Lighting circuits

Lighting circuits

There is nothing to prevent lights being taken from the ring circuit via a fused connection unit – and this is often a practical method for adding new lights. But it’s normal and much better practice to have the lighting circuits completely separate from the socket circuits. This makes the most economical use of cable and simplifies the circuit plans. Lighting circuits are a little more complicated than socket circuits, because the light­ing points are normally controlled by a separate wall switch and so extra connections are required.

There are two methods of wiring lights – by the junction box system or by the loop-in system.

With the junction box system, the circuit cable is taken to a series of junction boxes sited between ceiling joists close to where a cable is run to the switch. A further cable runs out to the ceiling rose.

In the loop-in system, which is more widely used, the junction box and the ceiling rose are combined. This method may use more cable than the junction box system – because ceiling roses are further apart than junction boxes – but it reduces the number of connections needed.

With the junction box system the circuit cable is taken from box to box and from each box cables are run to the switch and the lighting point.


Cable and fuses

Lighting circuits are normally run in 1mm2 cable and protected by a 5A fuse. The maximum number of light­ing points on such a circuit is usually ten. This allows for 100W light bulbs at each lighting point with 200W to spare. A circuit in 1 0mm2 cable can be protected by a IOA fuse and can then supply more lighting points, but the length of 1 0mm2 cable from the I OA fuse to the furthest lamp must not be more than 15m. For larger circuits you can use 15mm2 cable, but it’s usually better to install two small circuits instead.

Extra lighting points can be added to an existing circuit by connecting cable (of the same size as the existing circuit), usually via a junction box, provided the total number of lights on the circuit does not then exceed the total loading.

Two-way switching

Lights are often controlled by more than one switch. This is achieved by connecting further switches in the supply cable. Two-way switching is common and most switch plates are now supplied with the necessary extra terminal (if two-way switching isn’t needed this terminal simply isn’t

With the loop-in system the circuit cable is taken (or looped) from rose to rose and the switch cable is run from the rose to the wall switch.

used). The theory behind two-way switching is illustrated below – the live supply goes to the common terminals on the switches – the re­maining terminals LI and L2 are connected by strapping wires.

In practice the wiring is rather more complicated as the switch cable from the lighting point goes only to the first switch and three-core and earth sheathed cable is used to connect the two switches. The method is des­cribed on page 176.

For switching from more than two points, special intermediate switches (available as single switches only) are installed between the two-way switches, but this is rarely necessary for domestic installations.

Getting down to work

Much of the work in installing house wiring is structural, lifting floor­boards, drilling holes in Joists, cutting into walls -. for cable routes and for mounting boxes – and whoever does the work must be prepared for the upheaval.

Cable routes

In many homes it will be possible to run cable under the floor. Where there is a solid ground floor, the cable has to run in the plasterwork, in hollow metal or plastic skirting (to replace the old skirting) or along the existing skirting protected by plastic conduit. Cables up and down walls are usually recessed into the plaster and covered up.

All cables should run in straight lines vertically up and down – it’s then easier to work out where a cable might be hidden in a wall or under a floor.


Under the floor In floor spaces, there must be sufficient slack in the cable to make sure it is not strained. Under a suspended ground floor the cable should ideally be clipped to the joists. In the space under upper floors, cable that runs parallel to the joists should rest on the ceiling of the lower floor; cable that runs at right angles to the joists should loop from one joist to the next. Where cable has to pass through joists the holes should be drilled at least 50mm down from the top surface. This avoids the risk that nails will go through the cable when the floorboards are put back and does relatively little harm to the strength of the joists. However, if well-placed holes are not possible, cables can be run in notches cut in the joists, protected by conduit or sheets of metal nailed over each notch.

In walls Cutting plaster is not diffi­cult. Using a sharp knife score two lines 25mm apart along the proposed route. Use a club hammer and a brick bolster to chip out enough plaster to allow the cable to fit comfortably. (Another method is to use a plaster router bit fitted to the electric drill. It must be run at a slow speed and first you should cut a series of holes along the cable route.)

Fixing accessories

Flush fitting metal or plastic boxes are the neatest way of mounting ac­cessories to a wall but surface- mounting boxes are also available. To cut the hole use a masonry drill, marked to the correct depth, to cut a series of holes along the sides of the marked square and at intervals over the whole area. With a bolster chisel cut a hole about 8mm bigger all round than the box.

Check that the mounting box fits; knock out the required holes for access for the cable and fit a grommet (a rubber ring) into each hole to protect the cable. Drill and plug the holes for the box mounting screws. Check the box sits level. Feed the cable into the box, fix the cable in the wall with cable grips and replaster.

Cable through joists should pass through a hole 50mm down from the top, but this can be difficult.

Fixing a wall-mounted box is easier as you need channel out only enough of the wall for the cable. The plastic box is then screwed to the surface using wall plugs.

Hollow internal walls covered in plasterboard need more care. Cable access is easy in the hollow space (use flexible wire to pull cable through), but some scrap timber may be needed to form a mounting surface behind the plasterboard to take the box. The easiest method is to locate one of the timber uprights and mount the box against it – though it may be necessary to chisel away some wood to accommodate the box.

If you think ahead, mounting blocks can be incorporated as the plasterboard is fixed.

Making connections

Before connections can be made, the sheath and insulation of the cable have to be stripped back. The sheath is removed only from the length of cable which is within the steel box or the accessory itself: carefully slit the plas­tic lengthways with a sharp knife, keeping the cut away from the live and neutral conductors. Peel the sheath back and cut off the waste.

Insulation is best removed with special strippers. The amount to remove will depend on the type of terminal being dealt with, aim to leave virtually no bare conductor exposed once the connection has been made,

The alternative is to cut a notch and protect the cable from damage by a metal plate (or conduit)

but allow plenty of conductor for the terminal screw to grip. Where there is only one thin conductor to go in a hefty terminal, bend it double. When two or more conductors go in one terminal twist them together with pliers.

When installing any kind of circuit in which the conductors go on from one accessory to another, it is better to remove the sheath and insulation without breaking the con­tinuity of the conductors. The con­ductors may have to be squeezed into a tight loop with pliers before they will fit into the terminals. In this way, however bad the connection made at one point, there is still a good, low- resistance path on to the next point.

The earth continuity conductor has no insulation, but wherever the sheath is removed it should be covered in green and yellow sleeving (previously green) which can be bought for the purpose. This prevents accidental contact with the live or neutral termi­nals as well as indicating that it’s the earth.

Before an accessory is screwed in place, check that the terminal screws are tight and that all the conductors are firmly gripped. As the accessory is offered up to its box or pattress, help the conductors to bend into a shape which fits comfortably into the box – they should not be bent too sharply or trapped or pushed up hard against the metal surface of the box.

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