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Thread: Standby Power Capability-702.4

  1. #1
    Join Date
    Jun 2012
    Location
    Reading, PA
    Posts
    2

    Standby Power Capability-702.4

    Hello all,

    I have an application that have the following parameters. Please advice.

    Existing:
    500 KVA Pad-mounted transformer, 4160V-480/277V, Secondary connect to MLO Main Distribution Switchboard "MSB" rated at 1200A.
    Proposed:
    Disconnect TX-Secondary to MSB. Tapping off secondary of transfomer to a service rated ATS at 1200A with built-in GFI circuit breaker. Normal size of ATS will connect to MSB. Emergengy size will connect to gen set.
    Gen set will be 200KW at 480/277V, natural gas, outdoor.
    Questions
    230.82(5) allow standby power to be connect to the supply side of main service disconnect. Please correct me if i am wrong, as long as the ATS is service rated and have main disconnect, this is legal by code.

    Here is where it is get shady. Since this is an automatic transfer operation, 702.4 required that the standby equipment have to be able to handle "full load" or has "load management" built in to automatically manage the connected load. Our measured data is at about 100 kW peak (conducts 5days) for 1200A service.

    What is consider "full load" ? Is it the entire 500KVA (TX rating)? If so, does that mean i have to put in a gen set to match this 500KVA rating; or is the "full load" the peak measure load ?
    In the event we can't up size the generator to match the transformer size, What is consider "Load Management"? Any generator manufacturer offer this solution?

  2. #2
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
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    25,754
    Not my forte, can someone help this new member

  3. #3
    Join Date
    Nov 2010
    Location
    Fort Collins, Colorado
    Posts
    1,968
    Quote Originally Posted by ducceus View Post
    Hello all,

    I have an application that have the following parameters. Please advice.

    Existing:
    500 KVA Pad-mounted transformer, 4160V-480/277V, Secondary connect to MLO Main Distribution Switchboard "MSB" rated at 1200A.
    Proposed:
    Disconnect TX-Secondary to MSB. Tapping off secondary of transfomer to a service rated ATS at 1200A with built-in GFI circuit breaker. Normal size of ATS will connect to MSB. Emergengy size will connect to gen set.
    Gen set will be 200KW at 480/277V, natural gas, outdoor.
    Questions
    230.82(5) allow standby power to be connect to the supply side of main service disconnect. Please correct me if i am wrong, as long as the ATS is service rated and have main disconnect, this is legal by code.

    Here is where it is get shady. Since this is an automatic transfer operation, 702.4 required that the standby equipment have to be able to handle "full load" or has "load management" built in to automatically manage the connected load. Our measured data is at about 100 kW peak (conducts 5days) for 1200A service.

    What is consider "full load" ? Is it the entire 500KVA (TX rating)? If so, does that mean i have to put in a gen set to match this 500KVA rating; or is the "full load" the peak measure load ?
    In the event we can't up size the generator to match the transformer size, What is consider "Load Management"? Any generator manufacturer offer this solution?
    You can do this as you propose with a service rated ATS, and yes that can be your service disconnect if so rated. You'll have to change the neutral bonding around a bit to accomodate the new setup. You may want to investigate doing this with separate components (ATS and main disconnect) as this is going to be a very expensive service rated ATS with GF protection. On the other hand, the space and labor savings of the service rated ATS may be attractive. The code does require the standby capacity to equal the load calcs in art 220, unless as you say, you use auto load shedding. You can do this by using pilot contacts in the ATS to signal other equipment to shut down on stanby power. The Existing GF protection will have to be removed in favor of the new disconnect or, if you you use the service rated ATS to there.

  4. #4
    Join Date
    Nov 2010
    Location
    Fort Collins, Colorado
    Posts
    1,968
    Quote Originally Posted by texie View Post
    You can do this as you propose with a service rated ATS, and yes that can be your service disconnect if so rated. You'll have to change the neutral bonding around a bit to accomodate the new setup. You may want to investigate doing this with separate components (ATS and main disconnect) as this is going to be a very expensive service rated ATS with GF protection. On the other hand, the space and labor savings of the service rated ATS may be attractive. The code does require the standby capacity to equal the load calcs in art 220, unless as you say, you use auto load shedding. You can do this by using pilot contacts in the ATS to signal other equipment to shut down on stanby power. The Existing GF protection will have to be removed in favor of the new disconnect or, if you you use the service rated ATS to there.
    I'm assumming this is an optional standy system. FYI, best not to use the word "emergency", especially if the AHJ is involved as this implies other rules. I'm sure I missed things here as this is intended as a quick glance to get some comments going.
    Last edited by texie; 06-15-12 at 07:50 PM.

  5. #5
    Join Date
    Jun 2012
    Location
    Reading, PA
    Posts
    2
    First of all, texie, thanks for your response. I have the following question
    You can do this as you propose with a service rated ATS, and yes that can be your service disconnect if so rated.
    the incoming service from transformer secondary is 480/277V, since I am tapping transformer secondary and connect to intergral breaker within the ATS, does this mean that the ATS has to be a 4-poles ATS since we also switching the neutral in case standby power is on.

    The code does require the standby capacity to equal the load calcs in art 220, unless as you say, you use auto load shedding.
    This installation is for standby power with the intention of load shredding in the future.

    The Existing GF protection will have to be removed in favor of the new disconnect or, if you you use the service rated ATS to there
    Does this mean that if i have any existing GF protection in any of the breaker downstream, it need to disconnect it ?

    You can do this by using pilot contacts in the ATS to signal other equipment to shut down on stanby power
    So i take it that the ATS has to have built-in coil to send signal to circuit breaker downstream to let them know that standby power is on, and these CB need to shutdown, correct ? Which mean that the existing circuit breakers downsteam that shutdown during stadn by have to has shunt trip capability ? What other way could you shutdown these equipment ? such as Air Handling Unit or chiller.

  6. #6
    Join Date
    Jul 2007
    Posts
    3,100
    Quote Originally Posted by texie View Post
    ?......... The Existing GF protection will have to be removed in favor of the new disconnect or, if you you use the service rated ATS to there.
    The exsiting GF protection can remain, but the neutral/ground link will have to be removed in that section, and new equipment grounds run from the new main to the ground bar in the old.

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