What types of conduit are approved for direct burial?

The workhorses for direct burial under NEC 2023 are: Schedule 40 and Schedule 80 PVC (Article 352), rigid metal conduit (Article 344), intermediate metal conduit (Article 342), HDPE conduit (Article 353), and RTRC fiberglass conduit (Article 355). Each article specifically permits underground and direct burial use in its uses permitted section.

Burial depths from Table 300.5

Depth of cover for circuits over 30 volts, measured from the top of the raceway: PVC and other listed nonmetallic raceways need 18 in. RMC and IMC need only 6 in, which is why steel wins on shallow rock jobs. Under a driveway at a dwelling, raceways generally need 18 in. Residential branch circuits with GFCI protection at 20 amps or less get the 12 in allowance in some columns, and burial under a 4 in concrete slab reduces several values. Always read the column that matches your actual condition.

What about EMT?

Article 358 does technically permit EMT in concrete and in direct contact with earth where protected by corrosion protection judged suitable for the condition. In the real world, plain EMT and its setscrew fittings rust out underground, few inspectors accept it, and supplemental corrosion protection that satisfies 358.10(B) is more work than just laying PVC. Treat EMT as an above grade product.

Practical picks

Most underground residential and light commercial work is Schedule 40 PVC in the trench, transitioning to Schedule 80 where the conduit emerges and is subject to damage per 352.10(F), with rigid steel elbows on long pulls where the pull rope would burn through PVC sweeps. Use listed PVC female adapters at boxes, glue with the right cement, and remember expansion fittings on long runs per 352.44; buried PVC still moves with soil temperature. Pull in an equipment grounding conductor, since PVC provides no ground path.