Cables and flex run all round your home — and it’s important to know what each is for to make sure you use the right one for the right job.
Cable and flex are probably two of the most misused words when talking about electricity since, contrary to popular belief, they are not interchangeable. Cable is the main wiring carrying the supply to the many outlets in the home; flex is wiring that, for example, connects a lamp-holder to a ceiling rose or a vacuum cleaner to a socket.
Cables are not normally handled by the domestic consumer, since they have usually been built into the fabric of the house and make up the permanent wiring. Unlike flexes they are seldom moved during their working life; the expensive fine-wire construction of flex is not needed and many cables have single strand conductors running through them.
Cables connect the household electricity supply to flexes through suitable connection boxes such as ceiling roses or socket outlets. There are regulations to ensure cables are supported by clips at specific intervals depending on whether they run vertically or horizontally. PVC sheathed cable, for instance, can be buried in wall plaster without further protection.
Cable should never be laid in grooves cut into joists because of the danger of floorboard nails penetrating the cable. It should pass through holes drilled at least 50mm (2in) below the tops of the joists.
Colour coding has not changed on cables. Red is live, black is neutral and green is earth, although in some types of domestic cable a bare uninsulated wire is the earth. Interior house cable can be single core (one conductor), twin core, twin core and earth, or three core and earth.
If there is a bare earth when cable is attached to a domestic connection box it should be covered with a short length of green PVC sleeving to insulate and identify it.
In some older houses you could well find the cable made up with a number of strands. This cable usually runs in metal conduit which acts as an earth conductor. In modern homes you will also find cable with conductors made of up to seven strands. This is for circuits which carry heavy current, such as for cookers. All modern cable has an outer covering of PVC which is proof against moisture and most common chemicals and acids so it can be safely buried in walls.
Flex — or flexible cord—is a conductor of electricity which can be twisted and bent many times without breaking. It comprises metal conductors, each of which is made up of many strands of fine wire (rather than one thick one) encased in plastic or rubber. The finer the strands, the stronger the flex.
Flexes receive a lot of wear and tear, so it is very important they are connected properly and securely. In the case of a plug, the anchorage points must be secure enough to ensure any strain on the flex is taken by the tough outer covering (usually plastic or rubber) and not the metal conductors and the plug terminals. Similarly, connections to appliances must be through equally secure anchorage points to prevent strain at that end of the flex.
If the flex has to be lengthened at any time, it must be connected to another flex of the same type and joined by a proper connector.
Perhaps the most misunderstood aspect of flexes is the international colour code used for the PVC insulating covering. By law the three core flexes — whether sold separately or with an appliance — must have insulation coloured brown for the live wire, blue for the neutral wire and green-and-yellow bands for the earth. On older appliances you may find the live wire is red, the neutral black and the earth yellow.
There are, surprisingly, no regulations as yet covering two core flexes. You may buy an appliance with, for example, black and white plastic covering and no explanation about which is live and which is neutral. The reason is that here it does not matter which wire is connected to which plug terminal. Table and standard lamps are examples where, in most cases, plain two core flex can be used.
There are many variations in the types of flex and cables, although most have special uses in industry. For the domestic consumer flexes vary little. Cables are more varied; there are, for example, special cables with extra protection for outside lighting and garden use.
Cables and flexes are given a rating based on the area of the conductor’s cross-section and are described by this area when ordered from a supplier. A cable for a ring circuit, for example, is described as 2.5sq mm (or 2.5mm2), representing the cross-sectional area of one of the conductors available for carrying the current. The other conductor carries the same current but in the opposite direction. The earth conductor is slightly smaller than the other conductors in the cable and carries current only when there is a fault.
Cable and flex ratings do vary according to installation conditions, but this is unlikely to affect the domestic consumer. In a very hot situation, such as in an industrial process, a cable could be rated much lower than a similar cable in a domestic location because the rating is based on the rise in temperature that occurs when a conductor carries a given amount of current.