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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.)

Thanks!

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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).

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  • Thanks for the detailed explanation. I feel silly asking this, but if I use an existing lighting circuit for the lights in the bathroom and fork the MWBC for the bathroom receptacles, what do I use the other circuit from the MWBC for? One reason I am unsure is because my understanding is only one of the two circuits can have a GFCI when I fork it. The basement is decently sized, so maybe another lighting circuit, or maybe I simply do not fork it?
    – DaveS
    Apr 1 '21 at 22:50
  • A MWBC can have a GFCI recep anywhere if you don't use the "Load" terminals to extend GFCI protection. If you do, then the "Load" neutral cannot rejoin the MWBC and can no longer be shared. The other option for a MWBC is to use a GFCI breaker, in which case you can arrange it as you please. A MWBC is one circuit, but I believe you are allowed to use half as a mandatory bathroom circuit and half as something else. Apr 2 '21 at 0:01
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Generally you need to put the countertop receptacles on a separate circuit feeding only work surface receptacles, but an exception allows other outlets (hardwired fans and light) since your circuit feeds only one bathroom. (NEC definition of receptacle is distinctly different from "outlet". Receptacles are outlets, not all outlets are receptacles.)

210.11(C)(3) Dwelling Units. Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, one or more 120-volt, 20-ampere branch circuit shall be provided to supply bathroom(s) receptacle outlet(s) required by 210.52(D) and any countertop and similar work surface receptacle outlets. Such circuits shall have no other outlets.

Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).

The Italicized wording is new to 2020, and effects other receptacles in the bathroom that don't feed the work surfaces. 210.23(A)(1) and (A)(2) deal with allowed amperage on cord connected and fastened in place equipment. If you are going to have another wall receptacle in the bathroom that might feed a portable heater you could legally install it on either circuit. Do you want lights or hair dryer to go dead with it?

Identified Handle Ties are acceptable, the 2020 NEC says:

240.15(B)(1) Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiwire branch circuits that feed only single-phase line-to-neutral loads.

You can't use a nail or superglue some zip-tie in place. (Actually removed that once). The NEC hasn't always required handle ties, the previous owner may have run the circuit before the code required handle ties, grouping, and other identifying requirements.

As to whether it will trip both breakers when one trips? My experience hasn't been consistent, I don't know the code intent.

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Code still recognizes MWBC’s in NEC 210.4

Multiwire branch circuits when wired to code as as safe as any other circuit.

The 2 single pole breakers do require a handle tie when used as a mwbc 210.4.B. Since you are doing the work now it would be under your states current adopted code but handle ties have been required for a few code cycles so you will need a tie or a double pole breaker to be code compliant.

I understand your desire to have separate circuits for the receptacles and lights but a multiwire branch has required handle ties for safety for a decade or longer in some areas. I find GFCI’s fault more often than overloads or tripped breakers so having the receptacle that is first be a GFCI in the bathroom allows for a local reset. A receptacle GFCI provides a degree of isolation compared to using a double pole GFCI in the service panel and it is less likely to trip a 20 amp breaker unless both a hair dryer and curling iron are used at the same time and then they both would work if 1200w but higher values used at the same time will trip the breaker.

The reason multiwire branch circuits are not used as widely today Is because of GFCI’s. GFCI’s on both branches tend to have more false trips. But in your case if you have the lighting on the separate circuit you will not see problems as often as both legs being on a MWBC.

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