fusing 500 kva transformer

[Joe Corbin]

We are planning on using a 500 KVA pad mounted transformer for temporary start up and check out of some of the equipment in a water treatment plant. The permanent transformers are going to be way late on delivery 2- 3000 KVA stainless steel, expoxy coated, etc. The primary service is 13,800 volts the secondary is 480/277V. What rating should the primary MV disconnect be and also the 480v secondary disconnect? After the disconnect we will be feeding temporarily into the main distribution switch.

 

[don_resqcapt19]

See Table 450.3(A) for the protection.

 

[Joe Corbin]

With article 450.3A I calculated the secondary full load of the 500,000 transformer, as 500,000/1.732 x 480v = 500,000/831.38 = 601.40 amps X 125% = 751.76 amps. As I understand it the OCPD could be between 600 to 800 amps and of course the wiring would need to be rated in accordance with the OCPD used. In regard to the primary I calculated 500,000/13800x 1.732= 500,000/23901.6=20.919 amps x 300% = 62.75 amps and the transformer primary fuse would be a 60 amp or as high as a 70 amp. Is this correct?