Help with calc?

[pcsailor]

Hi All,
My boss asked me to size the conductor and conduit for an upcoming job and I'm wondering if I could talk through the calculation with you. I've not had to use the NEC codebook for my career (shipping-article 90), not done much new installs, and not done residential some I lack calculation-codebook experience and feel a bit handicap.
The question asked of me is " What size conductor & conduit for an underground 1000' run at 400KW, 480VAC for a solar system.
I've got 1500KCMil conductors, based on 75dC-CU with no derating of temp nor number.
I'm working on the conduit but a little lost on the underground, voltage drop, etc.
I haven't been told the conduit type but it'll be EMT or IMC probably.
I also haven't been told the terminal ratings (I went with 60dC), circuit breaker size, nor if this is a branch or feeder circuit.
I'm working on this now at a different job site and I'll post here what I come up with next.
Thanks all,
Phil

 

[Dennis Alwon]

For circuits that size 75C will probably be what you need unless it is very old equipment that is only rated 60C.

What is the likely load. Voltage drop is based on distance as well as the load. If you base it on 400kw then use an online calculator. Nobody will run 1500 kcm as a conductor.

BTW, are we to assume 3 phase? 400kw/480 x 1.73 = 482 amps

 

[pcsailor]

3 phase, yes
75dC noted, thanks
Uncertain of load so 400KW is what I'm using
My conductor calc is:
I=400000/(480x1.73) = 481amps
481 x 125% = 601.25 amps
Tble310.15(B)(16) @75°C-CU = 625a = 1500KCmil
Do I have this wrong?
Now I'm thinking I need to do a voltage drop calc and then determine conduit by conductor πr2

 

[gadfly56]

3 phase, yes
75dC noted, thanks
Uncertain of load so 400KW is what I'm using
My conductor calc is:
I=400000/(480x1.73) = 481amps
481 x 125% = 601.25 amps
Tble310.15(B)(16) @75°C-CU = 625a = 1500KCmil
Do I have this wrong?
Now I'm thinking I need to do a voltage drop calc and then determine conduit by conductor πr2

I believe Dennis' point regarding no one running 1500KCmil is that you could never pull that cable. You'd use parallel sets of something more convenient. 1500KCmil is about 1-1/4" in diameter, in solid, less insulation. It's running solid copper bars, more or less.

 

[mikeames]

To maintain less than 3% VD

cmills = 1.732 X 12.9 X 482 X 1000
-------------------------------------- = 747863 C mills
14.4 VD

3 parallel runs of 250Kcmills = 750,000Kcmills and 765 Amps



From there look in chapter 9 Table 4 for for conduit size OR ANNEX C if all conductors will be the same Size and type. Don't forget EGC or neutral if you need one, etc...