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Thread: Large Equipment Install MOCP and MCA Question

  1. #1
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    Large Equipment Install MOCP and MCA Question

    Hello,

    For a job I am working on, we need to install a large chiller. The chiller's spec sheet lists a MCA of 729.2 amps and a MOCP of 1000 amps. We designed our conductors for 760 amps, and specified a 1000 amp circuit breaker from the MDP. We got pushback from the electrical contractor, saying that our feeders were undersized (he did not give a code reference, just told us we were wrong and to redo it). We were referencing 110.3B, as the provided equipment listing we were required to follow was satisfied in both the MCA and MOCP. However, when I went back through the NEC to double check our work, I came across 240.4C. Per 240.4C, since the overcurrent device is over 800 amps, we need to size the conductors at or above the rating of the overcurrent device.

    I understand that since MCA is the minimum amount of ampacity the conductors need to be able to handle, conductors sized for 1000 amps would also satisfy the manufacturer's listed requirements; upsizing the conductors would carry a cost increase and new set of drawings that I'd like to avoid if possible.

    My question is: are we okay just following the manufacturer's listing or does 240.4C override that in our case?

    Thanks for the help!

  2. #2
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    Your application does not fall under 240.4(C), it is covered by 240.4(G) which says go Art. 440 and nameplate data from manufacturer applies.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    Your application does not fall under 240.4(C), it is covered by 240.4(G) which says go Art. 440 and nameplate data from manufacturer applies.
    Makes sense, I should have gone through that table! Thanks a ton!

  4. #4
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    Quote Originally Posted by s-putka View Post
    Makes sense, I should have gone through that table! Thanks a ton!
    I agree with Jumper. The contractor is wrong. Your design is correct-your conductor ampacity does not need to be larger that the MCA as stated on the data plate. The OCPD should be the 1000 amp as stated on the name plate. As a side note, on equipment this large, running larger conductors than required by the MCA can lead to other issues such as increasing available fault current. Larger is not always better.
    The contractor should hopefully use this as a learning experience and do some research on these rules.

  5. #5
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    Quote Originally Posted by s-putka View Post
    Makes sense, I should have gone through that table! Thanks a ton!
    Definitely read Art 440 Part I, all of it.

    You are welcome.
    "Electricity is really just organized lightning." George Carlin


    Derek

  6. #6
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    Quote Originally Posted by s-putka View Post
    Hello,

    For a job I am working on, we need to install a large chiller. The chiller's spec sheet lists a MCA of 729.2 amps and a MOCP of 1000 amps. We designed our conductors for 760 amps, and specified a 1000 amp circuit breaker from the MDP. We got pushback from the electrical contractor, saying that our feeders were undersized (he did not give a code reference, just told us we were wrong and to redo it).

    Thanks for the help!
    This is a common mistake by someone who doesn't understand the concept of what the MCA on the nameplate actually is. It's exactly what the name implies, it's the minimum conductor ampacity. Your 760 amp conductors are just fine and code complaint with any OCPD between 729.2 amps and 1000 amps.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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