A thermoplastic-sheathed cable (TPS) consists of a toughened outer sheath of polyvinyl chloride (PVC) thermoplastic, covering one or more individual annealed copper conductors, themselves insulated with PVC. This type of wiring is commonly used 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. Description 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), the protective earth conductor may be smaller, 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 conductors 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 armoured cable. North America In North America, this type of cable is designated as NM cable. NM means "nonmetallic", referring to the outer sheathing; the conductors are still metallic. NM was first listed and described in the NEC in 1926, but it was invented a few years earlier by the Rome Wire Company in 1922 in Rome, New York, and marketed under the trade name "Romex". Today, the name "Romex" is a trademarked brand of the Southwire Company. 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 colour coding. The following are nominal current ratings for copper conductors; long runs may require thicker wires to minimize voltage drop. White: 14 AWG wire (2.08 mm2) for 15-amp circuits. Yellow: 12 AWG wire (3.31 mm2) for 20-amp circuits. Orange: 10 AWG wire (5.26 mm2) for 30-amp circuits. Black: 6 or 8 AWG wire (13.3 mm2 or 8.37 mm2) for 60- and 45-amp circuits, respectively.