NM-B vs. UF-B: Which Wire Do You Need?
NM-B and UF-B are the two most common residential cable types, and choosing between them depends entirely on where the wire will be installed. NM-B is designed exclusively for indoor dry locations. UF-B is designed for outdoor, underground, and wet locations. Using the wrong cable type in the wrong environment violates the National Electrical Code and creates safety hazards. Here is a clear breakdown of each type.
NM-B (Non-Metallic Sheathed Cable)
NM-B, commonly known by the brand name Romex, is the standard residential wiring cable for interior circuits. It consists of insulated copper conductors wrapped in paper and enclosed in a thermoplastic outer jacket. NM-B is rated for dry locations at temperatures up to 90 degrees Celsius. It is used for virtually all interior wiring: bedroom outlets, kitchen circuits, lighting, switches, and panel connections within walls, ceilings, and floors. NM-B is color-coded by gauge. White jacket is 14 AWG (15-amp circuits). Yellow jacket is 12 AWG (20-amp circuits). Orange jacket is 10 AWG (30-amp circuits). Black jacket is 8 AWG or 6 AWG (40 to 60-amp circuits). IB Lighting stocks Southwire NM-B in all standard gauges and spool sizes at our electrical wire collection.
UF-B (Underground Feeder Cable)
UF-B is designed for direct burial in the ground without conduit, outdoor installations, and wet or damp locations. The key physical difference is construction: UF-B conductors are molded directly into the thermoplastic sheath with no air space between them, creating a solid, moisture-resistant cable that resists water penetration even when buried in soil. UF-B is used for outdoor lighting circuits buried in the yard, landscape lighting feeds, circuits to detached garages, sheds, or outbuildings when buried directly, circuits in damp basements or crawl spaces (where NM-B is not permitted), and any application where the cable may be exposed to moisture. NEC burial depth for UF-B is 24 inches minimum when not in conduit and 12 inches when run through rigid conduit. Browse Southwire UF-B cable at our electrical wire collection.
Key Differences
The construction differs fundamentally. NM-B has loose conductors inside a jacket with paper wrap and air space. UF-B has conductors embedded in solid thermoplastic with no air gaps. For moisture rating, NM-B is dry locations only. UF-B is rated for wet, damp, and direct burial. For flexibility, NM-B is easier to strip, bend, and pull through framing because the conductors move freely inside the jacket. UF-B is stiffer and harder to strip because the sheath is bonded to the conductors. For cost, UF-B costs approximately 20% to 40% more than equivalent gauge NM-B because of the more complex manufacturing process. For code compliance, using NM-B in a wet location or outdoors violates NEC and will fail inspection. UF-B can be used anywhere NM-B can (plus outdoor and underground), but the higher cost and reduced workability make it impractical for standard interior wiring.
Quick Decision Guide
If the wire runs inside interior walls, ceilings, or floors in a dry building, use NM-B. If the wire will be buried underground, installed outdoors, or exposed to moisture, use UF-B. If you are unsure about the moisture conditions, UF-B is always the safer choice since it is approved for both dry and wet locations.
IB Lighting stocks both NM-B and UF-B cable from Southwire in all standard residential gauges. Browse our complete electrical wire and cable collection or call (800) 674-9019 for help selecting the right cable for your project.