Thermoplastic-sheathed cable

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". The name "Romex" is a trademarked brand of the Southwire Company. Nonetheless, TPS cabling of this type, no matter the brand, at least in the US, is often colloquially called "Romex."
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.
The outer jacket is labeled with letters that show how many insulated wires are concealed within the sheath. This does not include the uninsulated ground wire. For instance, if the cable lists "12-2 AWG", it means there are two insulated 12-gauge wires (a black and a white wire), plus a ground wire. If the label says "12-3", this is a three-conductor, 12-gauge cable with a bare copper ground wire understood to be included. 
Different types of cables are approved for different applications; the cable used for interior wiring in dry locations is different from the types approved for underground burial, direct embedment in concrete, or service entrance use. 
Australia and New Zealand.
In Australia and New Zealand, the colour of the external sheath is usually white for flat TPS or black for circular TPS but several other colours are available. Wire sizes of 1–6 mm2 cross-sectional area (CSA) are available with the outer sheath covering the cores. TPS cable is available in several conductor configurations: single, twin, twin and earth, three and earth, and four and earth, the latter two for three-phase supply. Although available in the larger sizes, solid conductors are rarely used with wire sizes greater than 1 mm2 CSA, since the small increase in manufacturing cost of stranded conductors is far outweighed by the relative ease of working with them, especially at the points of termination.
Unlike in North America, the existence of the earth wire within the sheath is always specified if it is present (e.g. twin cable has two conductors and twin and earth cable has three.) The earth conductor is always stranded (unlike North American usage), with the exception of 1 mm2 cables, and covered with green-yellow striped plastic insulation. In older cables the plastic insulation of the earth conductor is green.
Prior to the introduction of TPS cable, tough rubber sheathed (TRS) cable was used. Because of this, TPS is sometimes referred to as "tough plastic sheathed".
Flat TPS is more common than circular, and is used for fixed wiring of domestic and industrial lighting, power outlets, appliances, and HVAC units. Circular TPS is common in industrial and commercial installations but generally not in domestic installations. It may be more difficult to strip the outer sheathing from circular TPS than from flat TPS.
United Kingdom.
In the UK, thermoplastic-sheathed cable in twin and earth (or T and E) format has the circuit protective conductor (Cearth) uninsulated (bare) and of reduced diameter compared to the main cores. Green and yellow sleeving is sold separately, to be applied at the ends.
The cross section on the main conductors is given first, and then the cross-section of the CPC.
Standard UK metric twin and earth cable sizes
In older properties (pre-1970) cable with imperial sizes are found, sometimes without CPC.
Mainland UK wiring regulations do not at present (BS 7671:AMD3) acknowledge twin and earth or flat TPS with a full sized and insulated (G/Y) earth conductor as a valid cable type, which may be awkward for contractors who work cross-border with the Republic of Ireland.
Also available in smaller conductor sizes are versions containing three current-carrying conductors and a circuit protective (earth) conductor. These configurations are commonly used for applications such as switched light circuits, battery-backed emergency lighting which requires a switched and unswitched supply, extractor fans with a run-on timer which require a switched and unswitched supply, mains-powered interlinked smoke alarms, and central heating thermostats.
There is an overall sheath of grey PVC (BS 6004), or white for low smoke compound (BS 7211), although prior to 2005 white PVC was also available.
Republic of Ireland.
In the Republic of Ireland the situation is different from that in the UK. Prior to 2013 IS 201-4:2001 ( I.S. 201 part 4: PVC and Low Smoke Halogen Free Sheathed cables for fixed wiring) permitted both the UK style of twin and earth, and also a version with a CPC with a cross-section equal to that of the main conductors and insulated in green and yellow inside the full length of the cable. 
However, from 2013, the option for the uninsulated and reduced CPC has been removed from the standard IS 201-4:2013, and as such is no longer permitted in new installations in the Republic of Ireland.