Exception cable space requirement in cable tray 3

[metta0810]

I use 2kV (client's requirement) 3/C-500 kcmil w/gnd (4 cond/phase) which runs 916 ft to feed 800Amps Bus in Motor Control Center. The cables are oversized to avoid the excess voltage drop especially during the motor starting. These cables run through a 36" multi lay cable tray. I calculate the space will be needed to accommodate the overall cable to be about 46". The calculation is based on the NEC 2008 Exhibit 392.3 which indicates that the separation between cables should not be less than one cable diameter spacing. Is there any exception that the space between cables can be less than one cable diameter since the cables are oversized with ampacity= 1337.6 but run in 800 amps only?
How much electrostatic flux does impact if we place the space between the cable at min of 1/2 of the one cable diameter spacing?

 

[dpc]

Review 392.9 and 392.11. You can place the cables closer together, but must derate the cables accordingly.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[resqcapt19]

You may also have an issue with 250.122(B). The typical equipment grounding conductor in a 3 conductor 500 kcmil cable would be a #2. Your application will require a 1/0 EGC in each of your multiconductor cables.

It appears to me that 392.11(A) will permit the full Table 310.16 ampacity for multiconductor cables in a cable tray without separation.

 

[7anoter4]

I think something is wrong in the open post data. I understood there are 4 conductor per phase [may be
Three-core cable may be 4*3 single core cables. According to Okonite catalog 3*500mcm 2.4 kV cable overall dia. is 2.61 inches [armour including].If there are single-conductor the o/dia.=1.29 inches [for 2.4 kV rated].
In the case of three-core cables-see the sketch-you need a 20.27 [minimum] inches cable tray width.
According to NEC [as declared by Okonite]
"Ampacities are in accordance with Table 310.75 of the NEC for a three conductor Type MV-90 or MC cable installed in uncovered cable tray in accordance with Section 392.13 of the NEC with a conductor operating temperature of 90°C
and ambient air temperature of 40°C. Where the cable tray is covered for more than six feet with solid unventilated covers, the ampacities shall not be more than 95% of the values shown above."
Permissible ampacity per cable will be 425 A [and you need only 200 A per one cable or total 800 A per phase].
If there are 12 cable single-phase then laying in triangle you need 20.06 or in straight line the required width will be 32 inches. But the ampacity will grow [520 A triangle 570 straight].
see:

http://www.okonite.com/Product_Catalog/section2/index.html

 

[metta0810]

Thank you for all the replies. I really appreciate it.