NM-B stands for Non-Metallic Sheathed Cable, Type B. It’s the most common type of residential electrical wire in North America, used for running power to outlets, switches, light fixtures, and appliances throughout a home. The “NM” refers to the plastic (non-metallic) outer jacket that bundles and protects the individual wires inside, and the “B” indicates a 90°C temperature rating on the conductor insulation.
What the “B” Actually Means
The “B” suffix has a specific technical origin. Before 1984, standard NM cable used conductor insulation rated for only 60°C. This caused problems as homes began using more thermal insulation in attics and walls. Recessed lighting fixtures in insulated ceilings generated enough heat to degrade the 60°C-rated insulation on nearby wiring, creating a fire risk.
The 1984 National Electrical Code addressed this by requiring NM cable to use conductors insulated at 90°C. Cable meeting this higher standard was labeled “NM-B” to distinguish it from the older type. Today, all NM cable sold is NM-B, so the distinction mostly matters when you encounter older wiring in a pre-1984 home.
There’s an important nuance here: while the insulation itself can handle 90°C, the NEC requires you to size NM-B cable using the 60°C ampacity column. The extra thermal headroom protects the insulation in high-heat situations (like running through blown-in attic insulation near a light fixture) rather than allowing you to push more current through the wire.
How To Read the Label on NM-B Cable
The printed text on the outer jacket tells you everything you need to know about the cable. A typical label might read “14/2 NM-B” or “12-2 w/G.” Here’s how to decode it:
- First number (14, 12, 10): The gauge of the individual conducting wires inside. Lower numbers mean thicker wire that can carry more current.
- Second number (2 or 3): The number of insulated current-carrying conductors. A “2” means two insulated wires (typically one black, one white). A “3” means three insulated wires (black, white, and red).
- “G” or “w/G”: Indicates a bare copper ground wire is also included, which doesn’t count in the conductor number.
So “12/2 NM-B with ground” contains two 12-gauge insulated wires plus a bare ground wire, all wrapped in a non-metallic jacket rated to the B standard.
Jacket Color Tells You the Gauge
The wire industry uses a standardized color-coding system for NM-B outer jackets, which makes it easy to identify what you’re looking at without reading the fine print:
- White jacket: 14-gauge wire, typically used on 15-amp circuits
- Yellow jacket: 12-gauge wire, typically used on 20-amp circuits
- Orange jacket: 10-gauge wire, typically used on 30-amp circuits
This color coding isn’t an NEC requirement, but it’s an industry convention that virtually all major manufacturers follow. If you open a residential electrical panel and see yellow-jacketed cables, you can quickly identify those as 12-gauge, 20-amp branch circuits.
Where NM-B Can and Can’t Be Used
NM-B is rated at 600 volts and approved for both exposed and concealed work, but only in normally dry locations. It’s the standard choice for running circuits through walls, ceilings, and attics in wood-frame residential construction.
It cannot be used in wet or damp locations, buried directly in the ground, or embedded in concrete. It’s also restricted in certain commercial buildings and structures above three stories, depending on the building’s construction type and local code amendments. For outdoor, underground, or wet-location wiring, you’d need UF-B (Underground Feeder) cable instead, which has a waterproof jacket designed to withstand direct soil contact and moisture exposure. UF-B costs more and is harder to bend, so using it indoors where NM-B would suffice is an unnecessary expense.
Using NM-B in locations where it doesn’t belong, particularly outdoors or underground, can lead to moisture damage, short circuits, and fire hazards.
Installation Basics
The NEC has specific requirements for how NM-B cable must be secured. Cable needs to be fastened within 12 inches of every electrical box and supported at least every 4½ feet along its run. The 2020 NEC added a clarification: no more than 18 inches of cable length is allowed between the point where the cable enters a box and the nearest support point. Fasteners can be staples, listed cable ties, straps, or hangers, as long as they don’t damage the cable jacket.
NM-B vs. Other Cable Types
NM-B is sometimes called by the brand name Romex, which is a Southwire trademark that has become a generic term (like calling all adhesive bandages “Band-Aids”). Other cable types you might encounter alongside NM-B include:
- NMC-B: Similar to NM-B but with a jacket that resists corrosion and fungi, approved for some damp (not wet) locations.
- UF-B: Waterproof cable designed for direct burial and outdoor circuits. The conductors are molded directly into a solid plastic sheath rather than wrapped in a loose jacket, which is why it’s stiffer and harder to strip.
- MC cable: Metal-clad cable with an aluminum or steel armor, used where NM-B isn’t permitted due to building construction type or local code requirements.
For the vast majority of single-family residential wiring in the United States and Canada, NM-B is the default. It’s affordable, flexible, easy to work with, and rated for the dry, interior environments where most household circuits run.