75KVA transformer primary over current protection

[DEW202001]

Can someone check my math?

75,000/480V x 1.733= 831.36

75,000/831.36= 90A

90A x 1.25 = 112.5




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[jumper]

Can someone check my math?

75,000/480V x 1.733= 831.36

75,000/831.36= 90A

90A x 1.25 = 112.5




Sent from my iPhone using Tapatalk

Your notation is a bit off, but 90A is correct.

What are multiplying by 125%

 

[templdl]

And the rest of the story can be found in the NEC art 450. Any wild guesses the the transformer has a common 208y/120 secondary which is all the more reason the read and understand art 450.

 

[infinity]

According to Mike Holt's transformer chart a typical 75 kva would use a 125 amp OCPD on the primary and a 225 amp on the secondary.

 

[jap]

According to Mike Holt's transformer chart a typical 75 kva would use a 125 amp OCPD on the primary and a 225 amp on the secondary.

I agree.
I got tired of always having to go to the charts for xfmrs.

for 3ph xfmrs for a shortcut I use 1.2 for 480
2.4 for 240
and 2.77 for 208v

In this example primary 75 x1.2 = 90 x 1.25 = 112.50
then secondary 75 x 2.77 = 207.75 x 1.25 = 259.69

JAP>