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Ampacity Rating for Cable in Direct Buried Conduit - Thermal Back Fill

Question asked by gekonrad on Oct 19, 2016

I have been working with an electrical engineering firm that consistently doubles the quantity of direct buried electrical feeders for a rated system in turn doubling the ampacity of the feeders. Where as three sets on 500MCM THHN inside a direct buried PVC conduit would be rated for 1290 Amps @ 480 volt according to NEC table 310.15. (B) (16) The engineer of record is specifying six sets of 500MCM The argument is the native soils thermal resistivity value or thermal RHO is to high which can cause the cable to overheat. If there are 3 conductors  at a minimum fill in direct buried conduit, then why the concern with the soils RHO value?

Attached is an article from Transmission & Distribution World which explains in detail the concerns of direct buried transmission cable. My question is related to direct buried cable in conduit at minimum fill less than 600V.