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Thread: High AMP feeders

  1. #1
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    High AMP feeders

    Hi There
    I am quoting an installation of 2 2000MW Emergency Generators from Cummings.

    It is my first time dealing with high AMP feeders.
    The system is 480/277 V. with aluminum conductors in raceway which type is not specified because
    is out of the my specific project's scope I mean doing by others.
    I received the electrical drawings with the information which include the in-line diagram
    for the generators, the feeder towards each EMSB ( Emergency Switch board), and from both EMSB to many ATS ( Automatic Transfer switch) down steam for feed different emergency loads.

    The drawing include a table with a list of different capacity CB and its corresponding AWG for feeders.
    More exactly the table specify Circuit Breaker Amps, # sets , and # of wires for each sett plus the AWG for grounding conductor.
    For example let's take one of those listed feeders:
    For 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral. plus 1 #3/0 grounding conductor.

    Among the notes included with the feeder table there is the following:
    Subscript "V" denotes an oversized feeder due to voltage drop. Increase equipment grounding conductor size per requirements of 250.122 (B)
    In total there are 12 feeders towards its respective ATS. Most of those feeders runs are in average 600 feet and have a tag "V" doing reference to the above note.
    Here is my first question: Is that table valid for 100' runs or it is already updated for long runs?
    Let's take the case of one of the feeders: 1600 A running 810 feet.
    They specify 6 sets of 3-400 Kcmil, 1_400Kmil for Neutral and 1 350Kcmil for grounding.

    I got that for one set , 480 v , four conductor in parallel (Phases and Neutral), PVC conduit(it's assumed), temperature 102 F deg then the voltage drop is 7.3 %. Is my calculation correct?

    Should I conclude that the Feeder table is for 100 feet run?


    My 2th question is regarding the equipment grounding conductor and table 250.122 B
    Let's take again the example feeder
    For 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral. plus 1 #3/0 grounding conductor.
    If I look up on the table 250.122(B) NEC 2011 I found that for Aluminum 3/0 conductor the corresponding CB should be 800A.
    This is 100 A more than the 700 A feeder.
    How should I interpret this?




    I would appreciate so much son help on this and Thank you all.

  2. #2
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    You sure about that 2,000 MW???

  3. #3
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    Quote Originally Posted by zog View Post
    You sure about that 2,000 MW???
    im sure he means 2000 kw if we are talking 480v. they dont make 480v gear rated for 2.4 million amps

    That said, for question 1, I would assume the voltage drops in the table are based upon the engineer's anticipated lengths of the cable runs, not a standard length. Depending on which state, plans are required to demonstrate that feeders and branch circuits comply with voltage drop requirements stipulated by building code, usually 2% feeder, 3% branch, 5% total. The calculation is done based on the anticipated loading of the conductors, the length of the run, type of conductor, number of parallel sets and type of conduit. You would need to know the demand load for the conductor to determine the voltage drop. Sometimes engineers use max conceivable loading of the conductor, which is not necessarily a realistic operating condition, but its also not wrong-you just might be penalizing yourself and upsizing for no real benefit.

    question 2, respectfully, i believe you are reading that table backwards in the way its described. the input to that table (250.122) is the circuit breaker size, the output is the minimum EGC size. meaning, the table is referred to in order to size the ECG based on the breaker size, not size the breaker based on the EGC. Where a feeder/branch is upsized for voltage drop, the ground must also be upsized by a minimum of the % increase in kcmil. as an example, if i need to upsize a 500 kcmil to 600 kcmil for voltage drop and i have a ground of 250 kcmil, i need to upsize that ground by a factor of 600/500 = 1.2. The result is a minimum ground size of 250 * 1.2 = 300 kcmil.

    hope this helps.

  4. #4
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    Hi all of you

    Yes, it is a 2000kw Emergency generator for 277/480 I have the submittals sheets and the feeder for one of the ESB is for 3000A and the other one is for 5000 A.
    A again yes you help me a lot.
    Regarding the firs question, that is something I was suspecting, the tables are already calculated for long runs.

