MWBC and different size wires

[TOOL_5150]

I ran into something I havent seen before, A MWBC with a #12 noodle and a #12 hot and a #14 hot. The house is K&T so it was 3 loom covered wires I was dealing with, the #14 being for the lights.

I was doing a service upgrade [whopping 4 circuits in this house!] I am still on the 2005 NEC, so I used SP breakers. The #14 was protected by a 20A FPE breaker.

When I hooked up my 4 circuits [2 MWBC's] I put both legs of that particular MWBC on 15A breakers. Could I use a 20A on the #12 hot, since the noodle is #12 also, while leaving the #14 leg on a 15A breaker? I am going back tuesday to finish up a few other things, so I could change it then. I just wanted to error on the safe side and used 15A breakers.

~Matt

 

[iwire]

Yes you can mix sizes.

On commercial job I re-used a 6/3 MC for one 20 amp 120 and 30 amp 120 circuits.

 

[480sparky]

You should check to see if the two circuits are not on the same yoke.

 

[hurk27]

The neutral only has to be sized for the largest breaker protecting the largest ungrounded circuit conductor in a multi-wire branch circuit, IE.
I had one that had a #10 with a #12 and then a #14 but with a #10 neutral and EGC, all legal # 10 fed a 30 amp 120 volt popcorn machine, the #12 fed a receptacle circuit on the same counter, And the #14 fed a 5 light wall sconce over the counter, from a 208/120 panel.

 

[mxslick]

I ran into something I havent seen before, A MWBC with a #12 noodle and a #12 hot and a #14 hot. The house is K&T so it was 3 loom covered wires I was dealing with, the #14 being for the lights.

I was doing a service upgrade [whopping 4 circuits in this house!] I am still on the 2005 NEC, so I used SP breakers. The #14 was protected by a 20A FPE breaker.

When I hooked up my 4 circuits [2 MWBC's] I put both legs of that particular MWBC on 15A breakers. Could I use a 20A on the #12 hot, since the noodle is #12 also, while leaving the #14 leg on a 15A breaker? I am going back tuesday to finish up a few other things, so I could change it then. I just wanted to error on the safe side and used 15A breakers.

~Matt

Yep like Bob said it is legal. My apartment was originally wired the same way, a single 12/12/14 NM cable from the meter doghouse up to the apt, where it split off in the first receptacle box with the 12 feeding ALL the recepts in the apt and the 14 feeding the lights.

Needless to say with me being me, as Bob will testify, it didn't stay that way long before I upgraded the entire service to my unit. :grin:

As for the FBE breaker, we all know that even the #12 wouldn't be protected by a 20 amp FPE.

And yes you can change the breaker on the #12 to a 20 amp.

 

[480sparky]

........., a single 12/12/14 NM cable ...........

Huh? An NM cable with two 12s and one 14?

 

[mxslick]

Huh? An NM cable with two 12s and one 14?

Yep, #12 black, #12 white, #14 red. No ground.

It was the MWBC that fed my apt. All of the units (4) in my building were home-run with it. They also had the old Cutler-Hammer double breakers fed directly off the old A-Base meters. At one time in the past the owners had replaced the C-H breakers with Homeline 2/4 space panels. I replaced mine with a Homeline 6/12 and added a main breaker to my panel. (The 5th unit in the pic is the "owner's" unit added later.)

Mine is unit "B". You can see the old C-H breaker "boxes" which are now used as j-boxes.

The old FPE panel on the right was recently replaced with an 6/12 Homeline like mine, and the meters and sockets were upgraded to new A-base to round socket adapters and Electronic meters.

 

[Split Bolt]

Interesting! Never seen cable like that! If it is FPE Stab-Lok, you can always put the wires under the main lugs if there are no more breaker spaces available. You'll get exactly the same amount of protection!

 

[TOOL_5150]

Thanks everyone!

Interesting! Never seen cable like that! If it is FPE Stab-Lok, you can always put the wires under the main lugs if there are no more breaker spaces available. You'll get exactly the same amount of protection!

Yeah really I changed the FPE out with a murray metermain. Ill add a few pics to this thread after tuesday.

~Matt