Motor Calculation

[starbright28]

I'm working on a motor calculation that I can't seem to get the right answer for. I think I am working it out correctly, and finally grasping the conception of how to do one, but frustrated because the answer I am getting seems to not be right.

Here's the question:
What size RHW aluminum conductor would be required for 4 - 230 volt 3-phase motors rated: 1 - 7 1/2 hp, 1 - 5hp, and 2 @ 25 hp?


My Math:
Full Load current amps for the following motors.
7 1/2HP - 22A
5hp -15.2 A
25hp - 68A
25hp - 68 A

I then took the 125% of the 68A and got 85 A.

Then I did the following math:
85+22+15.2+68=190.2A

Since you can round up to the nearest next fuse size - that is 200A

Looking at T310.16 in the AL column I get 250kcmil.

The Actual answer says 350kcmil. What did I forget to do?

I am not sure if the answer that I had provided is wrong, or if my math was wrong. I checked this with my dad and he also came up with 250 kcmil.

 

[don_resqcapt19]

Amanda,
It looks like the test writers used the wrong column in the table. 350kcmil AL would be required for conductors rated at 60?C, and it looks like it was set as a "trick" question. The 60? ampacity is 190 amps...0.2 amps too small. 250 is correct for RHW.
Don

 

[starbright28]

Thanks. At least I am not the only one who sees this answer is wrong. That is really frustrating when you think you understand it and can do it but then the answer key is wrong and you second guess yourself.