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Thread: Wire Size Min Cir ampacity

  1. #1
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    Wire Size Min Cir ampacity

    I have a project where the a HVAC compressor replacement unit for a building came in different than on the plans and what is existing. I have to tell the bad news. I am looking to see if I am correct first.

    The wiring is 500MCM THW ( short distance 15' from the MSB so not a real big deal )
    The conduit is inside.

    The nameplate is MCA 405a 208 3 phase
    MOCP 600a
    So THW is rated at 70 deg C only 380a ( does not have a 90 deg rating)

    Since it is less than the required MCA of 405a the wire must be up sized.
    I don't think there is a next higher rule for MCA-
    Am I correct?

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    I agree. You do not have enough ampacity. The next size up does not apply here.

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    Yep, MCA means MINIMUM Circuit Ampacity, not "More-or-less" Circuit Ampacity...
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    Thanks , so many have this confused and I wanted to be sure.

    So next question.
    I was thinking parallel 4/0 as the conduit is a full boat with 600 mcm and I really don't want to pull that.
    Do I need the 4/0 or can I go with 3/0 and the 90 deg column. I don't think the terminals are rated and I don't think there is room to splice a stub of wire.

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    Quote Originally Posted by Sierrasparky View Post
    Thanks , so many have this confused and I wanted to be sure.

    So next question.
    I was thinking parallel 4/0 as the conduit is a full boat with 600 mcm and I really don't want to pull that.
    Do I need the 4/0 or can I go with 3/0 and the 90 deg column. I don't think the terminals are rated and I don't think there is room to splice a stub of wire.
    The math says if you put this all in 1 raceway you will need 4/0 90 degree conductor. This would give you an adjusted ampacity of 416 that would be acceptable for 75 degree terminals.

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    What size raceway do you have?

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    3" PVC I believe

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    Quote Originally Posted by Sierrasparky View Post
    3" PVC I believe
    6, 4/0 THHN plus an EGC would fit in this OK.

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    Thanks to you all.

    One more question of course.

    So when using the 90deg column you use for wires inside conduit. When are you allowed to remove the derating in ampacity for conductor heating, more than 3 conductors in conduit?

    How much length must there be past the neck of the conduit to be considered no longer heating and then use the 70 deg column with no derating?

    Thanks

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    Quote Originally Posted by Sierrasparky View Post
    Thanks to you all.

    One more question of course.

    So when using the 90deg column you use for wires inside conduit. When are you allowed to remove the derating in ampacity for conductor heating, more than 3 conductors in conduit?

    How much length must there be past the neck of the conduit to be considered no longer heating and then use the 70 deg column with no derating?

    Thanks
    If you are no longer in raceway/cable then the adjustment no longer applies.

    Aux gutters/wireways - I think if longer than 30 feet you must apply the adjustment.
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