When to Use Underground Wiring
Underground wiring is required whenever electrical circuits run between separate buildings, to outdoor equipment (HVAC units, pool pumps, landscape lighting), or to detached garages and outbuildings. The NEC provides specific burial depth requirements based on conductor type, wiring method, voltage level, and whether the area is subject to vehicular traffic.
The primary governing reference is NEC Table 300.5, which provides minimum cover requirements for all underground wiring methods. 'Cover' is measured from the top of the conduit or cable to the finished grade — not from the bottom of the trench.
NEC Burial Depth Requirements
Direct-Buried Cables (UF-B): 24 inches minimum cover for residential branch circuits. Under a concrete slab at least 2 inches thick, this reduces to 18 inches.
Rigid Metal Conduit (RMC) or Intermediate Metal Conduit (IMC): 6 inches minimum cover. The superior mechanical protection of metal conduit allows the shallowest burial.
PVC Conduit (Schedule 40 or 80): 18 inches minimum cover for most residential circuits operating at 120V–240V. Under driveways, 18 inches minimum.
Under Streets and Highways: All wiring methods require 24 inches minimum cover. This applies to public roads, parking lots, and commercial driveways.
Under Residential Driveways: 18 inches minimum for all methods. The NEC recognizes that residential driveways typically see lighter vehicular loads than public roads.
Circuits operating at ≤ 20A, 120V with GFCI protection in RMC/IMC: Only 6 inches minimum cover — this is the most lenient requirement.
Wiring Methods Compared
UF-B Cable (Underground Feeder): The simplest method — direct burial without conduit. Must be UF-rated (moisture and sunspot resistant), not standard NM-B (Romex). Maximum voltage 600V. Most cost-effective for simple residential circuits but provides no mechanical protection and cannot be easily replaced.
PVC Conduit: The most common method for residential and light commercial. Schedule 40 for most burial applications, Schedule 80 where exposed above grade. Conductors can be pulled and replaced. Use PVC solvent cement joints — they do not require threading.
Rigid Metal Conduit (RMC): Provides the best mechanical protection and allows the shallowest burial depth (6 inches). More expensive and labor-intensive due to threading. Preferred for commercial and industrial installations.
LFNC (Liquid-tight Flexible Non-metallic Conduit): 18 inches minimum burial. Useful for short runs to equipment with vibration (HVAC compressors, pool pumps). Limited to 6 feet exposed above grade per NEC 356.10.
Installation Best Practices
Trench Preparation: Dig the trench 3–6 inches deeper than the required cover to allow for bedding sand. Remove rocks and debris that could damage insulation. Grade the trench bottom to prevent water pooling.
Warning Tape: Install detectable underground warning tape 12 inches above the cable or conduit. While not required by NEC for residential, it is standard practice and required by most utility companies.
Expansion Joints: PVC conduit expands approximately 4 inches per 100 feet for a 60°F temperature change. Install expansion fittings at building entries and every 100 feet of straight run.
Conductor Selection: Use THWN-2 or XHHW-2 conductors inside conduit — both are rated for wet locations. In direct burial, use UF-B or USE-2 rated cables. Standard THHN is not rated for wet locations and must not be used underground.
Transition to Above-Grade: When conduit transitions from underground to above grade, protect the exposed portion with Schedule 80 PVC or RMC to a height of 8 feet per NEC 300.5(D)(1).
Common Mistakes to Avoid
Using NM-B (Romex) underground — NM-B is not rated for wet locations or direct burial. Only UF-B cable may be directly buried. This is one of the most common DIY mistakes and a guaranteed inspection failure.
Insufficient cover under driveways — Many installations fail because the installer measured from the bottom of the trench rather than from the top of the conduit/cable to finished grade.
No ground wire with UF-B — Direct-buried UF-B circuits still require an equipment grounding conductor. UF-B cable with ground is available — do not use ungrounded cable.
Forgetting voltage drop — Underground runs are often long (50–200+ feet). Always calculate voltage drop and upsize conductors accordingly. A 200-foot run to a detached garage almost always requires upsizing beyond the ampacity minimum.