NM-B WIRE

Commonly recognized as Romex, NM-B wire is a popular type of non-metallic sheathed cable primarily used for residential electrical wiring. It's engineered for dry indoor environments, typically forming the branch circuits that extend from your home's breaker box to power outlets and light switches. The "B" in its name is significant, indicating a higher temperature rating of 90°C (194°F), which means it can safely carry more electrical current than older NM cable versions.

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NM-B wire, commonly known by the brand name Romex, is a type of non-metallic sheathed cable primarily used for residential electrical wiring. It consists of multiple insulated conductors and a bare ground wire encased in a durable, flame-retardant outer jacket. This wire is designed for dry indoor environments, typically forming the branch circuits that power outlets, switches, and light fixtures in homes.

The "NM" in NM-B stands for Non-Metallic, indicating that the cable's outer sheath is not made of metal. The "B" suffix is significant, denoting that the individual conductor insulation within the cable has a higher temperature rating of 90°C (194°F).

Yes, in everyday practice, when people refer to "Romex," they are almost always referring to NM-B cable. "Romex" is a registered brand name for non-metallic sheathed cable manufactured by Southwire, while NM-B is the generic, official classification for this type of wire.

NM-B wire is primarily used in dry, indoor locations for residential wiring applications. Its common uses include:

  • Branch Circuits: Extending from your home's breaker box to power outlets and light switches.
  • Lighting Fixtures: Wiring for ceiling lights and recessed lighting.
  • Appliances: Connecting various household appliances.
  • Dwellings: Permitted in one- and two-family homes, and their attached or detached garages and storage buildings.

No, NM-B wire is not permitted for outdoor use, underground installations, or in wet or damp locations. It lacks UV protection, is not moisture-resistant, and can degrade, leading to safety hazards. For outdoor or wet applications, cables like UF-B (Underground Feeder) or conductors in approved conduit are required.

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NM-B RESIDENTIAL WIRE

NM-B Cable, commonly referred to by the brand name Romex®, is the most widely used electrical wire for residential interior branch circuits. Engineered for use in dry, protected locations, NM-B features two or more insulated conductors and a bare equipment grounding wire, all encased in a flexible, flame-retardant non-metallic sheath. The "B" rating signifies that the internal conductors are rated for a maximum operating temperature of 90°C (194°F), though the ampacity is limited to that of a 60°C conductor per the National Electrical Code (NEC). Designed for easy pulling through wooden studs and joists, NM-B is the primary solution for wiring home outlets, lighting fixtures, and appliances.

Installation Note: NM-B is strictly for indoor, dry locations and must never be used outdoors or buried underground, as its paper-wrapping will wick moisture and cause insulation failure. When routing through wood studs, ensure the cable is set back at least 1.25 inches from the edge or protected by steel nail plates to prevent puncture damage from drywall screws. Use a dedicated Romex® stripper to avoid nicking the internal insulation, and ensure the cable is secured with approved staples within 12 inches of every box and at intervals no greater than 4.5 feet, taking care not to over-tighten and pinch the jacket.

Key Features
  • Simplified Installation: The flexible PVC outer jacket and integrated pull-string (on many sizes) make it easy to route through wall cavities and floor joists.
  • Industry Standard Color-Coding: Jackets are color-coded by gauge for instant identification: White (14 AWG), Yellow (12 AWG), Orange (10 AWG), and Black (8 AWG and larger).
  • 90°C Rated Conductors: Internal THHN/THWN conductors provide superior heat resistance, ensuring safety within crowded junction boxes.
  • Integrated Grounding: Every cable includes a dedicated bare copper ground wire to ensure a safe path for fault currents.
  • Paper-Wrapped Core: Conductors are wrapped in a moisture-resistant paper separator to prevent the jacket from sticking, allowing for faster stripping.
  • Flame Retardant: The outer sheath is designed to resist the spread of fire and is self-extinguishing.
Common Applications
  • General Purpose Outlets: The standard choice for wiring 15A (14 AWG) and 20A (12 AWG) receptacles in bedrooms, living rooms, and offices.
  • Lighting Circuits: Used for connecting switches to overhead lighting, recessed cans, and ceiling fans.
  • Large Household Appliances: Higher gauge NM-B (10 AWG and 8 AWG) is used to power water heaters, electric clothes dryers, and kitchen ranges.
  • Home Renovations & Additions: The go-to cable for DIYers and professional electricians when adding new circuits to existing residential structures.
  • Smoke Detectors: Providing the interconnected power and signaling required for residential life-safety systems.
  • Sub-Panel Feeders: Larger sizes of NM-B are often used to feed residential sub-panels in detached garages or basements (provided the run is indoors).