MWBC Questions

[Mr onetwo]

Good morning all,

I am finishing up an addition on my house and I hope I did not screw myself by roughing in a single 14/3 N-MB for outlets and lighting. The wire is buried in spray foam and the wall is finished. I can abandon 1 of the circuits and run everything off 1 15 amp arc-fault but I don't think that is a good idea with HALO Led puck lights and a Lutron dimmer. There is a total of 5 interior outlets and 1 exterior GFCI outlet for service on a mini-split mounted on the end of the addition.Have I screwed myself or will this all work out?

 

[Mr onetwo]

If the GCFI outlet is a violation I can abandon it and pull power off the mini-split if that is permissible.

 

[75gmck25]

Not sure, but I don't think you can combine a GFCI and a non-GFCI breaker with a handle tie for an MWBC, and not have it trip the GFCI.

 

[PCustoms]

Ditch the GFCI on the panel and just put it at the outlet.

Note: I feel like there is some screwy issue with mwbc and GFCI. I don't run MWBC so I'm not certain

 

[sparky 1971]

If you want AFCI's, you'll need a two pole breaker, handle ties won't do it.

https://www.supplyhouse.com/Square-D-HOM215GFI-Homeline-2-Pole-Miniature-GFI-Circuit-Breaker-120-240V-15A?utm_source=google_ad&utm_medium=pMax_General&utm_campaign=PMAX-New&utm_campaignid=20507293117&utm_adgroupid=&utm_targetid=&utm_product_id=HOM215GFI&utm_matchtype=&utm_keyword=&utm_adtype=pla&utm_category=General&gad_source=1&gclid=CjwKCAjwzMi_BhACEiwAX4YZUJ6Yf4NXnI91HPi7KJ3qflATFUfGvl5TQUQI5fWcUuJk_El_bHUJZhoCATkQAvD_BwE

The GFCI won't effect anything as long as it's connected to the line side only. It's when there's a load side connected that things get funny, the load would need a separate neutral from that point forward.

 

[Mr onetwo]

CB1 in the panel is an arc-fault breaker as required by code for the outlets. I watched a video on the Mike Holt forum and it stated that the real purpose of a handle-tie is to assure common shutoff when the handle is used as an "off" switch, not common "trip". Also to provide a visual cue that the circuits are related. Am I incorrect in this assumption? If I should go to a 2 pole arc-fault, is this going to damage the dimmer or LED's?

 

[sparky 1971]

CB1 in the panel is an arc-fault breaker as required by code for the outlets. I watched a video on the Mike Holt forum and it stated that the real purpose of a handle-tie is to assure common shutoff when the handle is used as an "off" switch, not common "trip". Also to provide a visual cue that the circuits are related. Am I incorrect in this assumption? If I should go to a 2 pole arc-fault, is this going to damage the dimmer or LED's?

Handle ties are for common "off" but if you want it to be two circuits and have AFCI you'll need to use a two pole breaker and have common off as well as common trip which won't hurt anything, including the LED's, but if one circuit trips they will both go off. That could also happen with a handle tie as well, but isn't the intent.

Two single pole AFCI breaker and a handle tie won't work. MWBC's with handle ties will work with normal circuits because the breakers are only monitoring a hot wire for short circuits and overloads. When it's an AFCI, they are also watching the neutral conductor but since it's a MWBC, there is one neutral for two breakers and it's just not going to work. If you want to use single pole AFCI breakers you will have to splice the hot wires together in the panel and make one circuit out of them. You should have used separate 14/2 cables.

 

[yatg]

 

[Mr onetwo]

You should have used separate 14/2 cables.

You are correct sir....an expensive mistake.

 

[wyliesdiesels]

You should have used separate 14/2 cables.

or this made specifically for AFCIs

Southwire 250 ft. 14/2/2 Solid Romex SIMpull CU NM-B W/G Wire 55048255 - The Home Depot

Visit The Home Depot to buy Southwire 250 ft. 14-2-2 NM Wire with Ground 55048255

www.homedepot.com

 

[reader2580]

My house has an existing MWBC. I had to change to AFCI so I had to get one of those expensive double pole AFCI breakers.

I would never install an MWBC as new.

 

[dscheidt]

My house has an existing MWBC. I had to change to AFCI so I had to get one of those expensive double pole AFCI breakers.

I would never install an MWBC as new.

MWBC have big advantages when dealing with conduit, because it uses fewer wires, and fewer current carrying conductors, both of which help with conduit fill. 1/2" EMT has a fill capacity of nine 12AWG wries. That gives you four two wire 120V 20A circuits, using 8 wires; or three 240V 20A MWBC used as six 120V circuits. In a big building, that can save a bunch of conduit runs and wire.

It also makes reuse of existing conduit more efficient. My house was built about 1925, with rigid conduit (emt hadn't been invented yet...), and has fewer trunk conduits going to the panel than you'd like. When we bought the house, we had it rewired, and they used a bunch of MWBC to cope with the limited conduit capacity. Sometimes, they ran both sides of the MWBC to the same area (the kitchen for instance has the two counter outlet circuits on an MWBC, with one side the left outlets, the other the right. In other cases, though, they're just using it get out of the utility room, and it splits at the first junction box. The back bedroom receptacles shares a MWBC with the lighting for front porch, front hall, and living room. three wires leave the panel, run a few feet up to the bedroom, and split there. All of this is done properly to the code in place at the time (which didn't require handle ties, let alone AFCI), and let two guys do the whole house in less than two full days[1]. But a typical DIYer from romex-land would have problems figuring out how it all works.


[1] it took longer install six new outlets in the living and dining room, and that made much more of a mess.

 

[Bert_]

You need a 2 pole because of the shared neutral. A GFCI and even afci monitors neutral current, if the current going out is different than the current coming back the breaker will trip.

Or if you can get away with it just toss the afci. I use them when I have too but there isn't a single one in my own house.

 

[reader2580]

I've tripped one AFCI. I have no issue with them other than the cost. I rewired at least 50% of my house after I bought it which triggered AFCI requirements to pass electrical inspection. No reason to take them out when they are paid for.

 

[reader2580]

MWBC have big advantages when dealing with conduit, because it uses fewer wires, and fewer current carrying conductors, both of which help with conduit fill. 1/2" EMT has a fill capacity of nine 12AWG wries. That gives you four two wire 120V 20A circuits, using 8 wires; or three 240V 20A MWBC used as six 120V circuits. In a big building, that can save a bunch of conduit runs and wire.

It also makes reuse of existing conduit more efficient. My house was built about 1925, with rigid conduit (emt hadn't been invented yet...), and has fewer trunk conduits going to the panel than you'd like. When we bought the house, we had it rewired, and they used a bunch of MWBC to c

That is not really new if using existing conduit. I am thinking of brand new wiring like the circuits I will be putting in my new garage. I am not going to install any MWBC circuits just to save a few bucks on wire.