Underground Cable Spacing and Tamb

[mesutphen]

I need to run underground cabling from my battery bank to my inverters. My inverters are rated for 6000VA and are 93.5% efficient, so I have calculated that the DC current will be 134A at 48Vdc. I have 2 inverters, but I plan to parallel the battery cables which, if I read the NEC correctly, I am allowed to do with 1/0 or larger wire. If I read further on, figure 310.60 details the required spacing for underground cabling, but this appears to be applied to 2001V-35,000V rated cables. Is there a horizontal spacing requirement for low voltage cables? The cables will be in 2 separate conduits, 2 wires to a conduit, if that makes a difference. The only thing I can find is 310.15(B)(3)(b) which states, “Spacing between conduits, tubing, or raceways shall be maintained.”

I’m also wondering how to determine the ambient temperature. I know that the cables need to be buried at least 18 inches below grade. What ambient temperature should I use as an adjustment factor? Air temperatures here, in the summer, typically reach 110F. Obviously, the temperature 18 inches below grade will be something lower than that. Do I need to measure the temperature of the soil at that depth? Or is there a thumbrule that can be applied? I’m thinking that the safest thing to do would be to use peak ambient air temperature. For this installation, it will not affect the cable size because a 110F correction factor puts me at a 1/0 cable; any lower ambient temperature would reduce my required cable size, but I would still need to use 1/0 in order to parallel them legally. However, I would like to know for future reference.

 

[7anoter4]

First of all the inverter efficiency is referring to input/output power so the input power has to be increased by 100/93.5=1.07
Then 134 A is a.c. supply source current. It seems to me the output current [d.c.]will be less[let's say 125 A]
Art. 310.15 A (2):
Selection of Ampacity. "Where more than one ampacity
applies for a given circuit length, the lowest value shall be used."
If the cable runs more than 20 ft. on cable tray the ampacity could be less.
Underground 30 dgr.C ambient
If the air temperature is 110 dgr.F [40 dgr.C] the soil temperature could be 30 dgr.C
Table 310.60(C) 83 for Detail 6 [2 cable 24" apart] indicates 215 A for 1/0 copper conductor 90dgr.C insulation rating [thermosetting as XLPE]
but for 20 dgr.C ambient.
According to Art.310.60 C 4
Inew=Itable*sqrt((Tc-Tanew)/(Tc-Tatable))
Tc=90 Tanew=30 Tatable=20
Inew=215*0.925=199 A
Cable Tray 40dgr.C
Table 310.60(C) 71 [ one cable diameter clearance from cable to cable has to be kept.]
I=185 A for 1/0 copper.
Now, according to Art.340.80 for UF cable [no mentioning of which conditions and what clearance one needs to keep]
from Table 310.15 (B) 16 [UF 60dgr.C] for 1/0 copper ampacity will be 125 A.