Understanding NM (Non-Metallic) Sheathed Cable

non metallic sheathing cable

Claire Cohen 

If your house was built in the last 50 years or so, it is likely that most of its wiring system consists of NM (non-metallic) sheathed cable. Sometimes known as "Romex," named for one of the common brand names, NM cable is a flexible electrical cable that features an outer plastic sheathing that protects two or more insulated conductors, as well as a bare copper ground wire.

parts of non metallic cable sheathing
Claire Cohen

 

non metallic white sheathing cable
Claire Cohen

History of NM Cable

NM cable was invented by the Romex company in 1922 and was first described and listed by the NEC (National Electrical Code) in 1926, but it did not come into prevalent use until the early 1960s, when plastics replaced woven rayon as the material used for the outer sheathing. From that point forward, NM cable became the standard for running residential electrical wiring in hidden locations, within walls and in floor and ceiling cavities. Its popularity stems from the fact that NM cable is very easy to use and fairly inexpensive when compared to other methods of running wire, such as metal conduit or armored cable. Today, NM cable is the standard for nearly all applications—other than in exposed locations such as against basement walls or along exposed exterior walls, where conduit is still the standard.

Anatomy of an NM Cable

For most circuit applications, the NM cable commonly used is described as "two-wire" or "three-wire" cable. This designation refers to the number of insulated wire conductors the cable contains. The designation is slightly misleading, however, as both two-wire and three-wire cables also contain an additional bare-copper grounding wire. Thus, two wire cable is often described as "two-wire with ground," while three-wire cable is sold as "three-wire with ground." In two-wire cable, one insulated conductor has black insulation (normally the hot wire), and the other conductor has white insulation (generally serving as the neutral wire). In three-wire cable, there is one white neutral wire, plus a black and a red hot wire.

Packaging for NM cable uses a convenient short-hand that allows you to recognize the characteristics of the cable at a glance:

  • A cable labeled "14/2 W/G" has two 14-gauge conductors plus ground
  • A cable labeled "14/3 W/G" has three 14-gauge conductors plus ground
  • A cable labeled "12/2 W/G" has two 12-gauge conductors plus ground
  • A cable labeled "12/3 W/G" has three 12-gauge conductors plus ground

Along with the bare copper grounding wire, there is also a paper wrapping woven through the interior of the cable, which serves to keep the wires from sticking together and making the cable easier to bend during installation.

The insulated conductors, the bare copper ground wire, and the paper wrapping are all contained within a tough PVC plastic sheathing that is non-conductive and heat resistant.

Types of NM Cable

NM cable is available in different forms, depending on its intended use. The standard NM cable used for interior residential wiring inside walls and floor and ceiling cavities is known as NM-B. This cable is approved for use in dry locations only; you will never see it used in outdoor locations or buried beneath the ground.

At one point, all NM cable used white sheathing, but the NM-B cable sold today has a color-coded outer sheathing to serve as a quick identification for the wire gauge of the cable.

  • Cable with 14-gauge wires has white sheathing (used for 15-amp circuits)
  • Cable with 12-gauge wires has yellow sheathing (used for 20-amp circuits)
  • Cable with 10-gauge wires has orange sheathing (used for 30-amp circuits)

Where cable needs to be run underground, a different type of cable is required. Known as UF-B cable, this underground feeder cable features wire conductors that are embedded in solid plastic rather than a hollow sheathing; the cable color is typically gray. UF-B cable is what is used when you are running underground circuits to a garage or shed, or when you are running power to a yard light or other landscape feature.

Service Entrance Cable

Yet another type of NM cable is used by the utility company to deliver electrical service to your home overhead or through the ground. Type SE cable is used for aboveground utility service, while Type USE is used for underground service wires. Homeowners almost never need to deal with this type of cable, as they are only handled by utility professionals.