Electrical – MWBC exists, use for bathroom


I am finishing my basement. The previous owner had ran a MWBC to the area that will be the bathroom. In the panel it is two 1-pole 20amp breakers. My questions are:

  1. First to confirm, it seems I should split/fork the MWBC into two circuits. One for the lights and exhaust fan, and the other for receptacles. On the receptacles circuit the first outlet should be GFCI. Correct?

  2. I am very unclear on if the two 1-pole breakers is considered safe or even to code. Should I be replacing the breakers with one that is combined, simply add a handle tie, or just leave it as is?

Relating to the last question, my understanding is its best to have two separate circuits in a bathroom. This way if the receptacle circuit blows when using a hair dryer, the lights also do not go out. For example, by using a handle tie don't I loose this — won't both circuits go out? Should I instead split the MWBC and use one circuit for the bathroom receptacles, and the other for lights in the other part of the basement? (This way if the breaker trips you aren't left in the dark in the bathroom.)


Best Answer

Ed Beal's answer covers the core matter.

The handle-tie is mandatory, period. So that's the end of that inquiry.

You're right that with handle-ties, one will probably trip the other. However FYI that's no guarantee*.

However, I agree with keeping bathroom lights off the receptacle circuit. What you might be overlooking is that a) lights and fan are quite small loads (less than 1 amp together)... and b) there are NO circuit restrictions associated with hardwired bathroom loads. They can be on any circuit**.

Since presumably you have basement lighting already existing, you can simply tap the basement lighting circuit for the hardwired bathroom lights and fan.

The only gotcha I see if that if you have lighting or fan inside a shower stall, it must be GFCI protected. However basement receptacles are also supposed to be GFCI protected, and you could always place the shower stall light/fan on the GFCI-protected "LOAD" side of a basement recep circuit.

* Factory 2-pole breakers rated for common-trip actually have an unseen internal mechanism which guarantees common trip. Handle-ties are not enough, because of breakers' "trip free" feature: they will trip even if the handle is held/locked in the "on" position. The handle moving to the tripped position is merely a courtesy indication; if a handle-tie were to tram or bind, the tripping breaker would just trip anyway and fail to trip the other breaker.

** Except for circuits reserved for other stuff, e.g. dedicated circuits required for kitchen, laundry room, garage, and bathroom (though see the 210.11(C)(3) exception NoSparksPlease mentions above).