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Thread: LV relation between t/f rating and load demand

  1. #1
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    LV relation between t/f rating and load demand

    I have been given a LV SLD of a housing scheme to study. In one of the drawings the transformer size is 750kVA, the incoming from the transformer connects to a busbar which further branches out into multiple MCCB'S. Now the load on the busbar comes out to be(MENTIONED IN THE DRAWING is 382.4kW(D) and 476.47kW(C), PLEASE TELL me how can i arrive at a value of 750kVA based on these loads
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  2. #2
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    What does C and D mean? Is it demand and / or continuous?
    Ron

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    Quote Originally Posted by ron View Post
    What does C and D mean? Is it demand and / or continuous?
    I believe its demand and connected load
    Last edited by jumper; 09-07-17 at 05:38 PM. Reason: fix quote

  4. #4
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    Quote Originally Posted by saad778 View Post
    I believe its demand and connected load
    Do they have calculations that show how they came to the demand value or the 750kVA service size? Is this in the US? Are the NEC 220 calcs provided?

    As you point out, there is no obvious connection.
    Ron

  5. #5
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    Perhaps there is no connection?
    they calculated the connected and demand loads
    assumed a pf
    and selected a xfmr
    eg, a 500 kva would be good for 450 kw at 0.9'pf
    went with a 750

    what voltage and phase?
    Last edited by Ingenieur; 09-07-17 at 05:21 PM.



  6. #6
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    Quote Originally Posted by Ingenieur View Post
    Perhaps there is no connection?
    they calculated the connected and demand loads
    assumed a pf
    and selected a xfmr
    eg, a 500 kva would be good for 450 kw at 0.9'pf
    went with a 750

    what voltage and phase?
    See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.

  7. #7
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    Quote Originally Posted by topgone View Post
    See the source 750kVA and the main cb is 1250A. It gives away the voltage as 480V! Crystal.
    in Pakistan?
    with 32 A cb's?
    I think 380/220 or something 400?



  8. #8
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    [QUOTE=Ingenieur;1857319]in Pakistan?
    with 32 A cb's?
    I think 380/220 or something 400?

    Voltage level is 440v here in Pakistan. IEC is followed mostly. Yes, another SLD shows the connected loads which adds up to the mentioned load on the bus bar. but what worries me is how a 750kVA T/F is selected.

    750,000/(root(3)*440) comes out to be 984A and for a 25% Overcurrent 1230A is the calculated value. I guess this is how they came up with the value.
    I have to design a LV network for the housing scheme, and this is the only source i have to study and make my own.

  9. #9
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    Quote Originally Posted by Ingenieur View Post
    Perhaps there is no connection?
    they calculated the connected and demand loads
    assumed a pf
    and selected a xfmr
    eg, a 500 kva would be good for 450 kw at 0.9'pf
    went with a 750

    what voltage and phase?
    Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value

  10. #10
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    Quote Originally Posted by saad778 View Post
    Please can you guide me on how 500kVA IS GOOD FOR 450Kw load. I want to know how to arrive at this t/f value
    Just an example
    if you were designing for a connected load of 476 kw
    assume a nominal pf of say 0.85 worse case
    kva = 476/0.85 = 560 kva
    if using demand 382/0.85 = 450 kva
    in my opinion a 500 kva would suffice
    most actual loads run nowhere near the connected or demand calculated values
    but with more data I may select a 750 to be conservative and not worry about it
    in the overall project cost (large housing complex) the overall cost dfference is small
    efficiency may be lower but since it may run coler it will last longer

    but consider, sometimes there is no exact mathematical way to determine things like this
    it is engineering judgement
    based on
    standard equipment ratings
    delivery times
    expansion
    etc
    Last edited by Ingenieur; 09-08-17 at 10:19 AM.



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