If you are running power to a shed, an outdoor outlet by the deck, or a landscape lighting transformer, you have probably seen UF cable on the shelf at Home Depot and wondered whether you actually need it instead of the cheaper NM-B you already own. The short answer to what is uf wire: it is underground feeder cable rated for direct burial in soil without conduit, and using NM-B in its place is both a code violation and a real safety problem. Understanding when and how to use UF correctly saves rework and keeps the inspector happy.
UF Cable Construction Explained
UF stands for Underground Feeder. The cable looks similar to standard NM-B (Romex) but with critical differences. The individual conductors are embedded in a solid PVC plastic compound rather than wrapped in paper and a thin jacket. This makes the cable waterproof and resistant to soil chemicals, fungi, and minor mechanical damage. The conductors themselves are typical THHN-style copper insulated to 600 volts and rated for 90 degrees Celsius.
NM-B by contrast has a paper-wrapped bundle inside a thin gray PVC jacket. It absorbs moisture, swells, and fails when buried. UF can be buried directly in trenches at the depths required by NEC 300.5 — typically 24 inches for branch circuits up to 120 volts and 18 inches for residential branch circuits with GFCI protection at 120 volts or less.
Where UF Is Required
UF is the right choice for any direct-burial residential application:
- Buried runs from the house to an outbuilding — shed, detached garage, workshop, well house
- Landscape lighting transformer runs over 25 feet
- Outdoor 120V or 240V outlets where the cable enters the ground
- Pool or pond equipment circuits where the line crosses the yard
- Pier or dock electrical (with additional protection per NEC 555)
For runs that are entirely inside conduit underground, you can use individual THWN-2 conductors instead. UF is most useful when you want to run cable directly through a trench without the cost and labor of pulling individual conductors through PVC conduit.
Where UF Should Not Be Used
UF is not a universal outdoor cable. It is not rated for exposure to direct sunlight for extended periods — UV breaks down the jacket within 5 to 10 years. If part of the run is above ground, you need to either protect it with conduit or use a different cable rated for sunlight exposure. Type SE service entrance cable handles aerial runs better.
UF is also not the right choice inside dry interior walls. NM-B is cheaper, easier to work with, and equally code-compliant for interior runs. Save UF for the wet and underground portions of a circuit only.
NEC Depth Requirements
NEC Table 300.5 sets specific burial depths based on cable type, voltage, and protection. For residential UF cable carrying 120-volt branch circuits:
- 24 inches — direct burial without conduit, no GFCI protection
- 18 inches — direct burial with GFCI protection ahead of the cable
- 12 inches — direct burial under a 4-inch concrete slab
- 6 inches — direct burial under a residential driveway with concrete or pavers above
For 240-volt feeder circuits to outbuildings, the depth requirement is 24 inches minimum regardless of GFCI status. Always check your local jurisdiction, because some areas add depth requirements beyond the NEC minimums in frost-heave zones.
Sizing UF Cable Correctly
UF is sold in the same wire gauges as NM-B: 14, 12, 10, 8, and 6 AWG. Wire size depends on the amperage of the circuit and the length of the run. For typical residential outdoor circuits:
- 14/2 UF — 15-amp circuits up to about 50 feet
- 12/2 UF — 20-amp circuits up to about 80 feet
- 10/2 UF — 30-amp circuits up to about 120 feet, or 20-amp circuits past 80 feet
- 8/3 UF — feeder circuits up to about 200 feet
- 6/3 UF — subpanels feeding detached garages and shops
For long runs, voltage drop matters more than ampacity. A 20-amp circuit at 80 feet on 12 gauge drops about 3 percent at full load. Beyond 80 feet, step up a gauge to keep voltage drop under 3 percent on lighting and motor loads.
Installing UF Properly
Dig the trench to the required depth plus 2 inches of clean sand or fine soil below the cable. Lay the cable flat without kinks or sharp bends. The minimum bend radius is 5 times the cable diameter. Cover the cable with 4 inches of sand or fine soil before backfilling with native soil, and place a continuous run of yellow Electrical Marking Tape 12 inches above the cable to warn future digging.
Where UF enters or exits the ground, run it inside a 1/2 or 3/4 inch PVC conduit sweep with a 90-degree elbow. The conduit protects the cable from physical damage and UV exposure at the most vulnerable transition point. Extend the conduit at least 12 inches below grade and to the inside of the receptacle box or panel.
GFCI and AFCI Requirements
NEC 210.8 requires GFCI protection on all 125-volt outdoor receptacles. The protection can be a GFCI breaker at the panel or a GFCI receptacle at the first outlet. For most outdoor runs, a GFCI breaker is the cleaner solution because all downstream outlets are automatically protected. A Square D Homeline HOM120GFI 20-amp GFCI breaker runs about $50 at Home Depot.
AFCI protection is also required for any branch circuit serving a 120-volt dwelling-unit receptacle, which includes outdoor outlets on the home or attached structures. Combination AFCI/GFCI breakers are the practical solution.
Connections and Splices
Splice UF cable only inside an approved weatherproof junction box. Direct-buried splices are allowed only when made with listed direct-burial splice kits like the 3M Scotchlok DBR/Y-6 or the King Innovation DryConn series. These submersible-rated kits include silicone gel that seals the connection against moisture for the life of the cable.
Never splice UF cable mid-trench without one of these kits. A standard wire nut buried in dirt corrodes within a year and fails dangerously.
Cost and Where to Buy
UF cable runs roughly 30 to 50 percent more per foot than NM-B in the same gauge. As of recent pricing, 12/2 UF with ground is about $1.30 to $1.80 per foot, versus $0.80 to $1.10 for 12/2 NM-B. Home Depot, Lowe’s, and Menards all stock common gauges in 25, 50, 100, and 250-foot rolls. For longer runs or unusual sizes, electrical wholesalers and Amazon offer better per-foot pricing on full reels.
When to Hire a Pro
Running UF cable is well within DIY scope for short outdoor circuits with a single GFCI outlet. For subpanel feeders to detached garages, 240-volt equipment circuits, or runs over 100 feet, hire a licensed electrician. Most jurisdictions require a permit for any new outdoor circuit, and the inspector will verify trench depth before backfill. Skipping the inspection means digging the trench up again later.
