How to work with domestic electrical cable and flex

The bulk of the work in any electrical project is installing wiring. In some North American localities such work must be undertaken by qualified electricians—though a knowledge of the system is still helpful to the layman. In Britain you can do your own wiring, provided the work is inspected by the supply authority.

Electrical wiring in the home is made up of fixed wiring and flexible wiring. Fixed wiring, housed within the ‘fabric’ of the home, carries electrical current from the consumer unit to fixed outlets such as sockets and lights. Flexible wiring provides the final link from a socket to an appliance, or from a ceiling rose to a lampholder.

Electrical cable

Fixed wiring consists of cable. This is made up of individual wires, or conductors, which carry the current, and insulation to prevent the current from leaking. In new wiring, the live and neutral conductors are usually insulated separately in colour-coded plastic sheaths then laid together with a bare earth wire in a common, outer sheath. In some older installations, each conductor runs separately within its own inner, colour-coded and outer sheaths. In others the live and neutral conductors are housed in the same outer sheath but the earth runs separately.

In Britain, the colour coding for cables is: red for live, black for neutral, and green/yellow striped for earth where this is insulated.

In the US and Canada, the colour coding is: black for live , white for neutral, and green for earth where this is insulated.

Modern cable is insulated with PVC which has an indefinite life and is impervious to damp and most common chemicals. Older installations may have been wired with rubber-insulated cable. This has a life of only about 30 years, after which it may begin to break up and become a potential danger. If your wiring is of this type, do not attempt to extend it.

Fixed wiring runs around the home hidden beneath floorboards and buried in walls. When recessed in solid plaster, the cable is protected by a length of plastic or metal tube called a conduit. Beneath floorboards, the cable can run freely along the sides of, or through holes bored in, the floor joists. When laid beneath floorboards, the cable should be slack, but well supported—either by lying on a ceiling of the floor below, or by being secured in place with cable clips.

Electrical flex

The conductors in flexible wiring, or flex, are made up of a number of thin wires. This enables them to stand up to repeated twisting and coiling, as occurs when the flex is wrapped around an unused appliance. But though flex is strong along its length, it should never be strained at the ends. Never pull a plug out of a socket by the flex or you may strain the terminals, making the plug and flex potentially dangerous.

In Britain, the conductors in three-core flex are colour coded brown for live, blue for neutral, green and yellow for earth. Older appliances may have flex with the same colour coding as cable: red , black and green. In the US and Canada, the live wire is black, the neutral white, and the earth green.

The two-core flex for double-insulated appliances requiring no earth is not always colour coded. The conductors in a table lamp flex, for example, are often insulated in transparent plastic because on non-earthed appliances, it does not matter which conductor connects to which terminal.

There are various thicknesses of flex conductor—appliances of high wattage ratings requiring thicker conductors than those of a lower rating.

If you need to lengthen a flex for any reason, make sure that the new length is of the same type as the existing flex and use a factory-made connector of the correct amp rating to connect the two. Never use insulating tape to join the conductors.

Lighting circuits

Older houses often have junction-box lighting circuits. In this system, the cable runs from the fuse box to junction boxes above the ceiling. Separate cables then run from the junction boxes to ceiling roses and light switches. You should not add a light to a junction-box circuit without seeking expert advice. In the modern loop-in system the wiring is continuous, running from the fuse box to each ceiling rose in turn. The switches are wired directly to the roses. A light controlled by its own switch can be added to a loop-in system by wiring an extension from the circuit cable running into an existing ceiling rose.

Installing an extra light connected to an existing one, and controlled by the same switch, is a straightforward project.

The main items you need are a lamp-holder to take the light bulb and a new ceiling rose. For connecting these two, buy 1mm2 flex. If the lampholder you plan to use is brass, it will have to be earthed, so buy three-core flex. Otherwise buy two-core. For connecting the new ceiling rose to the old one, buy 1mm- twin-with-earth cable, allowing about 0.5m more than the distance across the ceiling between the two lights. Also buy a short length of 2mm green/yellow PVC sleeving to insulate each earth wire.

