MWBC feeding a single 120vac sub?

I've installed an Outback GFX3524 grid-tie inverter and moved several critical circuits to a 120v sub panel to run on a 24v battery bank if/when the grid goes down. The sub is fed by a 60a breaker from the main panel per Outback's specs.

There are two more circuits I'd like to move to the sub but they are part of a multi-wire branch circuit. They are living room lights and dining room lights on two (adjacent) 15a breakers. I have not done a load calculation on the circuits but I can't imagine 8-9 lights being much of a load.

My question is can I combine the two hot wires of the two 15a MWBC onto a single 15a breaker (rated for two wires) in the 120v sub panel, or is that a code violation?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: MWBC feeding a single 120vac sub?

    First, is the branch wiring "sharing" a common neutral between a "black" and "red" 120/240 VAC branch circuit?

    If yes, then you must be sure that the 120 VAC breaker is 15 amps maximum as the two "Red and Black" wires would add current to a common nuetral if feed from a common 120 VAC source (i.e., if two 15 amp breakers on the same 120 VAC phase share a common neutral, then you would get 30 amps on the return).

    Since you are talking about just a single 15 amp breaker, then that is not an issue in itself (if you ever revert back to 120/240 VAC two 15 amp circuit breakers--then you have to make sure that the circuits are "converted" back correctly).

    Regarding loads--Many homes still have filament lamp based chandeliers--So they can add up quite quickly (6x 50 watt bulbs = 300 watts).

    Also, if there were a lot of CFL or LED lights with poor power factor (not that many have good power factor)--Then you should add up all the watts/volts (or current if listed on fixture/bulb) and multiply by two (for the typical worst case 50% PF) and make sure that the total current does not exceed 15amps*80%=12amps load (note, for CFLs and such that list rated current--you should not need to multiply by 1/PF -- That should already be taken into account when current is measured directly).

    In the end, can you do it safely (with a single 15 amp feed/breaker)--Yes. Will anyone ever find out you did such a dastardly thing. No--probably not.

    Does it violate code (it might, if there are Red and Black branch circuit wiring, those are supposed to be different phases).

    Are you comfortable (have you measured/estimated the current) with the amount of current flowing through the wiring? I believe you are not allowed to put two wires into one breaker opening--So you would have to add a wire nut and pigtail to make it legal (not a code guy--hopefully, somebody else here can give you that level of detail).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jkeenerjkeener Registered Users Posts: 19
    Re: MWBC feeding a single 120vac sub?

    Thanks for the reply.

    Yes, the conductors are red and black and share a common (white) neutral. It is more of a luxury thing to have those extra lights available. But it is not worth the added "luxury" if it violates code and can't pass inspection. Maybe a code person can chime in.... ;)
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: MWBC feeding a single 120vac sub?
    jkeener wrote: »
    I've installed an Outback GFX3524 grid-tie inverter and moved several critical circuits to a 120v sub panel to run on a 24v battery bank if/when the grid goes down. The sub is fed by a 60a breaker from the main panel per Outback's specs.

    There are two more circuits I'd like to move to the sub but they are part of a multi-wire branch circuit. They are living room lights and dining room lights on two (adjacent) 15a breakers. I have not done a load calculation on the circuits but I can't imagine 8-9 lights being much of a load.

    My question is can I combine the two hot wires of the two 15a MWBC onto a single 15a breaker (rated for two wires) in the 120v sub panel, or is that a code violation?

    You can certainly feed the two hot wires for the two circuits from a single two-circuit breaker (nominally designed for 120/240 operation.)
    In a standard breaker panel installation, the postion of the breaker on the bus will guarantee that the two hot leads are on separate phases. But if you are connecting both phase busses in the panel to one 120 volt source, you must make an adjustment to your wiring:

    Either you must run two separate neutrals, one for each circuit, or you must size the neutral wire for twice the current rating of the breaker for its entire length. I believe that either of those two options will be code compliant.
    You should probably, for the sake of future electricians working on the panel, put in a label which indicates that both busses are fed from a single phase.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: MWBC feeding a single 120vac sub?

    I think that "mixing" color codes on single circuit it "not legal"... However, you could black wrap the last few inches of red wire where ever it appears (not sure I would ever bother doing that).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: MWBC feeding a single 120vac sub?
    BB. wrote: »
    I think that "mixing" color codes on single circuit it "not legal"... However, you could black wrap the last few inches of red wire where ever it appears (not sure I would ever bother doing that).

    -Bill
    A very good question. The code says that colors or tags should indicate different voltages (and phases?), but I am not sure it prohibits using two different colors for the same phase. :-) The tape will definitely work. The only provision to take seriously is that white should only be the grounded conductor (not always the "neutral") in a circuit. For that reason you had to tape or paint the black wire white when using a two wire run between 3-way switches or in two switched feeds to something like a light/fan combination.
    SMA SB 3000, old BP panels.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: MWBC feeding a single 120vac sub?

    There is no problem combining the two circuits as long as the circuit breaker is rated no higher than either of the original circuit breakers.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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