Thermoplastic-sheathed cable

Thermoplastic-sheathed cable (TPS) consists of an outer toughened sheath of polyvinyl chloride (PVC) (the thermoplastic element) covering one or more individual cables which are PVC insulated annealed copper conductors. It is a commonly used type of wiring for residential and light commercial construction in many countries. The flat version of the cable with two insulated conductors and an uninsulated earth conductor all within the outer sheath is referred to as twin and earth. In mainland Europe, a round equivalent is more common.

Each of the current carrying conductors in the "core" is insulated by an individual thermoplastic sheath, coloured to indicate the purpose of the conductor concerned. The Protective Earth conductor may also be covered with Green/Yellow (or Green only) insulation, although, in some countries, this conductor may be left as bare copper. With cables where the current carrying conductors are of a large Cross Sectional Area (CSA) and current carrying capacity, the Protective Earth conductor may be found to be of a smaller CSA, with a lower continuous current carrying capacity. The conductors used may be solid in cross-section or multi-stranded.

The type of thermoplastic, the dimensions of the conductor(s) and the colour of their individual insulation (if any) are specified by the regulatory bodies in the various countries concerned.

Thermoplastic-sheathed cable is more vulnerable to rodent damage and accidental mechanical damage than wiring within electrical conduit or armored cable.

In North America, this type of cable is designated as NM cable. NM means "nonmetallic", referring to the outer sheathing; the conductors are of course still metallic. NM was first Listed and described in the NEC in 1926, but it was invented a few years earlier by Rome Wire Company in 1922 in Rome, NY, and marketed under the trade name “Romex.” The name "Romex" (a trademarked brand of the Southwire Company) is commonly used generically for NM cable from any source.

In modern products, the color of the NM cable sheath (or jacket) indicates either the gauge of the current carrying conductors within it, or special properties of the sheathing itself. Cables found in older installations may not conform with this color coding.

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