Quick voltage drop Q

[greenspark1]

Hi. It's been a while since I've done a voltage drop calculation. I tried using a couple of online calculators but they are giving different answers and not showing how they do the math. I want to make sure I know how to do it right.

Here's the situation: 480V/3ph 75kW load (=90A), 150' from the panel.

I also looked at this post: http://forums.mikeholt.com/showthread.php?t=83435&page=3&p=649546#post649546
But it uses DC resistance values from Table 8. Not clear why you wouldn't use the AC values from Table 9. I guess they're usually very close.

For #6 conductors:
Vd = 1.73 * 0.49/1000 * 90 * 150' = 11.44V dropped
480-11.44 = 468.6V
1- 468.6/480 = 2.5% voltage drop. This is within the 3% generally accepted branch circuit limit so I should be good. Does this make sense? Thanks for the help!

PS- This agrees with the calculator here, but not the second one here.

 

[charlie b]

Using the VD calculator I have on my computer, I get 11.49 volts. So I can confirm that your design selection is within the customary limit of 3%.

 

[charlie b]

I checked out the two calculation pages to which you provided links. I got the same answers you did, and I can't explain why the second one has the wrong answer.

 

[greenspark1]

Thanks, but I just realized (headslap) #6 is only rated for 55A so isn't an option. I need #2 for the 90A load. At 0.2 ohm/kft there will be less Vdrop so it'll work.