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Thread: MV Single Conductor Cable in Tray; NEC 392.22(C)

  1. #1
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    MV Single Conductor Cable in Tray; NEC 392.22(C)

    My application involves 10 sets of 3 single-conductor, 500kcmil, 5kV cable in a 36” cable tray. These cables have a diameter of 1.26”. Each triplexed set of cables has a #1/0 ground with an OD of .48”.

    NEC 392.22(C) states the following:
    The sum of the diameters of single-conductor and multi-conductor cables shall not exceed the cable tray width, and the cables shall be installed in a single layer. Where single conductor cables are triplexed, quadruplexed, or bound together in circuit groups, the sum of the diameters of the single conductors shall not exceed the cable tray width, and these groups shall be installed in a single layer arrangement.

    I take this to mean that we sum the diameters of all of our cables (10 x 3 x 1.26 + 10 x .48 = 42.6”). Therefore we’re over the 36” limit.

    However, one my colleagues disagrees. His interpretation is that you total the diameters of only the bottom layer when you have triplexed, single conductor cables. In other words, only the blue lines in the below sketch would be counted.

    Name:  Cable Tray.jpg
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    Point being, what's your interpretation of the above issue?

  2. #2
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    you're correct, you need more airspace for heat dissipation


    also remember
    392.80 Ampacity of Conductors.

    ....


    (B) Ampacity of Type MV and Type MC Cables (2001 Volts or Over) in Cable Trays. The ampacity of cables, rated 2001 volts, nominal, or over, installed according to 392.22(C) shall not exceed the requirements of this section.
    .....


    (2) Single-Conductor Cables (2001 Volts or Over). The ampacity of single-conductor cables, or single conductors cabled together (triplexed, quadruplexed, etc.), shall comply with the following:

    ....

    (3) Where single conductors are installed in a triangular or square configuration in uncovered cable trays, with a maintained free air space of not less than 2.15 times the diameter (2.15 × O.D.) of the largest conductor contained within the configuration and adjacent conductor configurations or cables, the ampacity of 1/0 AWG
    and larger cables shall not exceed the allowable ampacities in Table 310.60(C)(67) and Table 310.60(C)(68).

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    Thanks for the feedback.

    For this application we're looking use to the ampacities per 392.80(B)(2)(1) and not the free-air ampacities per 392.80(B)(2)(3). From what I understand, you can still use triplexed cable with a spacing of less than 2.15 x OD (between each triplexed bundle) assuming you follow the 392.80(B)(2)(1) ampacity values and meet the previously discussed fill requirements.

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