How deep does direct buried cable and conduit have to be?

Table 300.5(A) is the answer, and the number depends on the wiring method and on what is above it. For circuits rated 1,000 volts or less, direct buried cable or conductors require 600 mm, or 24 inches, of cover. Rigid metal conduit and intermediate metal conduit require 150 mm, or 6 inches. Nonmetallic raceways listed for direct burial without concrete encasement, which includes Schedule 40 and Schedule 80 PVC, require 450 mm, or 18 inches.

Reading the columns

The table has five location columns and they change the numbers substantially. Under a building, direct burial is not permitted at all and raceways only require the depth needed to be in or under the building. Under a minimum 100 mm, or 4 inch, thick concrete exterior slab with no vehicle traffic and the slab extending not less than 150 mm, or 6 inches, beyond the underground installation, direct buried cable drops to 450 mm, or 18 inches, and RMC, IMC, and nonmetallic raceway drop to 100 mm, or 4 inches.

Under a street, highway, road, alley, driveway, or parking lot, everything goes to 600 mm, or 24 inches, regardless of wiring method. One and two family dwelling driveways and outdoor parking areas used only for dwelling related purposes get 450 mm, or 18 inches, for every method. Airport runways and adjacent areas go to 750 mm, or 30 inches.

The residential exceptions that save trenching

Note 3 of Table 300.5(A) allows lesser depths where cables and conductors rise for terminations or splices or where access is otherwise required. Note 4 permits 300 mm, or 12 inches, of cover for a residential branch circuit rated 120 volts or less with GFCI protection and maximum overcurrent protection of 20 amperes. That is the note that lets a landscape lighting or a shed circuit go in at 12 inches.

Note 5 covers control and signaling circuits limited to 30 volts or less at 150 mm, or 6 inches, where in raceway, and 150 mm where direct buried with Type UF or other identified cable. Note 6 permits lesser depths where specified in the installation instructions of listed low voltage lighting systems.

Cover means cover

Cover is defined in 300.5(A) as the shortest distance in millimeters or 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. It is not the depth of the trench bottom. If you lay a 2 inch conduit in a 24 inch trench, your cover is 22 inches.

Protection, warning tape, and backfill

Section 300.5(D)(1) requires direct buried conductors and cables emerging from grade to be protected by an enclosure or raceway from below grade to a point at least 2.5 m, or 8 feet, above finished grade. Section 300.5(D)(4) requires conductors entering a building to be protected to the point of entrance.

Section 300.5(F) requires backfill that contains large rock, paving materials, cinders, large or sharply angular substances, or corrosive material to be excluded, or the raceway or cable to be protected by granular material, suitable running boards, sleeves, or other approved means. Section 300.5(D)(3) requires a warning ribbon placed in the trench at least 300 mm, or 12 inches, above underground installations for circuits over 1,000 volts.

Field takeaway

Direct buried cable at 24 inches, PVC at 18 inches, RMC and IMC at 6 inches, everything at 24 inches under a public driveway or road, 18 inches under a residential driveway, and a 12 inch allowance for a GFCI protected 20 ampere 120 volt residential branch circuit. Measure cover from the top of the raceway to finished grade, and keep sharp fill out of the trench.