overcurrent protection for 3-1 Phase transformers

[lowvoltman]

when using 3- 480/240-240/120 25 kva 1 phase to generate 3 phase should you base the primary overcurrent as if it were a three phase 25 kva transformer? might be a stupid question but i would like some confirmation.
example- 25000/480*1.732=30.08x125%=37amps-or 40amp c.b.
thanks!

 

[wirenut1980]

I believe you use the 3 phase kva rating, so in your example is would be 3-25 kva transfomers for a total kva rating of 75 kva.

75000/(480*1.732) = 90.21 Amps per phase.

 

[bob]

Would you explain how you take 1- 25 kva transformer and generate
480 volt 3 phase voltage. I may have misunderstood your question.

 

[cpal]

I believe you use the 3 phase kva rating, so in your example is would be 3-25 kva transfomers for a total kva rating of 75 kva.

75000/(480*1.732) = 90.21 Amps per phase.

I also agree with this proceedure.
But there is a flaw in the op

125% of 30.08 came to 37.6. (I'm not nit picking) the issue is that 125% is the maximum allowed setting (450.3 (B)) of the primary ocpd!!! You would install a 35A ocpd or you need to provide protection on the secondary side also.

 

[lowvoltman]

Would you explain how you take 1- 25 kva transformer and generate
480 volt 3 phase voltage. I may have misunderstood your question.[/QUOTE we are taking (3) 480/240-240/120v 1phase transformers with 480v 3phase primary and 120v 3 phase secondary.