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I am working on finishing my basement. In my basement there is 3 junction boxes fed by 12/3 wire. Each junction box feeds a single 12/2 line (White-White, Black-Black, Ground-Ground, Red-Nothing). My thought was this was done to allow expansion at a later date and my plan was to use the unused used red line.

I opened up the circuit breaker to see how they were wired (See picture). From my understanding of a MWBC each breaker should be on a different bus so that the load on the neutral would be the difference. If I'm looking at this right these are all wired wrong with both the black and red lines on the same bus bar.

In the picture:

Junction Box 1 - Breaker 1 + 5

Junction Box 2 - Breaker 2 + 6

Junction Box 3 - Breaker 4 + 8

If this is indeed incorrect, slightly more concerning is that 3 + 7 are on a 3-wire and both in use.

I bought this house new years back, so before I call up the builder. I wanted to verify that this was incorrect or is there some other reason they may have been done.enter image description here

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  • Probably because whoever set up those MWBC's was accustomed to GE panels with their thin breakers. That technique would have been correct there, in the 80s. However of late, MWBCs must be on 2-pole breakers or pairs of breakers with listed handle ties. And those force you to phase correctly.
    – Harper - Reinstate Monica
    May 18 at 1:20

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Yes, this is a big problem. Your understanding is correct. With full-size breakers (which is the case for everything in this picture), the hot legs alternate, so by skipping a space with an MWBC you end up with two hots on the same leg, potentially overloading the neutral.

Easy enough to fix by rearranging the breakers so that any MWBC pair of hot wires are on consecutive (vertically speaking) breakers.

There is one additional issue. For safety when working on an MWBC, code now requires common shutoff. That can be done either by using a double-breaker (which will also provide common trip, which is not required for MWBC but is perfectly fine) or by using a listed handle tie. Here is one Square D example from Home Depot:

handle ties

Depending on brand and where you get them, they might range from under a dollar each to $ 3 or more. Overpriced for what they are. But a lot cheaper than replacing the breakers.

In addition to safety benefits, handle ties (or double breakers) make it exceedingly clear to the next owner that these pairs of breakers need to be next to each other and not moved around separately.

An interesting test you could do if you have a clamp meter is to turn on a moderate amount of stuff on circuits 3 and 7 and then measure the current on the black, red and white wires. If everything is wired as you described, the white will show the sum of the black and red current readings instead of the difference.

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