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As I continue to make sense of the wiring in home, I have come across some shared neutral configurations. In one, I find two breakers that have been connected to one NM-3 line i.e. black and red are each connected to a separate breaker (on the same side of the box.) As I understand from here and other references, this is OK but it should be on a 'split-phase double breaker'.

First question: Am I right that this configuration is incorrect and should be fixed?

Second question: This suggests that these configurations are problematic for AFCI installations. I want to upgrade my breakers to AFCI. Will this kind of thing be an issue?

Lastly, for anyone so inclined, I have a Cutler-Hammer CH style board. If this needs to be fixed, which breakers would be acceptable?

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As long as the ungrounded (hot) conductors of the circuit are fed by different legs of the service, then there's no problem with the wiring. According to modern codes, the breakers need to at least have a handle tie. However, since the installation presumably predates the code, it does not have to be changed (unless you're doing work that requires it).

If you want to install AFCI breakers, you'll simply have to replace the two breakers with a double pole AFCI breaker.

  • When you say "fed by different legs of the service" I take that to mean that the breakers should be on separate bars. I just took a closer look and realized that the breakers alternate which bar they connect to. They are right next to each other so that means it's OK, right? – JimmyJames Jul 4 '16 at 19:19
  • Tester is spot on! I have heard folks say they have had AFCI Problems with a multi wire branch circuit but have not experienced this problem, the problems I have seen with AFCI's have been on light dimming circuits, flouresent ballast's and motor controls in furnaces. Not a MWBC issue. + – Ed Beal Jul 4 '16 at 19:26
  • @JimmyJames yes. One breaker gets power from one ungrounded (hot) bar, while the other breaker gets power from the other ungrounded (hot) bar. – Tester101 Jul 4 '16 at 19:36
  • Yeah -- most AFCIs (Mod3 or higher GE AFCI hardware is the sole exception to this I know of) on the market also have a 30mA differential (aka ground fault) trip on them, which means you need a two pole unit for MWBCs. – ThreePhaseEel Jul 4 '16 at 23:16

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