Cable derating

[bartb102]

I'm trying to size power cables between switchgear and two generators. Each generator is rated 722A at 2400V. I want to use 2 500MCM MV90 conductors for each phase, 3 conductors in each of four conduits. If I arrange them 2h x 2w, cable is rated 344A, not enough. If I had only one generator and arranged them 1h x 2w the cable is rated 417A, great. Where to I find the rating for 1h x 4w? Or how far apart do the two banks of 1 x 2 conduits have to be to be considered separate banks?
Thanks,
Bart B

 

[7anoter4]

So, the total current = 2*720 A=1440 A and will be 4 cables per phase of 500 MCM copper MV90.
NEC art. 310.60 TABLE for Detail 2 indicate 375 A per conductor for 3 single core cable in an Electrical Duct.
From :INFORMATIVE ANNEX B
1) The factors of 0.88 for 2*Detail 1 and 0.94 for 4 conductor in Detail 2 there are for 0-2000 V cable only.
2) B.310.15(B)(2) Typical Applications Covered by Tables.
"where adjacent duct banks are used, a separation of 1.5 m (5 ft) between the centerlines of the closest ducts in each bank or 1.2 m (4 ft) between the extremities of the concrete envelopes is sufficient to prevent derating of the conductors due to mutual heating".
If you will calculate the permissible ampacity as per IEEE-835-1994 [SEE:art 310.60 Conductors Rated 2001 to 35,000 Volts. Informational Note No. 1] for MV90 cable [shield grounded only one end] you'll get 370 A per cable for 4 embedded ducts in a straight line keeping only 30" distance between the centerlines of closest ducts of 2 duct banks in the same conditions as per NEC:
30" depth; RHO=90; 7.5" distance between center lines in each bank of two; LF=100%;Earth ambient temp.20oC.

 

[wareagle]

I assume these conduits are underground. Since you only need to supply 722 amps, you can use either 3 4/0 15 kv 90C @ 285 amps or
3 3/0 15 kv 90C @ 250 amps using detail 2 & 3 conduits per Table 310-60. 2 500 15 kv is not quite large enough.