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  • Writer's pictureTodd Engle

NM Cables

Many people use the name Romex® when referring to type-NM cable. Romex® is a trademarked name that has come into common usage for referring to plastic-covered wires, but type-NM just means "non-metallic" and also applies to other cable styles.

The earliest NM cables were, in fact, rubber-insulated copper conductors bound together as an assembly, with a woven-cloth sheathing. Originally approved by the NEC in 1928 as replacement for knob-and-tube wiring, it became the most common residential wiring used from the late 1940s, up to the introduction of modern thermoplastic (Romex®) type wiring of the early 1960s.

The image above shows some 1950s cloth-covered NM, with clear markings. The legend reads "Napax Type NM 14-2 600V." This is a non-metallic, 14-gauger two-conductor (no ground), rated for a maximum of 600 volts.

Prior to 1985, standard NM was rated for 60-degree applications, which was increased to 90 degrees and is now marked NM-B.

In type NM cable, conductor insulation is color-coded for identification, typically one black, one white, and a bare grounding conductor. The National Electrical Code (NEC) specifies that the black conductor represent the hot conductor, with significant voltage to earth ground; the white conductor represent the identified or neutral conductor, near ground potential; and the bare/green conductor, the safety grounding conductor not normally used to carry circuit current.

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