NM is a type of cable that contains insulated conductors enclosed within an overall nonmetallic jacket. It is commonly known as “Romex®”, which is the brand that is most widely used. NM cable contains two or more insulated wires and a bare ground wire. NM is normally used in one- and two-family dwellings, and certain multifamily dwellings. NM-B is what you will see when shopping for NM cable, which is rated for 90 degrees C (194 degrees F).
There are many different ways to wire a dwelling during initial construction using NM cable. The most common type of NM used are 2-wire and 3-wire.
A 2-wire NM contains one black wire (usually hot) and one white wire (usually neutral) and a bare ground wire. When used in switch wiring, the white wire is sometimes used as a hot wire.
A 3-wire NM contains a black, red, white, and a bare ground wire. The black and red wires can be used for 120 volts A and B phases, or as travelers in 3- or 4-way switch wiring.
14 AWG NM rated for 15 amps
12 AWG NM rated for 20 amps
10 AWG NM rated for 30 amps
A 12-2 NM cable (2-wire) contains 2 insulated 12 AWG current carrying conductors (black and white) and a bare ground wire.
A 14-3 NM cable (3-wire) contains 3 insulated 14 AWG current carrying conductors (black, red, white) and a bare ground wire. The 2 hot wires are different phases (A and B) and share the neutral wire for return current.