Nick,
This sounds like it would be considered a fountain, which, as you know is regulated by Part 1 of 680. As far as the Underground Wiring Locations, it may seem contradictory, but it does provide for relief in instances just like yours. On the other hand, my experience is limited to 600 volts, and 18" for a NM raceway seems a bit shallow for MV feeders. Could you double the depth?
