Grey: usage for underground installations, designated as "underground feeder" (UF) cables. 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