RE:240.21 (C) (6) Calc

[LYLE2231]

RE:240.21 (C) (6) Calc

Good Morning Friends, Could someone show/illustrate the calc for this article.
Thanks, Lyle

 

[jumper]

Here is a MH video on this subject that you may find helpful.

http://forums.mikeholt.com/showthre...Transformer-Secondary-Conductors-240.21(C)(6)

 

[augie47]

Good video, but, IMHO Mike kinda skimmed past the OP's calculation question.
I believe the situation to be as such:
Assume you have a 75 kva 3 phase 480/208Y1220 transformer with a 125 amp primary OCP device.
1/3 of the primary OCP device will be 42 amps.
The transformer ratio 480/208 is 2.31
42 amps X 2.31 is 97 amps so the minimum conductor ampacity would be 97 amps.

(I hope I did that correctly...the ratio throws me sometimes... if I'm wrong someone here will correct me)

 

[raider1]

Gus your calculations look good to me.

Chris

 

[infinity]

Good video, but, IMHO Mike kinda skimmed past the OP's calculation question.
I believe the situation to be as such:
Assume you have a 75 kva 3 phase 480/208Y1220 transformer with a 125 amp primary OCP device.
1/3 of the primary OCP device will be 42 amps.
The transformer ratio 480/208 is 2.31
42 amps X 2.31 is 97 amps so the minimum conductor ampacity would be 97 amps.

Does anyone have an idea why we use this ratio?