Underground wire: which types to use and how deep to bury them
Published Sep 30, 2026
Running power underground comes down to two choices: a cable or conductor rated for burial, and a depth that matches it. Get either wrong and the inspector will have you dig it up. This guide covers the common wire types and the minimum cover from NEC Table 300.5.
Wire types for underground runs
| Type | What it is | Direct burial | In underground conduit |
|---|---|---|---|
| UF-B | Copper branch-circuit cable, solid PVC jacket | Yes | Yes |
| USE-2 | Single conductor, cross-linked insulation, often dual rated RHH/RHW-2 | Yes | Yes |
| URD | Aluminum duplex, triplex or quadruplex made for direct burial | Yes | Yes |
| THHN/THWN-2 | Building wire, 90 °C wet rating | No | Yes |
| XHHW-2 | Building wire, cross-linked insulation | No | Yes, and well suited to it |
| NM-B | Indoor cable | No | No |
Conduit below grade is a wet location (NEC 300.5(B)), so anything pulled into it needs a wet rating, which is why NM-B is never allowed there. For a closer look at the two branch-circuit cables, see UF-B vs NM-B; for conductors in conduit, THHN vs XHHW-2.
Minimum cover depth, NEC Table 300.5
Cover is measured from the top of the cable or raceway to the finished grade. These are the values for the general location (column headings of Table 300.5), for circuits of 0 to 1,000 V:
| Wiring method | Minimum cover |
|---|---|
| Direct-burial cable or conductors (column 1) | 24 in. |
| Rigid metal or intermediate metal conduit (column 2) | 6 in. |
| PVC or other nonmetallic raceway listed for burial without concrete encasement (column 3) | 18 in. |
| Residential branch circuit, 120 V or less, GFCI protected, 20 A or less (column 4) | 12 in. |
| Irrigation and landscape lighting circuits of 30 V or less, in UF or other identified cable or raceway (column 5) | 6 in. |
The table has more rows for other locations, and they change the numbers:
- Under a building: no minimum cover, but the wiring must be in a raceway or be a cable identified for direct burial under a building.
- Under a 4 in. concrete slab with no vehicle traffic: shallower cover is allowed.
- Under streets, highways, driveways and parking lots: 24 in. for every method.
- One- and two-family driveways and outdoor parking areas used only for dwelling purposes: 18 in. for most methods.
Read the whole table and its notes for your situation before you trench.
Other rules that catch people
- Where it comes up: direct-buried cable that emerges from the ground must be protected from the required cover depth to at least 8 ft above grade, or up to the enclosure it enters (300.5(D)(1)).
- Warning tape: service conductors buried 18 in. or more below grade need a warning ribbon at least 12 in. above them (300.5(D)(3)).
- Keep large rocks, paving and sharp material out of the backfill against the cable (300.5(F)).
- Direct-burial splices need a method listed for burial (300.5(E)).
- Sealing: a raceway where moisture could reach live parts must be sealed or plugged at one or both ends (300.5(G)).
Choosing a wire for a common job
- Detached garage, 20 A, 120 V: 12/2 UF-B at 12 in. with GFCI protection, or at 24 in. without it.
- Subpanel in a shop, 60 to 100 A: aluminum URD direct buried at 24 in., or THWN-2 or XHHW-2 in PVC at 18 in.
- Well pump at 240 V: UF-B at 24 in. The 12 in. allowance applies only to 120 V circuits.
- Low-voltage landscape lighting: landscape lighting cable at 6 in.
Shop underground wire
- 12/2 UF-B and 10/2 UF-B
- 2 AWG USE-2/RHH/RHW-2 aluminum
- 4-4-4 URD aluminum and larger URD
- 6 AWG XHHW-2 copper for conduit
Verify with your local code. Burial depth, GFCI requirements and approved wiring methods vary by NEC edition and by local amendment, and utilities set their own rules for service laterals. Call 811 before you dig and ask your inspector before you backfill.