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randypape

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hello,
i have some questions on a school homework project i am working on. i have a 480 75hp 3 phase motor ,91 FLA on nameplate, and it's fed from a 208/120 3 phase panel. when i figure out what size 208/480 xfmr i need i come up with 75653va (480x91x1.732). in my homework packet i can only use 75 or 112.5 kva. so i feel i must go up to the 112.5 xfmr. i want to protect the primary only so do i size the overcurrent protection for the primary 450.3(B) by 125% of the FLC from nec 430.250(96)? i have heard conflicting ways to do this. some guys say wire and protect the xfmr to it's full capabilities,but our teacher wanted us to size things only for the load at this time,this motor above. i need to size the conductors for this feed also. thanks Randy
 
randypape said:
hello,
i have some questions on a school homework project i am working on. i have a 480 75hp 3 phase motor ,91 FLA on nameplate, and it's fed from a 208/120 3 phase panel. when i figure out what size 208/480 xfmr i need i come up with 75653va (480x91x1.732). in my homework packet i can only use 75 or 112.5 kva. so i feel i must go up to the 112.5 xfmr. i want to protect the primary only so do i size the overcurrent protection for the primary 450.3(B) by 125% of the FLC from nec 430.250(96)? i have heard conflicting ways to do this. some guys say wire and protect the xfmr to it's full capabilities,but our teacher wanted us to size things only for the load at this time,this motor above. i need to size the conductors for this feed also. thanks Randy
Click to expand...

It is probably beyond your scope at this time, but the inrush current of a transformer usually requires a breaker not much smaller than 125% of FLA.
 
what?

what?

i don't get your answer? i know in 450.3(B) protecting just the primary side over 9 amps requires 125% of FLC and primary and secondary protection says 250% for primary and 125% for secondary. how does this answer my question of onlyprotecting the xfmr for kva used not for full rating of xfmr? thanks
 
I'll admit your question is way over my head, but you might be better off posting it in a different area of this website.

Maybe Electrical Calculations, or even Continuing Education.

Maybe I'm wrong, but I'm hoping that you get your question out to the most people that can answer it as possible.
 
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