Transformer OCPD Sizing, Please Help

[padawan learner]

I recently came across an installation where a 400A fused disconnect feeds a 440kVA transformer. There is no secondary protection. The FLA on the high side is 529A.

According to 450.3(B) the overcurrent protection should be 125% X 529 =662 A, which rounds up to 700A. It has been in service for several years and hasn't blown because I believe the connected load to be no more than 300kVA (361A).

Am I correct in calculating the OCPD to be 700A or is sizing the transformer's primary protection based on the calculated load acceptable somewhere in the code???

 

[david luchini]

450.3(B) says that the OCPD for primary only protection shall be a "maximum" of 125% of the rated primary current. Nothing prohibits it from being smaller. As long as the OCPD and conductors are sufficient for the load to be served, there is no Code issue.