Charlie, Thank you for your service...enjoyable and very informative. Section 300.3(B) discusses conductors of the same circuit and in general, requires that all conductors (including the grounded conductor and all equipment grounding conductors and bonding conductors) to be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4). An exception under 300.3(B)(1) - Parallel Installations - allows conductors that are installed in underground, non-metallic raceways to be arranged as isolated phase installations as long as the raceways are installed in close proximity and comply with 300.20(B). Section 300.5(I) discusses conductors of the same circuit in underground installations. This section also requires all conductors of the same circuit (including the grounded conductor and all equipment grounding conductors) to be installed in the same raceway or cable "or shall be in close proximity in the same trench." Exception No. 2 under this section, however, allows isolated phase, polarity, grounded conductor, and equipment grounding and bonding conductor installations in non-metallic raceways in close proximity where conductors are paralleled as permitted in 310.10(H), and where the conditions of 300.20(B) are met.
Now to my question: For an underground 3 phase, 5 wire installation (3 phase conductors, an equipment grounding conductor, and a grounded conductor), can the 3 phase wires and the equipment grounding conductor be installed together in a single underground PVC conduit and the grounded conductor be installed in a separate, adjacent underground PVC conduit? The conductors are "installed in close proximity in the same trench" per the last sentence of Section 300.5(I) and this also appears to meet the general intent covered by Exception No. 2 under Section 300.5(I). However, this is not a parallel installation and each phase is not isolated. All phase conductors (and the equipment grounding conductor) are grouped together in one underground PVC conduit and the grounded conductor (the neutral) is installed in a separate underground PVC conduit with both conduits being side by side (i.e. in close proximity in the same trench). Thanks, Chris DeWeese