Thermoplastic-sheathed cable

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A thermoplastic-sheathed cable (TPS) is a durable PVC outer jacket that covers one or more copper conductors, each insulated in PVC. It is commonly used in homes and light commercial buildings. The flat form, called twin and earth, has two insulated current-carrying conductors and an uninsulated earth conductor inside the sheath; circular TPS is also common in many places.

Each current-carrying conductor is insulated with color-coded PVC to show its function. The earth conductor is usually green/yellow striped insulation, though in some countries it may be bare. If the earth conductor is larger than the others, it may have a different size or rating. Conductors can be solid or stranded, with stranded wires often preferred for easier termination in larger sizes. The exact plastic type, conductor sizes, and insulation colors are set by local electrical standards. TPS is more vulnerable to rodent damage and other accidental damage than cables inside conduits or armored cables.

In North America this cable is called NM cable (nonmetallic jacket). “Romex” is a well-known brand name. The jacket color can indicate gauge or other properties, and some older installations don’t follow current color codes. Wires are labeled by a gauge and count, like 12-2 or 12-3: “12-2” means two insulated 12-gauge wires plus a ground; “12-3” means three insulated wires plus a ground. The jacket notes only the insulated conductors; the bare ground isn’t counted. Cables for underground or special uses are often labeled UF (underground feeder) and have grey jackets.

Australia and New Zealand use TPS with outer jackets that are usually white (flat cables) or orange (circular cables). Sizes range from 1 to 6 mm2. Configurations include single, twin, twin and earth (2C+E), three and earth, and four and earth (for three-phase). The earth conductor is green-yellow insulated and is typically stranded; very small sizes (around 1 mm2) have special arrangements.

In the United Kingdom, twin and earth cables typically have a bare CPC (earth) conductor inside the outer sheath, with green-yellow sleeving applied at the ends. Older UK designs used imperial sizes. Some UK and cross-border installations may still differ from newer standards. The outer sheath is usually grey (BS 6004) or white for low-smoke versions (BS 7211).

In the Republic of Ireland, pre-2013 standards allowed both the UK-style twin and earth and a version with an insulated green-yellow CPC. Since 2013, uninsulated CPC options have not been permitted in new installations there.