Parallel conductors direct buried. (is there a multiconductor derate?

[southernviewenergy]

I am installing a 600 amp panelboard in a solar farm that is located 135' from the main disconnect. For this installation, I have chosen to run parallel 350 MCM aluminum for each phase condctor, (1) 4/0 Alu for the Neutral, and (1) #2 Cu for the EGC to feed this panelboard that is fused at 500 amps. I am having a difficult time interpreting the code on the effects of multiconductor derating for conductors which are direct buried. Table 310.15(B)(16) mentions the ampacity of the conductors when installed as (not more than 3 current carrying conductors installed in Raceway, Cable, or Earth. However, when you go to 310.15(B)(3)(a), it specifically mentions that the multiconductor derate is for conductors installed in a Raceway or Cable and mentions nothing of in Earth.

My assumption would be that if the conductors are not bundled in a group so that there is no opportunity for heating of conductors, then the multiconductor derate would not apply. However, the way that the header on table 310.15(B)(16) is written, it implies that you would need to.

I am being told by another engineer that I have to derate the conductors and increase the size of my phase conductors or seperate out each bundle of what is existing and run seperate EGC and Neutrals to accomodate article 300.5 (I). I think that the other enginner is incorrect in his assesment of the code that I need to size the phase conductors larger to offeset the multiconductor derate as mentioned earlier. Any help on this topic would be greatly appreciative.

Scott Earp

 

[kwired]

Read 310.15(B)(3)(a) carefully again.

More Than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

If direct buried don't just throw them in a trench and cover them up, route them in the trench so there is some maintenance of spacing.