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October 12, 2007 A Penton Media Publication Vol. V No. 19

310.4 Conductors in Parallel

What's Wrong Here?

Code Q&A

Code Quiz

EC&M Code Change Conferences

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    Top 50 NEC Rules

    310.4 Conductors in Parallel

    By Mike Holt
    Ungrounded and grounded neutral conductors sized 1/0 AWG and larger can be connected in parallel (i.e., electrically joined at both ends).

    When conductors are run in parallel, the current must be evenly distributed between the individual parallel conductors. This is accomplished by ensuring that all ungrounded and grounded neutral conductors within a parallel set are identical. Each conductor of a parallel set must:

    1. Be the same length.
    2. Be made of the same conductor material (copper/aluminum).
    3. Be the same size in circular mil area (minimum 1/0 AWG).
    4. Use the same insulation material (like THHN).
    5. Terminate in the same method (set screw versus compression).
    In addition, raceways or cables containing parallel conductors must have the same physical characteristics and the same number of conductors in each raceway or cable. Conductors for one phase (ungrounded conductor) or the grounded neutral conductor, aren't required to have the same physical characteristics as those of another phase or grounded neutral conductor to achieve balance.

    The equipment grounding (bonding) conductors for circuits in parallel must be identical to each other in length, material, size, insulation, and termination. In addition, each raceway (where required) must have an equipment grounding (bonding) conductor sized in accordance with 250.122. The minimum 1/0 AWG rule of 310.4 doesn't apply to equipment grounding (bonding) conductors [250.122(F)(1)].

    When more than three current-carrying conductors are run together in a raceway longer than 24 inches, the ampacity adjustment factors of Table 310.15(B)(2)(a) must be applied. See 310.10 and 310.15 for details and examples.

    Editor's note: This information was extracted from Mike Holt's textbook, Understanding the National Electrical Code

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    Code Challenge
    What's Wrong Here?

    By Joe Tedesco

    Think you know how this installation violates the NEC? Visit EC&M's Web site to see the answer.

    Hint: Think Too-Tall Jones

    Code Q&A
    By Mike Holt
    Q. When are insulated bushings required on raceway terminations?

    Visit EC&M's Web site to see the answer.

    Code Quiz
    By Steven Owen
    Q. A cable tray system has been installed to support service entrance conductors. The service entrance conductors consist of six sets of three-conductor 500kcmil interlocked armored cables installed in parallel in a single layer in the tray. An additional feeder, consisting of three sets of parallel three-conductor 500kcmil interlocked armored cables, has been installed in a single layer in the same cable tray. The cable tray is wide open (i.e., no barriers or dividers). Is this installation NEC compliant?

    1. Yes, because all cables are interlocked armored cables.
    2. No, because conductors other than service entrance conductors are not permitted in the same cable tray with service entrance conductors, unless a solid fixed barrier of a material compatible with the cable tray is installed to separate the service entrance conductors from the other conductors.
    3. No, because interlocked armored cables are not permitted to be used as service entrance conductors.
    4. No, because cable trays cannot be used to support service entrance conductors.

    Visit EC&M's Web site for the answer and explanation.

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    For additional information on the dates and locations of these events, click here.

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