I am a Building Inspector and recently have came across a question about splitting the neutral wire between the garbage disposal and the dishwasher. I am unable to locate this in the 2011 NEC. Does anyone know where I may locate this?

Thank you, Wendy

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    What exactly do you mean by splitting the ground wire? Do you mean a multiwire branch circuit? – Grant Jun 16 '16 at 16:22

Yes, the proper term for that is "multi-wire branch circuit". Two hot wires share a neutral. The neutral handles only the difference in current, becuase the hots are on opposite poles. If one hot is loaded to 15A and the other is loaded to 13A, the neutral only carries 2A. Several things for an inspector to watch for:

  • the hots MUST be on opposite poles. If they are on the same pole, the neutral will carry the sum of currents (28A) instead of the difference (2A).

  • In MWBC, neutral wires do not count as conductors for anything thermal. E.g. There is a maximum of 9 conductors allowed in one conduit before derating is required. If you see 12 wires in four MWBCs, that is OK because it's only 8 conductors. Why? See note at bottom.

  • Wires in MWBCs must be marked as "together" in the usual manner of marking wires as groups, e.g. entering the box in the same Romex.

  • starting at NEC 2002, neutrals in MWBC's must be pigtailed. To be more precise, wiring must be done so that any device (outlet, switch, lamp, etc.) can be removed for servicing without severing the neutral for the other side of the MWBC. That's so if the power is still turned on for the other half of the MWBC, the serviceman doesn't interrupt its neutral. This is not a factor after the hots go their separate ways, e.g. if they split into two outlet strings.

  • Starting at NEC 2011, the maintenance shut-off must disconnect all legs of the MWBC. On common circuit breaker panels, the breakers need handle ties. A 2-pole breaker will do the job, but it is not required. Overcurrent protection does not need to trip together, so if a trip on one leg fails to trip the other handle-tied breaker, that is not a violation.

  • Beware of MWBC's connected to a tandem/duplex breaker. People often confuse these for 2-pole breakers, but they are on the same pole, so they create the dangerous situation in my first point. One hint is that they don't make handle ties for that.

  • There is such a thing as a 3-phase MWBC. This involves 208 "wye" and three hots and a neutral. With these, you can get 3 MWBC's (e.g. 9 lighting zones) into a single conduit without derating.

Why do 3 wires count as 2? Heat is a function of current squared, and in any MWBC load, the three wires' heat is equal or cooler than two wires carrying the same current. Three wires carrying 15, 13 and 2 amps run cooler than two wires carrying 15 and 15 amps. Hence they are only counted as two wires.

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NEC 210.4 multi wire branch circuit. There are 2 hots 1 neutral and 1 ground. the breakers supplying this configuration are required to be handle tied or Double pole breakers. Many electricians use these to save wire especially on dishwasher/garbage disposal circuits. Also for the 2 required small appliance circuits.

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    As of NEC 2008, yes. Legacy work does not need handle ties. However what's absolutely required is that they be punched down on opposite poles. If they are on the same pole, it will overload the neutral. A voltage measurement between the two "hots" must be 240V. If it is 0V, it is punched down wrong and must be taken out of service immediately. – Harper - Reinstate Monica Jun 17 '16 at 0:59

If you're asking about supplying a disposal and dishwasher with a multiwire branch circuit. Then there's no problem at all, as long as the MWBC is properly installed.

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