Transformer primary feeder sizing.

[GadreP]

Dear Friends, I have a question regarding sizing of the breaker for primary side of transformer.
I have 160kVA, 415 to 11kV, Z=4.5%, ONAN transformer. Its full load current is 223A. What should be the size of the feeder. I referred to NEC & it says 125% of FLC of transformer. I have chosen Schneider Electric's CVS 320A MCCB with 35kA for 1 Sec & LSI protections. Just wanted to cross check with you..

 

[RRaghunath]

The application seems to be for a step-up trafo connected to 415V switchboard.
Current rating seems OK if you are ensuring that the 320A rating corresponds to the site rated design ambient temperature.
Make sure the MCCB fault level matches with the 415V bus fault level.
Don't you want to have earth fault protection for the feeder. LSI release doesn't include the same (LSIG release is the right one)!!

 

[GadreP]

Hi Raghun, thanks for your reply. Bus fault level & breaker fault levels are matching. I will definitely take a note of LSIG protection but is assumption of 125% margin is correct? In that case transformer upto 175kVA can work on this breaker.

 

[RRaghunath]

125% - That is what we also follow. Transformers can take overloads in accordance with IEC60076-7.
125% margin in the feeder rating ensures that the feeder is not going to impose any constraint in utilising the overload withstand capabilities of the trafo.
IEC says the busbars etc. in switchgears cannot have any sustained overload ratings.