sizing line and load breakers for kva trans.

[fhelectric]

I am installing a 75kva trans from a 480y/277v to a 120/208v panel. My calculations show that I need a 100a breaker on the line side, and a 200a on the load side. On the previous commercial jobs that I have worked on, I found a 45kva trans. with a 70a line, and a 150a load. This does not add up when I do the math so I am wondering what I am missing???

 

[jim dungar]

I am installing a 75kva trans from a 480y/277v to a 120/208v panel. My calculations show that I need a 100a breaker on the line side, and a 200a on the load side. On the previous commercial jobs that I have worked on, I found a 45kva trans. with a 70a line, and a 150a load. This does not add up when I do the math so I am wondering what I am missing???

NEC 450 deals with the maximum value that a protective device can be not the minimum. Per 450.3(b), a transformer must have a primary device not larger than 125% of the primary FLA. Only if a secondary protective device of not more than 125% FLA is provided, 450.3(B) then allows (but does not require) the primary device to be sized up to 250% instead.

 

[fhelectric]

that answers the mystery, thank you:grin: