nec 450.3 sizing of breaker for a transformer

Discussion in 'The Projects Forum' started by Anirudh Sharma, Jan 23, 2017.

  1. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    sir,
    for an example,
    if a transformer rated 160kVA 11kV/.433kV the full load current of the transformer would be 223A on the LV side.
    the breaker henceforth downstream would be sized to 125% as per NEC thereby 250A or 400A.

    my doubt here, how is it that the breaker for a demand load to be sized for 100kW
    what should be the LV breaker size for 100kW of demand load??

    if an SLD is shown would be helpful

    Also if the transformer standard rating is 160kVA why should the breaker be rated at 125%??
    wont the winding start to melt at 100% or 110%??
    is the actual design of the transformer more than what is specified??
    if yes then how much more...

    please help...
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,176
    4,739
    If you have a 160Kva transformer then 125% allows for inrush and short occurrences of O/L etc.
    You would size the secondary side based on the actual demand on the secondary.
    Max.
     
  3. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    sir,
    but nec 450.3 states that the secondary side breaker should be sized for 125% of the full load secondary current??
    Also
    is the actual design of the transformer more than what is specified??
    if yes then how much more...

    Anirudh..
     
  4. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    sir,

    can i know the answer???
    it is quite confusing that is why...
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,176
    4,739
    What is your final load on the secondary?
    You size the secondary breaker for the full Anticipated load.
    Max.
     
  6. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    sir,
    as you can see in the attached drawing the demand load is 215kW
    the size of the transformer is 315kVA
    but the secondary breaker is sized at 630A which is as per NEC 450.3
    my doubt here is the full load current of the transformer would 440A
    will the transformer windings not burn when it is overloaded more than 440A??
    why is NEC stating 125% more??
    is the actual transformer designed more than rated??
    is yes, how much more??

    sir
    kindly help...

    Anirudh
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,176
    4,739
    Can't quite make out the numbers in the pic, what is the pri/sec voltages for the 315Kva Tfmr?
    Max.
     
  8. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    sir,

    they are 11kV/433V
    50Hz Dyn11
     
  9. Anirudh Sharma

    Thread Starter New Member

    Mar 11, 2016
    10
    0
    SIR,

    ATTACHED IN PDF FORMAT
     
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