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...

 

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.

 

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..

 

sir,

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

 

What is your final load on the secondary?
You size the secondary breaker for the full Anticipated load.
Max.

 

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

 

Can't quite make out the numbers in the pic, what is the pri/sec voltages for the 315Kva Tfmr?
Max.

 

sir,

they are 11kV/433V
50Hz Dyn11

 

SIR,

ATTACHED IN PDF FORMAT