This something I found and it does answer my question. It’s very subjective but an answer nontheless. See general note 3.
Table 300.50 Minimum Covera Requirements
| Circuit Voltage | General Conditions (not otherwise specified) | Special Conditions (use if applicable) | | | | | | | | | | |
| Column 1 | Column 2 | Column 3 | Column 4 | | Column 5 | Column 6 | | | | | |
Direct-Buried
Cablesb mm in. RTRC, PVC, | and HDPE | Conduite mm in. Rigid Metal Conduit and | | | Kaceways
Under Buildings or Exterior
Concrete Slabs,
100 mm (4 in.)
Minimum
Thicknessd | | Cables in Airport
Runways or
Adjacent Areas Where Trespass Is Prohibited | | | Areas Subject to Vehicular
Traffic, Such as Thoroughfares and Commercial Parking Areas | | |
| mm | in. | mm | in. | mm | in. | mm | in. | | | | | |
Over 1000 V
through 22 kV | 750 | 0E | 450 | 18 | 150 | 6 | 100 | 4 | 450 | 18 | 600 | 比Z |
Over 22 kV
through 40 kV | 900 | | 600 | 24 | 150 | 6 | 100 | 4 | 450 | 18 | 600 | 24 |
| Over 40 kV | 1000 | 42 | 750 | 30 | 150 | 6 | 100 | 4 | 450 | 18 | 600 | |
General Notes:
- Lesser depths shall be permitted where cables and conductors rise for terminations or splices or where access is otherwise required.
- Where solid rock prevents compliance with the cover depths specified in this table, the wiring shall be installed in a metal or nonmetallic raceway permitted for direct burial. The raceways shall be covered by a minimum of 50 mm (2 in.) of concrete extending down to rock.
- In industrial establishments, where conditions of maintenance and supervision ensure that qualified persons will service the installation, the minimum cover requirements, for other than rigid metal conduit and intermediate metal conduit, shall be permitted to be reduced 150 mm (6 in.) for each 50 mm (2 in.) of concrete or equivalent placed entirely within the trench over the underground installation.
Specific Footnotes:
"Cover is defined as the shortest distance in millimeters (inches) measured between a point on the top surface of any direct-buried conductor, cable, conduit, or other raceway and the top surface of finished grade, concrete, or similar cover.
*Underground direct-buried cables that are not encased or protected by concrete and are buried 750 mm (30 in.) or more below grade shall have their location identified by a warning ribbon that is placed in the trench at least 300 mm (12 in.) above the cables.
Listed by a qualified testing agency as suitable for direct burial without encasement. All other nonmetallic systems shall require 50 mm (2 in.) of concrete or equivalent above conduit in addition to the table depth.
*The slab shall extend a minimum of 150 mm (6 in.) beyond the underground installation, and a warning ribbon or other effective means suitable for the conditions shall be placed above the underground installa
