How do you determine if an outlet is part of a unidentified multi-wire branch circuit? Also, does code say MWBCs need to be on double throw circuit breakers and is it permissible to link CBs with that hole in the handle?

The easy way to tell if an outlet is powered by an MWBC is to turn off the breaker(s) known to supply power to that MWBC and see if the outlet's power goes away.

The US National Fire Protection Agency's National Electrical Code does specify that all nongrounded conductors in an MWBCs must be controlled by a double-pole breaker or marked pair of linked breakers.

It's absolutely permissible to link breakers together using that hole - that's what the hole is there for. In most cases, though, it's a better idea to use a unitized double-pole breaker; that way you're likely to get the best possible match and service life between the two individual internal breakers.

Note that it is NOT permissible to link two breakers with different CURRENT ratings together in that manner - all nongrounded conductors in an MWBC must be the same size wire, and so if two linked individual breakers are used, then they MUST have the same current ratings.

  • It is also the case that a three phase MWBC must have a triple pole breaker ganged together. – wallyk Aug 17 '14 at 6:24
  • The nec does say that the link needs to be listed for the purpose but yes I always felt that a 2 (or 3) pole is better. – user24125 Aug 17 '14 at 8:51
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    @user24125 can you show me the code reference that disallows a 20A and 30A linked if#10 AWG is used for all three conductors even though I have never done that. – user24125 Aug 17 '14 at 9:03
  • What if they are improperly installed? I am dealing with many unidentified not in tandem, nor pigtailed, MWBCs. How to test if I'll regret disconnecting the neutral from an outlet? Will a clamp on amp meter show a reading over the neutral? – Mazura Aug 17 '14 at 20:26
  • @mazura "not in tandem", I meant not in double pole breakers. Also not necessarily adjacent either. – Mazura Aug 17 '14 at 21:34

You need to identify each pair of hot wires with there corisponding neutral first try starting at the romex sheath in the panel with the power to the panel OFF and follow each to there termination and label them. And relocate breakers as necessary. If this is not possible it is much more difficult .If you can find the first box that each MWBS is run to you can use a breaker finder tracer and a continuity tester to locate each wire at the panel. Tracer for the hots and continuity for the neutral with the wire taken loose at both ends. Continuity test needs to be done with all power off. Continuity can be used for all wires and is easiest with 2 people and a 2way radio or cell phone. If you aren't sure of how to test continuity tell me and I'll explain.

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