Two types of wiring are used in the domestic electrical circuits. Fixed cables are normally concealed, and carry electrical current to switches, ceiling lights, and socket outlets. Flexible cords (flexes) connect portable appliances and light fittings to the fixed wiring.
Fixed wiring
This consists mostly of PVC-sheathed cable containing three copper conductors (cores). The core insulated in red PVC is the live and the one in black the ‘neutral’ though in lighting circuits in the cable to the switch both the black and red are live. (The black core is required to have a piece of red sleeving on it to indicate this, although this is often omitted by incompetent electricians. Cables having two red conductors are made for contract work, but rarely stocked and sold retail).
The third core is the earth and this is uninsulated, but when exposed after the sheathing is removed ready for wiring up it must be sleeved in green/yellow striped PVC before being connected to the earth terminal. In some wiring circuits PVC-sheathed cables having one core only are used, for example, where a live core is looped out of a switch or a neutral core is looped out of a light, to supply an additional light or lights. Three-core and earth cable is used for two-way switching. The conductors are insulated in red, blue and yellow; the colour coding is for purposes of identification only.
Cables for fixed wiring
Most domestic wiring is now supplied in metric sizes which refer to the cross-sectional area of one of the conductors, whether it is composed of one or several strands of wire. Most common sizes of cable are 1.0mm2 and 1.5mm2 used for lighting, and 2.5mm2 used for power circuits. Cable with grey sheathing is intended to be concealed in walls or under floors; white sheathing is meant for surface mounting.
Flexible cords
Flexible cords are made in various sizes and current ratings and the types you’ll most often come across are: parallel twin unsheathed, circular PVC-sheathed, circular braided, unkinkable and heat-resisting. Each conductor is made up of a number of strands of copper and it is this which gives the cord its flexibility. The insulation used round the conductors now conforms to an international colour coding standard – brown denotes the live wire, blue the neutral, and green/yellow the earth, when it is part of the flex. Transparent or white insulation is used for a flex that carries a low current and where it doesn’t matter which wire is connected to the live and neutral terminals of an appliance.