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Thread: Neher Mcgrath and duct bank derating

  1. #11
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    Quote Originally Posted by publicgood View Post
    I understand your notes, but if the NEC is not applied appropriately, I’d say someone has grounds to stand on in a court room setting. For instance if you use the 310.60 tables for underground medium voltage duct banks. It says 90 RHO. That may or may not be applicable to your specific installation; therefore, engineered calcs may be the NEC requirement.

    Have a read here:
    http://manuals.decagon.com/Applicati...tivity_Web.pdf

    “...I’ve acted as an expert witness in cases where [the original designer] assumed a thermal Rho of 90. These cases are in litigation because of failure.”



    Engineering judgement. Talk to the client. Understand their operations. Makes reasonable assumptions based on your years of experience and baseline metrics available via engineering references. Document your design decisions.

    If there is ever a failure, you will get to judge your reasonable design considerations against what actual loads where that caused the failure.

    That’s the best we can do and is the industry standard of care.
    For that reason, I do Neher McGrath calcs every time, and we do a lot of UG ductbanks.
    Ron

  2. #12
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    One code cycle, I think in the 90s, had underground duct bank ampacities for under 600 volt systems. They were put in the code with an effective date of 3 years later. The action on the next code cycle deleted those tables, and mostly because the load calculations in Article 220 result loads that are substantially greater than the real world loads. However for industrial, data center and loads like that, you really need to do the required calculations if you don't want early failure of conductors.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  3. #13
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    In my opinion, you may use this 15 kV 750 MCM MV 105 copper cable in duct bank of 9 ducts [three cables per duct] if the maximum temperature will be 105 oC [Earth 90 RHO, 25 oC].
    The cables close to terminals-I think-they have to be collected in a 24 inch width cable tray for 10 ft. minimum in order to reduce the temperature under 90oC.
    However, 90 RHO it is not sure. I think-if you don't know the actual earth thermal resistance- to take 120.

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