I was trying to replace a fan/light in my living room with a new led light, and have found that the living room branch circuit (lights and outlets) is actually fed by a Multi Wire Branch Circuit (14-3) that powers both the living room and a second branch circuit that only powers two kitchen lights. As an MWBC, The two circuits share one neutral and one ground.

I had suspected something might be funky, because a tester in a living room outlet had indicated a bad ground, which I understand would happen in this case, though it is also my understanding that both circuits would actually have a working ground.

At the panel, the MWBC is properly attached to two adjacent slots but the two breakers are not linked together, as code says they should be. After turning off the living room circuit, I was fortunate to check for hot wires before touching anything, so I was able to figure this out and turn off the kitchen lights branch circuit before I touched a hot wire.

Anyway, my question is, how can I fix this? Two options I was thinking about were: replacing the two breakers with a dual pole breaker so that they trip together and no one risks touching a hot wire like I almost did. I have been slowly adding GFCI/AFCI protection to other circuits; would I be able to find a dual function, dual pole 15A breaker like that? (Square D Homeline panel). Would the shared ground and neutral being shared cause problems with this? Secondly, I was thinking about just eliminating the branch circuit that powered the kitchen lights (disconnecting and wire nutting the hot wire at the panel and the junction box, and labeling them as such), and combining the kitchen lights and living room lights and outlet onto one circuit. I will be replacing the two kitchen lights with leds totalling only about 35 watts, so am I correct in thinking that the extra power draw from adding the kitchen lights to the living room circuit would be negligible and OK to do?

up vote 1 down vote accepted

You'll have to give up on ground fault protection here, unless you are willing to splice an extra box into the homerun or scrap the MWBC

There are three possible fixes for the base Code violation here. You can:

  • handle tie the existing breakers with a Square-D HOM1HT,
  • replace the existing breakers with a Square-D HOM215, or
  • move the kitchen lights onto the living room lights circuit with a pigtail and wirenut (thus getting rid of the MWBC entirely)

However, these approaches have different limitations when it comes to adding additional protection to the circuit:

  • With the first approach, you're basically stuck in a "stop gap" state until you replace the breaker anyway
  • With the second approach, you can use a Square-D HOM215CAFI to provide AFCI protection; however, the only way to add GFCI protection atop that for this circuit would be to splice a HOM24L70S/F "spa panel" box with a HOM215GFI in it in line with the homerun of the circuit.
  • With the third approach, you lose the MWBC, but you do get the benefit of being able to use a HOM115DF for AFCI and GFCI protection in a single package. Since you can't double-tap a Homeline AFCI, GFCI, or DFCI (the regular HOM115 supports it, but the HOM115DF can't because it has to use a different lug configuration to make room for the load neutral terminal), the best bet would be to use a pigtail and wirenut to combine the two hot wires in the panel.
  • Thanks ThreePhaseEel, I think I'm going to go with the third option. There's no useful reason for them to have run a MWBC here in the first place, and the additional draw from the kitchen lights circuit should be minimal with led. The outlets in the living room will be used for tv and stereo equipment that will be connected. – Mr McBanners and Cheese Oct 20 at 22:04

Make sure those "adjacent breakers" are in fact not "double-stuff" breakers. That's especially a problem in GE Q-line panels with their 1/2" tall breakers, where 2 adjacent breakers may in fact be on the same pole. If each of the two breakers is 3/4" or 1" tall, you are in the clear of that problem.

Yes, the simplest way to eliminate a MWBC is to nut all the hots together and pigtail them to a single 1-pole breaker. Now it is 1 plain circuit with much lower circuit capacity, of course.

Indeed that could be a 1-pole GFCI/AFCI breaker.

However, for about twice the cost, they do make 2-pole AFCI/GFCI breakers. These are great for protecting MWBCs and also NEMA 10-wired ranges or dryers.

If you don't want to do that, two single breakers can be tied with a listed (safety certified) "handle tie" made for that panel and breaker type. These do not provide common trip (much) but do provide the Code-required common maintenance shutoff. Handle-ties are hard to find, much easier to find are 2-pole breakers, at about $9, which do provide common trip.

MWBCs do not need common trip but do need common shutoff for the reason you now know.

1-pole GFCI breakers cannot be handle tied to serve a MWBC. Most 1-pole AFCI breakers cannot be handle-tied to protect a MWBC, but a new type is emerging which can.

  • You can get AFCI or GFCI in 2-pole, but not both at the same time. His panel's Square-D, so we don't have to worry about THQP shenanigans, though, at least! – ThreePhaseEel Oct 20 at 4:31
  • Yes, it is a square-D Homeline panel, with 1" tall breakers. I've been using the HOM120DFC and HOM115DFC to replace existing breakers that don't have Arc or Ground Fault protection, and the HOM115CAFIC and HOM120CAFIC to add combination arc fault protection to circuits that already have a gfci in the outlet. – Mr McBanners and Cheese Oct 20 at 4:41

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