Running service conductors underground presents many advantages, perhaps most notably increased protection from the elements. Among the disadvantages is the increased “protection” from maintenance crews. For example, you can get some good partial discharge images on overhead conductors but the camera can’t see those conductors through cement or topsoil.
The NEC doesn’t provide clearances for underground conductors, the way it does for overhead conductors. This doesn’t mean clearances don’t apply as engineering requirements. For example, it’s sound engineering to not only route underground conductors away from vehicle traffic but to also allow sufficient distance for that traffic to flow uninterrupted if the day arrives that someone must excavate the cables for testing or repair.
Part III of Article 230 does have other requirements, however. These begin with the requirement that the conductor be insulated for the applied voltage [230.30]. But note that you determine the type of conductor per Chapter Three (it must be listed for use as a service conductor). If it’s not a direct burial conductor, then you’ll also need to use a Chapter Three raceway method permitted for underground services.
You must size the conductors so they have sufficient ampacity for the load (as calculated per Article 220) [230.21(A)] but they can’t be smaller than 8 AWG if copper or 6 AWG if aluminum.
