerous about multi-wire branch circuits as long as the person working on the circuits understands their construction and usage. Unfortunately, most homeowner and "weekend warriors" do NOT understand them and that can cause problems when they try to modify the circuits. I personally have no problem with MWBCs being used in commercial and industrial situations but I think they should be restricted in residential service because of the possibility of an unknowing person making dangerous changes.
 
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Old 07-02-11, 07:58 PM
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ThomasE
?%#@ another complicating factor I just discovered is that this MWBC wire feeds the two outside outlets in my back yard (seems like there was an old Jacuzzi area there which previous owners took with them). Apparently the MWBC travels to the outdoor outlets through underground PVC conduit. I do not really need those outdoor outlets so I could disconnect them really but if there is a way to preserve them I would rather keep them. Perhaps irrational fear but I would like to have CGFI protection on a wire that travels underground.

I get the feeling that these MWBCs have significant shortcomings. I wish whoever wired this would have just run two separate circuits after the first outlet in the chain.
Thomas.,

Before we can go further on this one can you verify the breaker size and the conductor { wire } size for the former Jacuzzi or spa tub ? look at the unused breaker somecase they may unhook in the panel or you can able look at the former Jacuzzi or spa connection box if that was a spa genrally will have 16mm² { # 6 AWG } conductor with either 50 or 60 amp breaker the Jazzuci genrally will have much smaller conductor so it can go anywhere from common 4.0mm² { 12 awg } all the way to 16mm².

Merci,
Marc
 
  #12  
Old 07-02-11, 08:23 PM
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Typo corrected.
Thanks, Ray. I shouldn't be watching the TV when commenting on here.
 
  #13  
Old 07-02-11, 08:27 PM
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Originally Posted by Justin Smith View Post
$60 for the gfci receptacles, $ 80-170 for a gfci breaker depending on the company.

If it is properly grounded, It doesn't need gfi protection.
It's in a garage, it needs to be GFI protected.


In my garage, a 2 phase Rommex wire (Black,Red,White,Ground) comes, presumably from the attic, and feeds two separate outlets in a two gang outlet.
 
  #14  
Old 07-02-11, 08:34 PM
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If two neutral pigtails are added to the incoming double gang box GFI receptacles can be used and can also have the downstream protection.

The conduit should already be grounded to the panel. You are providing the GFI for personal protection, not the ground the conduit.
 
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Old 07-03-11, 07:51 AM
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It's in a garage, it needs to be GFI protected.
Sorry, I was talking about the conduit, not the receptacles.
 
  #16  
Old 07-03-11, 01:28 PM
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Sorry for mentioning Jacuzzi. I do not know for a fact what exactly was hooked up in those outdoor outlets. I thought previous owners had mentioned something about a Jaccuzi they had disconnected… but perhaps I assumed too much. In any case, the wire feeding those 2 outdoor outlets is 12-3 and the main panel breakers for that circuit are 20A. There are no other breaker boxes.

In an ideal world I would be confident that all the metal conduits and boxes I see in the garage are grounded. However my house is from the 50s and the original wiring seems to have no ground from what I can tell. Many of the original outlets are black and white wire only, (though in the remodeled part the green wire is present). In summary, I feel rather unsure about whether every piece of conduit in the house is actually grounded and with low enough resistance to trip the fuse. Hence the attractiveness of the GFCIs.to watch not only for leaks after the outlet but in the wiring too, especially that long run into the wet outdoors.

I’m thinking that it may be best to eliminate (disconnect) one phase from that MWBC circuit (or at least carry the second phase only up to the first 2 outlets in the chain and then continue single phase. I can probably live with the 20A limit further downstream, including the outdoor outlets. I’d rather have the safety than the ampacity, (if I’m using the correct terms).
 
  #17  
Old 07-03-11, 03:24 PM
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If you are worried about the conduit being properly grounded you could pull an insulated green conductor and attach to all the boxes using a ground screw.
 
  #18  
Old 07-04-11, 09:53 AM
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Originally Posted by pcboss View Post
If two neutral pigtails are added to the incoming double gang box GFI receptacles can be used and can also have the downstream protection.
Using GFCI receptacle units and multiwire branch circuit, only one half of the MWBC can continue downstream with protection (not needing another GFCI at each box).

Do not tie together the incoming neutral going to the GFCI line terminal with the continuing neutral attached to the GFCI load terminal.

If two GFCI receptacle units are in the same box, do not tie togehter the load side neutrals.
 
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