UF-B Wire: The Definitive Guide to Direct Burial Underground Cable
If you are running electrical power to a detached garage, workshop, shed, pool equipment, landscape lighting, irrigation controller, or outdoor outlet that requires underground cable, UF-B is the wire the NEC specifies for direct burial without conduit. Using the wrong cable underground is one of the most common and most dangerous DIY electrical mistakes. This guide covers everything you need to know about UF-B cable including what makes it different from Romex, where it is required, how deep to bury it, and how to size it correctly.
What Makes UF-B Different from NM-B (Romex)
The critical difference is in the jacket construction. Standard NM-B (Romex) has individual conductors wrapped in paper or plastic separators inside a thin outer thermoplastic jacket. The jacket provides no meaningful moisture protection. If buried underground, water infiltrates the jacket within weeks to months, causing insulation breakdown, ground faults, and fire hazard.
UF-B is constructed completely differently. The conductors are embedded directly into a solid PVC plastic compound that fills every void around and between the conductors. There is no air space, no paper wrapping, and no separate inner sheathing. The result is a cable that is inherently waterproof throughout its cross-section. Water cannot infiltrate UF-B the way it penetrates NM-B because there is no space for water to enter. This construction also gives UF-B excellent resistance to soil acids, sunlight (UV-rated for direct exposure), and physical compression from soil load.
Where UF-B Is Required
Direct Earth Burial
Any circuit buried directly in the earth without conduit must use UF-B or another listed direct-burial cable. This includes feeders to detached garages, workshops, pool houses, and other outbuildings, circuits for outdoor outlet boxes installed on posts or stakes, landscape lighting circuits run underground, irrigation controller power circuits, and outdoor HVAC disconnect sub-feeds where the underground portion is direct burial.
Wet and Damp Locations
UF-B is also appropriate for above-ground installations in wet or damp locations where NM-B is prohibited. Examples include crawl spaces with moisture, outdoor surface-mounted conduit that may collect condensation, and garage and outbuilding wiring in unheated structures with high humidity.
Generator Connections
The underground portion of a generator fuel line trench often includes an electrical conduit run for the low-voltage control wiring between the generator and the automatic transfer switch. While conduit is typically used for these generator electrical runs, the feeder from the main panel to an outdoor subpanel near the generator is frequently run in UF-B direct burial to avoid the cost of trenching separate conduit.
NEC Burial Depth Requirements
The NEC specifies minimum burial depths for UF-B cable based on the installation conditions. Direct burial of UF-B without conduit requires a minimum depth of 24 inches in general residential applications. When run under a concrete slab that is not subject to vehicle traffic, the minimum is 12 inches. When installed in conduit (which is not required but sometimes done for additional protection), the minimum depth drops to 12 inches. When run under a driveway subject to vehicle traffic, conduit is required and the depth must be at least 24 inches.
Many local jurisdictions require depths greater than the NEC minimums. Always confirm local requirements with your building department before digging. Marking your burial route with a line locator or flags before digging helps avoid existing utilities.
Sizing UF-B for Common Applications
UF-B is available in the same gauge and conductor configurations as NM-B. 14/2 UF-B (15-amp circuits) works for small shed circuits with lighting and a few outlets. 12/2 UF-B (20-amp circuits) is the most common size for standard outdoor outlet circuits, irrigation controllers, and small outbuilding feeds. 10/2 UF-B (30-amp circuits) covers larger outbuilding feeds and single-appliance dedicated circuits. For larger outbuildings requiring 60 amps or more, a subpanel fed with larger conductors (8 AWG, 6 AWG, or larger) in conduit is the standard approach rather than large-gauge UF-B.
When sizing UF-B for a long underground run, calculate voltage drop in addition to ampacity. Underground runs of 100 feet or more may need to be upsized one gauge to maintain acceptable voltage at the far end. A 12/2 circuit carrying 20 amps over 150 feet of UF-B experiences approximately 5% voltage drop, which is at the upper limit of acceptable. Upsizing to 10/2 for that same run reduces voltage drop to approximately 3%.
Can UF-B Be Used Indoors?
Yes. UF-B is listed for indoor use as well as outdoor and underground applications. It can be used anywhere NM-B is used indoors, with the additional capability of being used outdoors and underground. The tradeoff is cost: UF-B costs approximately 20% to 40% more per foot than equivalent NM-B, so using it throughout an indoor installation is more expensive than necessary. Most projects use NM-B indoors and UF-B for the outdoor and underground portions.
IB Lighting carries 12/2 UF-B and 14/2 UF-B from Southwire in standard spool sizes with free freight insurance on every order. Browse our electrical wire and cable category or call (800) 674-9019 for contractor pricing and custom length orders. Visit the Southwire brand page for the complete UF-B lineup.