perfectly balanced wye system

[CJ357]

Recently on my JW county test, I had a question that kind of threw me for a loop. Here goes to the best of my memory;

You have a perfectly balanced 120/208v wye system at 35,000VA, what is the maximum amperage you will see?

I cant remember the exact VA for the question. Now I know this is something I should know, I remember doing it in class ( 3 years ago), but I just cant remember how to do it.

Ive talked to a few brothers and they said something to the effect that i needed to multiply by 1.73 due to the wye configuration, but that just doesnt click in my head.

Thanks in advance.

 

[Smart $]

Recently on my JW county test, I had a question that kind of threw me for a loop. Here goes to the best of my memory;

You have a perfectly balanced 120/208v wye system at 35,000VA, what is the maximum amperage you will see?

I cant remember the exact VA for the question. Now I know this is something I should know, I remember doing it in class ( 3 years ago), but I just cant remember how to do it.

Ive talked to a few brothers and they said something to the effect that i needed to multiply by 1.73 due to the wye configuration, but that just doesnt click in my head.

Thanks in advance.

You divide 35kVA by (120 x 3) or (208 x 1.732)... your choice. You may see some slight discrepancy in the results because 120 x 3 = 360 while 208 x 1.732 = 360.256. It is due to rounding of...

√3 = 1.7320508075688772935274463415059... to 1.732​

and

120 x √3 =207.8460969082652752232935609807... to 208​

 

[charlie b]

. . . they said something to the effect that I needed to multiply by 1.73 due to the WYE configuration, but that just doesn?t click in my head.

You will need to get used to the fact that the square root of three (approximately 1.732) is going to show up on almost any calculation you do for a three phase system. In your example, it goes like this:

Apparent Power (VA) equals 1.732 times Voltage (V) times Current (A).

That is the same as saying,

Current (A) equals Apparent Power (VA) divided by { 1.732 times Voltage (V) }

Therefore,
Current equals (35000) / (1.732 x 208)
Current equals (35000) / (360)
Current equals 97.2 amps.

 

[ryan_618]

In my opinion, it is worth memorzing the fact that 208*1.732 is 360, and that 480*1.732 is 831.