Preparing to wire

Decide where you want to situate the light and drill a small pilot hole up through the ceiling.

From the floor above, locate the hole and the existing ceiling rose. Unless you are working in the attic, this necessitates the removal of floorboards. Enlarge the hole with a 13mm bit. If the hole has emerged in an awkward position, such as too close to a joist, alter its position slightly—the original hole can be filled in later.

To mount the new ceiling rose base, a piece of 150mm x 25mm blockboard is fitted between the joists over the hole. Measure the distance between one of the joists and the hole and transfer the measurement to the piece of board. Drill a 13mm hole in the board, fit it over the hole in the ceiling and attach it with angle brackets to the joists on either side. You can now fix the rose base to the ceiling below. Make sure that its hole is in line with the hole in the ceiling and screw it through the plaster into the blockboard .

Go back into the ceiling and run a length of cable from the existing light’s ceiling rose to the new rose, allowing a generous amount at either end for cutting and wiring-in. If the cable route is parallel with the joists the cable can lie undipped between them. If it runs at right angles to joists which are carrying floorboards, however, you will have to drill holes through the joists. Drill these at least 50mm from the tops of the joists to prevent damage by nails when the floorboards are replaced.

Next, prepare the cable for connection. First slit it for about 50mm from the end by sliding the blade of a handyman’s knife between the live wire and the bare earth wire. If you angle the blade slightly against the bare wire as you go, you will find you can split the outer insulation sheath without cutting through the red insulation on the live wire itself. With the sheathing removed for about 50mm , strip about 6mm of insulation from the end of each wire with a pair of wire strippers.

Wiring the new ceiling rose

Return to the floor below and prepare both ends of the flex that will run from the ceiling rose to the lampholder. Remove about 75mm of sheathing from the end to be connected to the ceiling rose and 50mm from the end for the lampholder. Bare the ends of the wires. Sleeve the bare earth wire of the new cable and connect the cable and the flex to the ceiling rose. It does not matter which of the ceiling rose terminals you use for which group of wires as long as you group the wires correctly in the same terminal block—the two lives together; the two neutrals together; and both earth wires together if your flex includes an earth wire.

Tighten all the terminal screws and hook the flex wires over the two anchor pieces in the base of the ceiling rose. These are designed to take the weight of the hanging light off the terminal screws. Slip the ceiling rose cover up over the flex and screw it onto the base. Then connect the lampholder to the other end of the flex, again hooking the wires over the anchor pieces provided and making sure that the terminals are fully tightened.

Wiring-in the extension

Before wiring-in the extension, you need to isolate the electric circuit you plan to work on. To do this, turn on the light to which the extension is to be made, and keep pulling fuses out of the consumer unit until the light goes off.. Once you have found and removed the fuse for the circuit you want to work on you can turn the main switch on again. This allows you, if you need light during the wiring process, to plug a standard lamp into a wall socket.

Since both lights are to be operated by the same switch, they are wired-in in parallel—that is, connected to the same terminals in the lighting circuit.

So the first thing to do is to identify the terminals to which the flex of the existing light pendant is connected. These are the terminals to which the live and • neutral wires of the new cable must also be connected.

If the existing pendant flex has coloured conductor wires, wire the red live wire of the new cable to the same terminal as the brown flex wire of the old light pendant. Similarly, wire the black neutral wire of the new cable to the same terminal as the blue flex wire.

If the existing flex is not colour-coded, you cannot tell which side is the live. This does not matter, so long as your new cable is connected to the same terminals as the old flex, and not to any other terminal.

In either case, you may find you have to push one old wire and one new one into the same terminal hole. This again does not matter, so long as both are secure.

Before connecting the bare earth wife of the new cable, sheath it in sleeving.

Make sure that all the wires in each terminal block are securely connected, replace the ceiling rose cover and you are ready to switch on.

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