Wiring a converted loft

Extending existing electrical wiring into the roof space to allow extra lighting points and socket outlets is generally similar to wiring extensions in other parts of the house.

Switches, ceiling roses, socket outlets and other wiring accessories can be similar to those used in other parts of the house.

They will usually be made of moulded plastic and you can locate them in positions of your own choice.

There are no special regulations for loft conversion schemes in IEE Wiring Regulations, but the whole conversion must conform to them as in other parts of the house. Where the scheme includes a bathroom, special electrical precautions must be taken; these are the same as for a conventionally situated bathroom.

Lighting wiring restrictions

It is usually a simple job to connect new wiring to the lighting circuit supplying the upper floor of a two-storey house, but you must consider the restrictions in terms of the electrical load. Before running new cables, check how many lights are already supplied from the circuit. The normal 5amp lighting circuit has a maximum load of 1200 watts: this figure is obtained by multiplying the current rating (5amps) by its voltage (240 volts). The figure does not mean 1200 watts of lighting is actually connected to the circuit at any one time; in practice it is somewhat less.

Assessing load

The regulations stipulate you treat each lamp holder, containing a lamp of 100 watts or smaller. As 100 watts for the assessment. Lamp-holders containing lamps larger than 100 watts are assessed at their actual wattage; most lamps used on a domestic lighting circuit are 100 watts or less, including common fluorescent tubes (fluorescent tubes are rated above the stated wattage. So check on this when buying them). A 150 watt lamp is equivalent to one-and-a-half lampholders and a 200 watt lamp is equivalent to two. Count the number of lampholders on the circuit, add the number you wish to include in the loft conversion and adjust your figures for any 150 or 200 watt lamps. If the final figure is no more than 1200 you can add the chosen number of new lights.

If there are two lighting circuits, use the most convenient: in a bungalow for example. The cables will already be in the roof space. Running across and between the joists.

Socket wiring restrictions

You can normally add extra sockets to a ring circuit, but you will again have to assess the number already in existence. The cables forming the ring circuit on the floor below the loft conversion will normally be under the floorboards. It is rare to find a ring circuit cable which has already been run into the roof space. Except possibly to supply a heat/light unit mounted on a bathroom ceiling. Such a cable is most likely to be a spur; as such, you should not use it to supply any other outlets apart from the one appliance it serves.

Assessing load

Before you run new cables from an existing ring circuit you will have to check on the location of existing socket outlets to ascertain the floor area already served. A domestic ring circuit can supply an unlimited number of 13amp socket outlets and fixed appliances supplied from fused connection units; the latter should have an individual rating of not more than 3000 watts, but an immersion heater or water heater should preferably be wired on a separate radial circuit. Any one ring circuit, however, is limited to supply an area of not more than 100sq m (or 1000sq ft). Before you extend the ring circuit, measure the area of the rooms which have socket outlets run from the circuit and add the area of the proposed loft conversion; if the total is below 100sq m. you can extend this circuit into the loft conversion.

Most three or four-bedroomed houses have two ring circuits; one supplies outlets on the ground floor and the other supplies those on the upper floor. In this case it is a simple matter to measure the area of each; in most cases it will be no more than about 50sq m (or 500sq ft) so you can add the loft space and still not exceed 100sq m (or 1000sq ft). If there is only one ring circuit, you will almost certainly need a new circuit for the loft conversion.

Check which sockets are on which circuit; if you have two ring circuits, remove each of the relevant 30amp circuit fuses in turn and test each socket by plugging in a table lamp. Any fixed appliances. Supplied from fused connection units, have to be checked individually as the circuit fuses are removed.

Extending a ring circuit

Use 2.5sq mm twin core and earth flat PVC-sheathed house wiring cable when extending a circuit. The type of extension will depend upon how many outlets you require. If you only need two single socket outlets or one double socket outlet. With no lighting or fixed appliances included in the power circuit, you only need a single length of cable forming a spur. This is connected to the ring circuit at an existing ring socket outlet or at a junction box inserted into the ring cable.

Extending cable

If the loft space is being converted into a living and/or sleeping area, you will need more than two single socket outlets; you may also need one or more fixed lights as recommended and possibly one or more fixed appliances.

When extending the ring circuit for a full-scale conversion such as this, you will need to extend the ring cable itself. This is done either by opening it up at a socket outlet on the floor below or by cutting cable under the floorboards and running two cables from the break; choose the most convenient method. The two cables resulting from this operation will be the outward and return legs of your ring cable extension. One cable goes to the first new outlet and the other to the last: the remaining outlets are linked together by cable in the form of a ring between these two cables, which are connected into your existing ring circuit at the brea k in the cable on the floor below. Use two single sockets mounted on a dual box to restore the final ring if connecting in at a socket outlet or two 30amp junction boxes if cutting into the cable.

Running cable

The two cables of a ring circuit extension (or the single spur cable if you are using this method) have to be run up the wall from the floor to the loft space via the ceiling. You could run the cables up through a cupboard, but not against a hot water cylinder since the cable will only withstand a maximum temperature of 65°C (150°F). Since structural work will be necessary during the conversion, you should be able to cut a chase into the wall to bury the cables. Make sure you fit all your circuits before decorating.

