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Thread: Parallel Conductor Question

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    Parallel Conductor Question

    I'm trying to expand my knowledge of conductor selection for high amp systems and would like your assistance in clearly understanding how an equipment manufacture selected the recommended conductor and conduit sizes for the equipment.

    Equipment Specs:
    * Three Phase Input
    * Nominal Input Current - 311 VAC
    * Maximum Input Current - 389
    * Recommended cable sizes are 167F (75C) (THW) wire at 86F (30C) ambient

    Manufacturer Recommended input conductor and conduit:
    * Conductor
    - Phase: 3-250kcmil
    - Ground: #1/0AWG
    * Conduit
    - (2) 2.5C

    My questions are:
    1. Based on the maximum input voltage, I would have expected the conduit size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil?

    2. How is #1/0AWG ground calculated?

    3. How is (2) 2.5C conduit calculated?

    Note: This is not for a specific project that I'm working on, it's strictly for research and my own increased knowledge.

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    ...see sec. 310.4 of the '08 NEC (conductors in parallel). See 250.122 where parallel equipment grounding conductors are used.

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    Quote Originally Posted by jmp46205 View Post
    I'm trying to expand my knowledge of conductor selection for high amp systems and would like your assistance in clearly understanding how an equipment manufacture selected the recommended conductor and conduit sizes for the equipment.

    Equipment Specs:
    * Three Phase Input
    * Nominal Input Current - 311 VAC
    * Maximum Input Current - 389
    * Recommended cable sizes are 167F (75C) (THW) wire at 86F (30C) ambient

    Manufacturer Recommended input conductor and conduit:
    * Conductor
    - Phase: 3-250kcmil
    - Ground: #1/0AWG
    * Conduit
    - (2) 2.5C

    My questions are:
    1. Based on the maximum input voltage, I would have expected the conduit size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil?

    2. How is #1/0AWG ground calculated?

    3. How is (2) 2.5C conduit calculated?

    Note: This is not for a specific project that I'm working on, it's strictly for research and my own increased knowledge.
    In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.

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    Quote Originally Posted by jmp46205 View Post
    ...

    My questions are:
    1. Based on the maximum input voltage [current] I would have expected the conduit [conductor] size to be either 1-600kcmil or 2-#3/0AWG. How does the manufacturer calculate 3-250kcmil

    2. How is #1/0AWG ground calculated?

    3. How is (2) 2.5C conduit calculated?
    Can't answer any of your questions based on the provided data. Spec's are ambiguous in that the current's are not provided in a manner consistant with the NEC. All the Recommendations just lead to more questions...!!!
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by texie View Post
    In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.
    That's a good guess... but then I have to ask about the nominal current. I'd have to guess nominal current would be the continuous portion if energized for more than three hours. Max current should be something that only occurs occasionally, and thus non-continuous. You shouldn't have to use that value times 120%.

    1/0 ground would be the minimum if you used aluminum conductor. Using copper, #2 is good for a 500A ocpd.
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by texie View Post
    In the absence of more detail, I'm guessing, but a max current of 389 amps X 1.25% is 486. Next size up would be a 500 amp breaker. You would then need 2, 250's per phase at 75 degree. 1/0 would be the minimum parallel for the EGC. I'll let you find all the relevent code references.
    I mispoke on the EGC. It would only need to be a #2 in each raceway in this case. Maybe that 1/0 is a neutral? But smart$ is right. We don't have enough info and this is all speculation.

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    Quote Originally Posted by Smart $ View Post
    1/0 ground would be the minimum if you used aluminum conductor. Using copper, #2 is good for a 500A ocpd.
    But you still need 1/0 minimum if you have parallel conductors.

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    Quote Originally Posted by kwired View Post
    But you still need 1/0 minimum if you have parallel conductors.
    I'm aware of the 1/0 minimum for paralleled circuit conductors, but I cannot locate any such requirement for EGC's. Please enlighten me...
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by Smart $ View Post
    I'm aware of the 1/0 minimum for paralleled circuit conductors, but I cannot locate any such requirement for EGC's. Please enlighten me...
    You are probably correct 300.3(B)(1):
    (1) Paralleled Installations. Conductors shall be permitted to be run in parallel in accordance with the provisions of 310.10(H). The requirement to run all circuit conductors within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord shall apply separately to each portion of the paralleled installation, and the equipment grounding conductors shall comply with the provisions of 250.122.

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    Quote Originally Posted by kwired View Post
    You are probably correct 300.3(B)(1):
    But the wording of 310.10(H)(5) muddies the waters a bit.
    (5) Equipment Grounding Conductors. Where parallel equipment grounding conductors are used, they shall be sized in accordance with 250.122. Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables in accordance with 310.104, provided the combined circular mil area of the sectioned equipment grounding conductors in each cable complies with 250.122.

    It is my opinion that EGCs smaller than 1/0 are permitted to be installed in parallel, but this section casts some doubt, especially with a number of the CMPs saying that the words "shall be permitted" act to prohibit that that is not specifically permitted.

    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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