NEC Table 300.5(A) sets the minimum cover — the distance from the top of the buried wiring to the finished grade — for underground installations up to 1000 volts. The general case: 24 inches for direct-buried cable, 18 inches for PVC conduit, and 6 inches for rigid metal or IMC. A GFCI-protected 20 A residential branch circuit gets a reduced 12 inches.
| Location of wiring method or circuit | Direct burial cables or conductors | RMC or IMC | Nonmetallic raceway listed for direct burial (PVC) | Residential branch circuits ≤120 V, GFCI, max 20 A | Irrigation and landscape lighting ≤30 V, GFCI, max 20 A |
|---|---|---|---|---|---|
| All locations not specified below | 24 | 6 | 18 | 12 | 6 |
| In trench below 2 in. thick concrete or equivalent | 18 | 6 | 12 | 6 | 6 |
| Under a building | 0 (in raceway only) | 0 | 0 | 0 (in raceway only) | 0 (in raceway only) |
| Under min. 4 in. thick concrete exterior slab with no vehicular traffic, slab extending 6 in. beyond the installation | 18 | 4 | 4 | 6 | 6 |
| Under streets, highways, roads, alleys, driveways, and parking lots | 24 | 24 | 24 | 24 | 24 |
| One- and two-family dwelling driveways and outdoor parking areas, used only for dwelling-related purposes | 18 | 18 | 18 | 12 | 18 |
| In or under airport runways, including adjacent areas where trespassing is prohibited | 18 | 18 | 18 | 18 | 18 |
300.5(D) — protection from damage. Direct-buried conductors emerging from grade must be protected by enclosures or raceways from below grade to at least 8 feet above finished grade. The raceway itself must extend at least 18 inches below grade or to the required cover depth, whichever is less.
300.5(F) — backfill. Backfill may not contain large rock, paving, cinders, or corrosive material that could damage raceways or cables. Sand bedding or a suitable running board is the standard fix in rocky soil.
300.5(J) — earth movement. Direct-buried conductors and raceways subject to settlement or frost heave must have an S-loop or expansion fitting where they enter a building or fixed structure. This is why underground feeds to a detached garage are looped at the riser.
300.5(I) — all conductors in the same trench. All conductors of the same circuit — including the equipment grounding conductor — must be in the same raceway or trench. Running the ground in a separate trench creates an inductive loop and is a violation.
300.5(D)(3) — warning ribbon. Underground service conductors that are not encased in concrete and are buried 18 inches or more below grade need a warning ribbon at least 12 inches above them. Other circuits do not require one, but many jurisdictions add it by local amendment.
The cover depth is only meaningful if the wiring method is rated for the location. Underground is a wet location by definition under NEC 300.5(B), and every conductor in a buried raceway must be listed for wet locations — THWN-2, XHHW-2, or USE-2, never plain THHN or NM-B.
UF-B cable is the standard direct-burial cable for branch circuits and takes the 24-inch general column, or 12 inches on a GFCI-protected 20 A residential circuit. USE-2 is the direct-burial service-entrance conductor. NM-B (Romex) is never permitted underground — not in conduit, not in a sleeve, not for a short run.
Per NEC Table 300.5(A), direct-buried cable needs 24 inches of cover in the general case, PVC conduit 18 inches, and rigid metal or intermediate metal conduit 6 inches. A GFCI-protected residential branch circuit of 120 V or less and 20 A or less may be buried at 12 inches.
Under streets, highways, roads, alleys, driveways, and parking lots, every wiring method requires 24 inches of cover. The exception is a one- or two-family dwelling driveway used only for dwelling-related purposes, which drops to 18 inches — or 12 inches for a GFCI-protected 20 A residential circuit.
No. NM-B cable is rated for dry locations only under NEC 334.12(B)(4). Underground is a wet location, and conduit does not make it dry — water gets in. Use UF-B cable for direct burial, or pull THWN-2, XHHW-2, or USE-2 conductors through the conduit.
Cover is measured from the top surface of the cable, conduit, or raceway to the finished grade above it — not from the bottom of the trench and not to the centre of the conductor.