transformers

[Matt_hndrx]

The minimum size 75 degrees C transformer secondary conductor not over 7.5m long for a 480 volt primary to 208 volt secondary transformer supplied by a 100 ampere primary overcurrent device is: (a) 6 Awg XHHW aluminum. (b) 4 Awg RHW aluminum. (c) 3 awg THW aluminum. (d) 2 AWG USE aluminum.

Just wondering what the answer is and how you figure this out, thanks

 

[George Stolz]

It has been decided I was in error closing this thread. I apologize to the original poster for the delay.

 

[S'mise]

I'll take a stab at it; 7.5m= 24.6 feet so I know they are refering to the 25ft tap rule, see 240.21(3)

480/208 gives you the ratio of 2.3:1 And 1/3 of the 100a is 33.3A
So, 2.3X33.3=76.6A Table 310.16 75c collum says #4 is good for 85amps

So I'd say it's B

The only thing not mentioned in the question that is required, is that the secondary conductors must land in a over current protection device.

 

[Matt_hndrx]

Thanks

Thanks

yeah that makes since I wasn't understanding the whole voltage ratio then multiply by the 1/3 I was trying to figure out the amps by cross multiplying and it wasn't making since I think your right on other than I think the answer is D because it says there aluminum conductors. Thanks for the help!

 

[S'mise]

Your welcome, I didn't catch the aluminum part. The devil is in the details.