What Is NM Cable & 4 Options For Your Home
October 9, 2020 | Cristina Dinulescu
Acircuit for residential electrical services like GFCI outlets, wall outlets, or lights is made of copper wires. These have to run in parallel, in a tightly packed bundle, without making any contact with each other. Because running every wire would be difficult, the need for a tight bundle, such as the NM cable, appeared. If these wires touched, the circuit itself would not function.
Since electricity has been powering homes, parallel wires have been separated and bundled up in plenty of different ways. However, the non-metallic (NM) sheathed electrical cable is the most common and efficient way to create the bundle.
Most of your home’s wiring system likely consists of NM sheathed cable if your house was built or rewired after 1965. Before the late 80’s there was no ground wire, and most homes had a two wire system. After this time, the Electrical Code required the ground wire, which meant a switch to the three wire system.
The NM cable, referred to as Romex, after one of the most popular brands, is a flexible electrical cable, featuring an outer plastic sheathing, which protects two or more insulated conductors and a bare copper ground wire.
What Is NM Cable?
The non-metallic mention in the name refers to the outer sheathing which packs individual wires like a cable, in contrast with the metallic sheathed cable in which the individual conductors are protected by some form of metal coil or conduit.
In 1922, the Romex company invented the NM cable and in 1926, it was first described and listed by the National Electrical Code (NEC). However, it wasn’t until the 1960s that it started being used. It was then that plastic replaced woven rayon as the material used for the outer sheathing and the NM cable quickly became the standard for running residential electrical wiring within walls and in floors or ceiling cavities. Nowadays, the NM cable is used for all applications, except exposed locations, where conduit is still required.
The NM cable has three parts: the wire insulation, outer cable sheathing, and wire.
- the cable sheathing: this is a 30 mil-thick PVC jacket, extremely tough, bundling the individual wire conductors and protecting them. The sheathing needs to withstand the stress inflicted on it when the cable is pulled through holes in studs. A cable ripper, which is a metal device, is used to rip the cable when making connections with devices. When the ripping is over, the remaining attached sheathing is cut off with scissors or a utility knife.
- the wire insulation: the wires in the sheathing are insulated with color-coded PVC (polyvinyl chloride). Individual conductors have black, white and red insulation. Within the NM cable, there is a copper grounding wire, left uncoated and bare. Sometimes, however, this is coated in green PVC and often, paper is added as a separator.
- the wire: the individual electrical conductors within the NM cable are about 65% copper. This cable goes in many wire gauges. However, most household circuits use 12-gauge or 14-gauge wire, with two or three conductors inside (and the bare copper ground wire).
1. Two-Wire & Three-Wire NM Cable
The NM cable is referred to as “two-wire” or “three-wire” cable for most circuit applications. This designation has to do with the number of insulated wire conductors the cable contains, even though it is a bit misleading. Both versions have an extra bare-copper grounding wire.
In terms of insulation, in the two-wire cable, one insulated conductor has black insulation (the hot wire) and the other white insulation (the neutral wire). In the three-wire cable, you will find one white neutral wire, plus a black and a red hot wire.
NM cable packaging makes it easy to recognize its characteristics:
- Two 14-gauge conductors plus ground are labeled “14/2 W/G”.
- Three 14-gauge conductors plus ground are labeled “14/3 W/G”.
- Two 12-gauge conductors plus ground are labeled “12/2 W/G”.
- Three 12-gauge conductors plus ground are labeled “12/3 W/G”.
Through the interior of the cable, along with the bare copper grounding wire, there is paper wrapping which keeps the wires from sticking together. Plus, it makes the cable bend easier during installation.
The paper, the bare copper ground wire, the insulated conductors, they are all contained in a strong PVC plastic sheathing, which is both heat resistant and non-conductive.
2. NM Building Cable
Depending on what it is for, the NM cable comes in different forms. Standard NM cable is suitable for interior residential wiring, within ceiling cavities or walls and it is known as NM-8. This type of cable cannot be used in outdoor locations or buried underground. It is approved for use only in dry areas.
The NM-8 has a color-coded outer sheathing to quickly identify the wire gauge of the cable:
- The 14-gauge wires cable has white sheathing and is used for 15-amp circuits.
- The 12-gauge wires cable has yellow sheathing and is used for 20-amp circuits.
- The 10-gauge wires cable has orange sheathing and is used for 30-amp circuits.
3. Underground Feeder NM Cable
In case the cable needs to run underground, a different type is required. The UF-B cable is the safest option to use underground, as it features wire conductors, enabled in solid plastic. The color of this cable is typically gray and it is used to run underground circuits to a shed or garage or for running power to a landscape feature.
4. Service Entrance Cable (SE)
The SE cable is used by the utility company to deliver service to your home. This is used above ground, while the USE type is used for underground service wires. But you don’t have to worry about this type of cable, as it is only used by utility professionals.