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Cable Sizing Query

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  • 16-07-2018 6:30pm
    #1
    Tibulus
    Registered Users Posts: 203 ✭✭


    Am sizing a cable between a generator and LVAC panel, want to verify two items:

    1. I have protection at both the generator and LVAC. The LVAC overload is set just above Ib and the generator higher to coordinate with it. When I verify that my cable Iz is higher than the protection do I use the maximum MCCB setting (In) of MCCB or the actual setting (Ir)?

    2. Again when I verify that my Iz of the cable is greater than the protection setting must I use the MCCB at the generator or can I use the LVAC MCCB which is set lower?


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Tibulus wrote: »
    Am sizing a cable between a generator and LVAC panel

    Is it just one generator and just one distribution board?

    What size is the load?
    1. I have protection at both the generator and LVAC.

    What type of protection?
    Have you an MCCB at both ends of the same cable?


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    #3
    Tibulus
    Registered Users Posts: 203 ✭✭Tibulus


    The generator is 300kVA,

    630A LSI MCCB both ends of the same cable. Have overload at the LVAC set at 434A and the generator set as 541A. This is the lowest possible without setting with the tolerance bands of each MCCB.

    Load is been designed as the full capacity of the generator, Ib = 433Amps

    Is a single generator straight into its own incomer on the LVAC panel.


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Tibulus wrote: »
    The generator is 300kVA,

    630A LSI MCCB both ends of the same cable.

    Why have you an MCCB at both ends of the same cable?
    The MCCB at the generator will protect the downstream cable. What will the other MCCB (in the distribution board) protect?
    You could replace this with an isolator. Perhaps that is too much hassle?

    Have overload at the LVAC set at 434A and the generator set as 541A.

    These may be too close to ensure discrimination.
    Is the cable sized for 541A ?


    This is the lowest possible without setting with the tolerance bands of each MCCB.

    Why do you care which one trips? If either trips you have lost everything.

    I know I haven't really answered your question, will see what I can find out today.


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    #5
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Tibulus wrote: »
    630A LSI MCCB both ends of the same cable. Have overload at the LVAC set at 434A and the generator set as 541A. This is the lowest possible without setting with the tolerance bands of each MCCB.

    Load is been designed as the full capacity of the generator, Ib = 433Amps

    Is a single generator straight into its own incomer on the LVAC panel.

    Normally the design current (IB) is calculated based on the load.

    Perhaps I am misunderstanding you but the way I read it is that you have calculated the design current based on the capacity of the generator. This approach may result in an oversized cable. A larger cable will of course cost more, but can be handy for future expansion.
    When I verify that my cable Iz is higher than the protection do I use the maximum MCCB setting (In) of MCCB or the actual setting (Ir)?

    In is the actual setting.
    Remember, In < or = Iz
    2. Again when I verify that my Iz of the cable is greater than the protection setting must I use the MCCB at the generator or can I use the LVAC MCCB which is set lower?

    You must use the MCCB at the generator. Why? Because an MCCB can only protect a cable downstream of it.


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