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Thread: Shared Neutral

  1. #1
    Join Date
    Mar 2003
    Location
    Nebraska
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    2

    Shared Neutral

    What section of the NEC covers shared neutrals?

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    16,724

    Re: Shared Neutral

    The phrase “shared neutral” does not appear in the 2002 NEC. However, the phrase “common neutral” appears in several places. Here are two:

    215.4 Feeders with Common Neutral.
    (A) Feeders with Common Neutral. Two or three sets of 3-wire feeders or two sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.
    (B) In Metal Raceway or Enclosure. Where installed in a metal raceway or other metal enclosure, all conductors of all feeders using a common neutral shall be enclosed within the same raceway or other enclosure as required in 300.20.

    225.7 Lighting Equipment Installed Outdoors.
    (A) General. For the supply of lighting equipment installed outdoors, the branch circuits shall comply with Article 210 and 225.7(B) through (D).
    (B) Common Neutral. The ampacity of the neutral conductor shall not be less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit.

    Is that what you were looking for?
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

  3. #3
    Join Date
    Mar 2003
    Location
    Nebraska
    Posts
    2

    Re: Shared Neutral

    Partially, I am trying to narrow down how many circuits can share the same neutral and ground. For example, can a circuit containing six lights share the neutral and ground with a circuit containing two receptacles. Keeping in mind that both circuits are on their own breaker and on opposite sides of the bus. Also, can three receptacle circuits share the same or common neutral and ground. Also keeping in mind that all current carrying conductors, grounds and neutrals are in the same raceway.

  4. #4
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,569

    Re: Shared Neutral

    Look at the definition of Branch Circuit, Multiwire in Article 100. If it is a single phase system, then 2 hots can share a common grounded conductor. If it is a 3 phase system, then you can use 3 hots with a single neutral.
    As far as the equipment grounding conductor, a single green wire can serve all of the circuits in the same raceway as long as the EGC is sized based on the largest OCPD protecting any one of the circuits in the raceway.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #5
    Join Date
    Feb 2003
    Location
    Florida
    Posts
    7,258

    Re: Shared Neutral

    Installing multiwire branch circuits needs to be done in compliance with 210.4 of the 2002 NEC. The important issue here is ensuring the ungrounded conductors end up on different phases. If not, the neutral currents will not cancel out and will overload the neutral conductor. However, placing the ungrounded conductors on the different phases also has its problems. If the neutral were to open during operation, you essentialy have placed the two circuits in series with each other. This in effect cause low and high voltages outside of nominal ranges that can damage equipment. Great care must be taken when installing multiwire branch circuits.
    Bryan P. Holland, MCP

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