    Regarding the second question, I give you this case of 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral plus 1 #3/0 grounding conductor just and my intention in backward reading was about verify if the grounding ECG was already oversized.

    Again, thank you so much for help me both of you.

  5. #5
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    Quote Originally Posted by Piramyd View Post
    Hi all of you

    Yes, it is a 2000kw Emergency generator for 277/480 I have the submittals sheets and the feeder for one of the ESB is for 3000A and the other one is for 5000 A.
    A again yes you help me a lot.
    Regarding the firs question, that is something I was suspecting, the tables are already calculated for long runs.

    Regarding the second question, I give you this case of 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral plus 1 #3/0 grounding conductor just and my intention in backward reading was about verify if the grounding ECG was already oversized.

    Again, thank you so much for help me both of you.
    The EGC has been increased in size. The standard EGC for a 600 amp OCPD is 1 AWG with a circular mil area of 83,690. Parallel 350 kcmil would be code complaint for the 600 amp circuit. The ungrounded conductors were increased in size to 600 kcmil for voltage drop. 250.122(B) requires a proportional increase in the size of the EGC. (600/350) * 83690 = 145,469 circular mils. This is the minimum size of the EGC. The next standard size conductor is 3/0 with a circular mil area of 167,800. 2/0 is too small having an area of 133,100.
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  6. #6
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    Thank you very much for this additional explanation.
    You are teach me a lot.

  7. #7
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    Hi drktmplr12

    Regarding this part of your answer:
    " The calculation is done based on the anticipated loading of the conductors, the length of the run, type of conductor, number of parallel sets and type of conduit. "

    You were absolutely right regarding the engineered feeder table, Thank you.

    Now, I have other question, it is about a wording on the same project' specification:

    "Provide wire and terminations from the transfer switches to their respective ESB switchboard."

    What does the meaning of the word termination?
    It's about hook up those parallel feeders to the corresponding ESB or ATS or is some kind of device attached to the end of the parallel wires run?
    Thank you in advance.

  8. #8
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    Quote Originally Posted by Piramyd View Post

    Now, I have other question, it is about a wording on the same project' specification:

    "Provide wire and terminations from the transfer switches to their respective ESB switchboard."

    What does the meaning of the word termination?
    It's about hook up those parallel feeders to the corresponding ESB or ATS or is some kind of device attached to the end of the parallel wires run?
    Thank you in advance.
    Many switchboards and transfer switches come without lugs or terminals needed to connect the conductors.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  9. #9
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    Quote Originally Posted by Piramyd View Post
    Hi drktmplr12

    Regarding this part of your answer:
    " The calculation is done based on the anticipated loading of the conductors, the length of the run, type of conductor, number of parallel sets and type of conduit. "

    You were absolutely right regarding the engineered feeder table, Thank you.

    Now, I have other question, it is about a wording on the same project' specification:

    "Provide wire and terminations from the transfer switches to their respective ESB switchboard."

    What does the meaning of the word termination?
    It's about hook up those parallel feeders to the corresponding ESB or ATS or is some kind of device attached to the end of the parallel wires run?
    Thank you in advance.
    it means labor and materials required to physically connect (terminate) wire onto the equipment. Terminations can be either to lugs installed in the gear or bus stabs. Many times you can purchase lugs and bolt them onto the stabs, then terminate the wires on the lugs. Or you can hi-press connectors and bolt them right to the bus.

    either way, the specs are tell you to make sure that EVERYTHING (labor and materials) required to make those connections is included in your price. if you are lucky the engineer is telling you what is/will be there. without knowing you can either price the two options and pick the higher of the two or try to coordinate with the supplier of the gear to see what they are providing, so you can mate up your stuff with theirs.

  10. #10
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    Thank you sir
    Again you are help me lot.
    Best regards,

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