Different methods

If the existing ring circuit cannot be extended into the loft space because of the area calculation, there are three other possible methods of providing a power circuit for the conversion; you can provide a new ring circuit, a 30amp radial power circuit or install a new one or two-way consumer unit in the loft space. The last method is particularly suitable if the conversion is to be a self-contained flat.

New ring circuit

You can fit any number of socket outlets and fixed appliances on a ring circuit. To install it, run two 2.5sq mm twin core and earth PVC-sheathed cables up into the roof space and connect them to a spare 30amp fuseway in the consumer unit. If there is no spare fuseway then you will have to fit a one-way consumer unit or switched fuse unit beside the main consumer unit; this should be connected to the meter by the electricity company. Run the ring cables from this unit, using 2.5sq mm twin core and earth PVC-sheathed cable, and protect the circuit with a 30amp fuse. You should provide two lengths of 16sq mm single core PVC-sheathed cable to connect the switched fuse unit to the meter.

Radial power circuit

This will only permit six outlets, made up of socket outlets and fixed appliances. One outlet could comprise a fused connection unit supplying the lighting for the loft conversion. This method will also require its own 30amp fuseway in the consumer unit, but in this case the cable size depends on the type of fuse used. With a miniature circuit breaker (MCB) or a cartridge fuse, 4.0sq mm twin core and earth PVC-sheathed cable is used; but with a rewirable fuse, 6.0sq mm cable must be used. The latter is extremely difficult to work with and this method is best avoided. The same size cable is used to link all the outlets on the circuit.

If there is no spare fuseway in the consumer unit. You will have to fit a switched fuse unit as before; buy an MCB or cartridge fuse type and use 4.0sq mm cable.

Separate consumer unit

Install a two-way consumer unit in the loft space and connect it with 4.0sq mm twin core and earth The main consumer unit. The switched fuse unit should be fitted with a 30amp fuse; the two-way unit should have a 5amp fuseway for the lighting circuit and a 30amp fuseway for the ring main in the loft. Use 1.0sq mm twin core and earth PVC-sheathed cable for the lighting circuit and 2.5sq mm cable of the same type for the ring main.

Extending a lighting circuit

Use 1.0sq mm twin core and earth PVC-sheathed cable. It is normally only necessary to insert a junction box into the cable running between the joists in the roof space. Alternatively you could connect the new cable to a loop-in ceiling rose mounted on the ceiling immediately below the roof space; pass the cable through the existing hole in the ceiling after you have moved the ceiling rose.

However, since the construction of the loft conversion will probably require re-routing of existing lighting cables and possible ‘partial rewiring, make provision to connect the PVC-sheathed cable to an MCB or cartridge type switched fuse unit next to roof space lighting wiring at the appropriate stage.

Using power circuit

Although technically correct. It is not always the best installation practice to have all the new lights on the same lighting circuit. If the lights immediately below, including those serving the landing or hall, are on the same circuit and the fuse blows, the exit route and the loft conversion itself will be in darkness. Check whether the hall and landing lights are on the same circuit as the bedroom lights by withdrawing the bedroom lighting circuit fuse and trying the landing and hall lights. If these are on a different circuit to the bedrooms. Connect into the bedroom circuit; if they are on the same circuit, it is a good idea to supply one or more of the fixed lighting points from the power circuit in the loft conversion.

Connect these lights to the power circuit via a fused connection unit containing a 3amp fuse. If you supply only one light in this way. Its location will depend upon the nature and layout of your loft conversion scheme; the light should be near the entrance. You could fit another light on the stairway outside the entrance to the loft area. This should be controlled by two-way switching. With one of the two switches fitted at the bottom of the stairs. If your conversion warrants only one light of this kind, the fused connection unit can be a switched version so you can use it to control the light. If you require two-way switching in this case. Fit a special switched fused connection unit which has a two-way switch instead of the usual one-way switch.

The above considerations are obviously unnecessary if you are installing a separate two-way consumer unit in the loft since the lighting circuit will be independent of those below.

Wiring alterations

Existing wiring in the loft space will usually be lighting circuit wiring; although it is unlikely to be associated with your new wiring, it is almost sure to be disturbed as the structural alterations are made. This will probably mean all cables are re-routed: where they cross joists they should be threaded through holes drilled in the joists at the regulation (minimum) depth of 50mm (2in) below the tops of the joists.

If all the existing wiring is in metal conduit, it can be difficult and often impossible to move the conduit: in some cases you may be able to let the conduit into notches cut into the tops of the joists. Such conduit installations in the home are generally old and will require rewiring and the fitting of outlet boxes at lighting points. The better and less costly alternative is to replace the conduit with a new PVC-sheathed cable installation. This is still more desirable since earthing is often ineffective or non-existent in an old conduit installation. It-conduit drops to switches are buried in plaster, you can leave them and run the new cables down the conduits to the switches.

If your wiring is old, the sheathing will be tough rubber or lead; in either case the insulation is likely to have perished and you should renew the installation. It is therefore important to have the present wiring surveyed before you start the conversion, even if rewiring or re-routing is not required. Make sure you make any wiring additions or changes needed in the bedrooms below your conversion before the conversion is finished; once it is completed, the changes will become difficult and costly.

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