Running power to a detached garage, shed, or landscape lighting always comes down to one decision: which cable can legally go in the dirt and how deep does it need to sit. Underground wire is a category, not a single product, and picking the wrong type or burying it too shallow is the fastest way to flunk a permit inspection or create a shock hazard after the first hard freeze.
The Three Wire Types You Will See
Residential underground runs typically use one of three products. Each has a different jacket, approved use, and trench depth under the National Electrical Code.
- UF-B cable: Grey jacket, direct burial rated, carries hot, neutral, and ground in one sheath. Most common for 15, 20, and 30 amp circuits
- THWN-2 in conduit: Individual insulated conductors pulled through schedule 40 or 80 PVC. Standard for service feeders and higher amperage
- USE-2 or SER: Service entrance cable used for main feeder runs from meter to subpanel, often at 100 or 200 amps
NEC Burial Depth Requirements
Depth is not a suggestion. Table 300.5 of the National Electrical Code sets the minimum cover based on circuit type and voltage. Frost line is a separate concern, and several northern states require burial below frost regardless of the NEC minimum.
- UF-B direct burial, residential 120V circuit with GFCI: 12 inches of cover
- UF-B direct burial, no GFCI: 24 inches of cover
- THWN in rigid metal conduit: 6 inches minimum
- THWN in schedule 40 PVC: 18 inches
- Under driveways or parking areas: 24 inches for most types
How to Trench the Right Way
Rent a walk-behind trencher from Home Depot or United Rentals for about $90 per day. A Ditch Witch 1030 cuts a 4-inch wide trench up to 24 inches deep in roughly one minute per linear foot in average soil. For short runs under 50 feet, a sharp spade and a mattock will do the job in an afternoon.
Call 811 at least 3 business days before digging. The free utility locate service marks gas, water, telecom, and existing electric lines with color-coded paint. Hitting a gas line with a trencher is a six-figure mistake.
Conduit vs Direct Burial
Direct burial UF-B is faster and cheaper for short, simple runs. A 250-foot spool of 12/2 UF-B with ground runs about $180 at Lowes. But once buried, any damage means re-trenching the whole run.
Conduit is more work up front but future-proofs the installation. Pulling THWN-2 through schedule 40 PVC lets you upgrade amperage later without digging. Use long-sweep 90s at direction changes, never tight elbows, or you will fight the pull. Lubricant like Ideal Yellow 77 makes conductors slide through 100-foot runs without scraping insulation.
Sizing Wire for the Load
Voltage drop matters on long runs. Over distances above 100 feet, the conductor size often needs to bump up one or two gauges from what the breaker would suggest.
- 20 amp circuit, 100 feet: 12 AWG copper, 3 percent drop at full load
- 20 amp circuit, 200 feet: step up to 10 AWG
- 50 amp subpanel feeder, 150 feet: 6 AWG copper or 4 AWG aluminum
- 100 amp subpanel feeder, 200 feet: 2 AWG copper or 1/0 aluminum
Splicing and Terminations
Every splice in underground wire must happen in an accessible, weather-tight box. Buried wire nuts inside an unprotected junction will corrode within two winters and start tripping the GFCI intermittently. Use a NEMA 3R rated outdoor box with weatherproof connectors, and fill splices with gel-filled connectors like the 3M DBR/Y series for direct-buried joints.
At the house end, bring the cable up through schedule 80 PVC conduit, which is required for any wire emerging from the ground into the first 18 inches of exposure. The grey schedule 80 elbow is the tell at every properly permitted job.
Permits and Inspection
Any circuit leaving the main structure usually requires a permit. Expect a $75 to $200 fee and two inspections: a trench inspection before backfill and a final after terminations. Do not backfill until the inspector signs off. Burying an open trench and then being asked to re-dig is a real scenario.
Common DIY Mistakes
Homeowners routinely run NM-B indoor Romex underground, thinking the plastic sheath is enough. It is not. NM-B will absorb moisture and short within months. Only UF-B or conduit-protected conductors belong in soil.
Another frequent miss is forgetting the ground rod at a detached structure. A subpanel in a garage or shed needs its own grounding electrode, typically an 8-foot copper-clad rod driven at the structure, bonded to the subpanel ground bar with 6 AWG bare copper.
Cost Breakdown for a Typical Run
A 100-foot run from a house to a detached shed with a 20-amp circuit, done right with UF-B direct burial, costs roughly $420 in materials. That breaks down to about $180 for the 12/2 UF-B cable, $60 for the schedule 80 PVC riser and fittings, $25 for the GFCI breaker, $45 for the exterior NEMA 3R box, $30 for the ground rod and clamp, and $80 for trencher rental. Pulling the same run in conduit roughly doubles the material cost but lets you upgrade later without digging.
A 200-amp subpanel feeder to a detached garage runs $1,800 to $3,200 in materials alone when you include 2-inch conduit, 2/0 aluminum SER, a 200A disconnect, and a subpanel. Permit and inspection add $150 to $350. Expect $2,500 to $5,000 total for a DIY install and $6,000 to $10,000 turn-key with a licensed electrician.
Tools You Actually Need
- Fish tape or nylon pull rope for conduit runs over 50 feet
- Ideal Yellow 77 or equivalent wire-pulling lubricant
- Klein NCVT-3 non-contact voltage tester before and after every splice
- 4-inch spade bit or rotary hammer for penetrations through the foundation
- 8-pound sledge for driving the ground rod
- Clamp meter to verify load balance on multi-wire circuits
Done right, underground wire runs for 40 or 50 years without a single issue. Done wrong, it becomes the mystery fault that haunts the house for a decade.
