Some appliances need to be wired direct into the domestic supply. These include items such as showers, immersion heaters and cookers.
A shower unit has an electrical loading of either 6kW or 7kW and therefore requires a circuit of 30A current rating, since the current demand is 25 or 29 amps respectively. The cable for this 30A circuit may be 4.0mm2 provided it is supplied from either a cartridge circuit fuse or an MCB. Were the circuit fuse of the rewirable type, a 6.0mm2 cable would be necessary since the current at which it would blow would be more than the current rating of 4.0mm2 cable.
Materials required: A quantity of 4.0mm2 (or 6.0mm2) twin and earth PVC sheathed cable; green/yellow PVC sleeving; one 30A double-pole cord-operated ceiling switch; one 30A main switch and fuse unit preferably fitted with an MCB or a cartridge fuse; two 1 metre-long 6.0mm2 single core PVC insulated and sheathed cables, one red, the other black for connecting to the main switch and fuse unit; about 2m of 6.0mm2 green/yellow PVC insulated cable.
Install the main switch and fuse unit next to the existing consumer unit and connect the mains leads and earth conductor to the mains and earth terminals respectively.
Run the twin and earth PVC sheathed cable to a point above the bathroom ceiling and pass the end through a hole pierced in the ceiling at the position of the switch. Run another length of the cable from the switch position to the shower unit. Connect and fix the ceiling switch in a position normally inaccessible to a person using the shower. Connect the cable to the shower unit.
At the mains end connect the circuit cable to the load or output terminals of the main switch and fuse unit. Fix all cables in position and get the electricity board to connect the mains leads to the meter.
An electric cooker of 12kW or under can be supplied from a 30A circuit, for although this loading is equivalent to 50 amps the regulations take account of the diversity of use of the boiling rings and oven and a kettle plugged into the control unit when assessing the current demand on the circuit. A cooker in excess of 12kW requires a 45A circuit.
Materials required: A quantity of twin and earth PVC sheathed cable (6.0mm2 for a 30A circuit; 10mm2 for a 45A circuit); one cooker control unit with auxiliary 13A socket outlet, or a 50A double-pole cooker switch if an auxiliary socket outlet is not required; mounting box for the above; one cable outlet unit and box if a free-standing cooker; cable clips, wood screws and wall plugs. One main switch and fuse unit.
Decide the position for the cooker control unit which should be about 300mm to one side of the cooker and traditionally about 1.5m above floor level. Knock out two cable entry hole blanks from the mounting box and fit PVC grommets if a metal box. Fix the box to the wall or if a flush type fix it into a chase cut into the wall. Similarly, fix the box of the cable outlet unit about 500mm above floor level.
Install the main switch and fuse unit next to the consumer unit and connect the pair of tails and earth conductor to the unit. From this, run the circuit cable to the cooker control unit following the most satisfactory route. From the cooker control unit, run a length of the same cable down to the cable outlet unit box. Prepare the ends of the cables. Connect the two cables in the control unit box to the respective terminals in the control unit and fix the unit to its box.
Connect the cable outlet unit to the cable running down from the control unit. If the cooker is in position, also connect the cooker trailing cable to the cable outlet unit. Fix the unit to its box. Connect the other end of the circuit cable to the main switch and fuse unit.
A fixed appliance such as a wall heater, panel heater, towel rail, heat/light unit or shaver unit can be supplied from a ring circuit spur where the loading of the individual appliance does not exceed 3120 watts (13 amps). No socket outlets are permitted in bathrooms.
The basic materials required are: a quantity of 2.5mm’ twin and earth PVC sheathed cable; green/yellow PVC sleeving; one 13A fused connection unit and a mounting box; as well as cable clips, wood screws and wall plugs.
The spur cable is connected to the ring circuit at a socket outlet or at a 30A joint box inserted into the ring cable. For wall heaters, towel rails and similar fixed appliances including a high level radiant heater in the bathroom, the spur cable terminates in a switched fused connection unit fixed near the appliance as it cannot be reached from the bath) and connected to it by the appliance flex. In a small bathroom where a fused connection could be reached from the bath, the appliance must be wired as for a heat/light unit.
A heat/light unit requires a separate isolating switch which, in a bathroom, must be a double pole cord-operated switch. For this circuit, fix a non-switched fused connection unit next to the socket outlet to which it is connected and from the unit run 1.5mm’ twin and earth PVC sheathed cable to the ceiling switch. From the switch run a length of the cable to the heat/light unit.
A spur cable feeding a shaver supply unit in the bathroom is run direct into the unit, no fused connection unit being necessary. The cable must be 2.5mm2.
Alternatively, the shaver unit can be supplied from the lighting circuit as should shaver sockets not of the bathroom type, and extractor fans.
Built-in split-level cooker
The one cooker control unit may serve both sections of a split-level cooker provided each is within 2m of the control unit. Otherwise, two units are required, these being linked together. The circuit cable and control unit is exactly as for a free-standing cooker but two cables are run from the control unit, one to each section of the cooker, and connected direct to the terminals of the section.
All cables connecting a cooker and cooker sections must be of the same size as the circuit cable, unlike other appliances which may be connected with flex.
An immersion heater installed in a hot-water storage cylinder has a loading of 3kW (maximum) and requires a 15 or 20A circuit. There are two principal types of top-entry immersion heater. One has a single element which extends to about 75mm from the base of the cylinder. The other type has two elements: a long one of the same length as a single-element heater, the other one about 300mm long.
Both types are supplied from a single circuit and both require a double-pole 20A isolating switch fixed in close proximity to the heater. The two-element immersion heater also requires a changeover switch. This switch, together with the isolating switch, forms a dual switch assembly on the one mounting box.
Other materials required are: a quantity of 2.5mm2 twin and earth PVC sheathed cable; green/yellow PVC sleeving; a length
of 1.5mm2 three-core heat resisting flexible cord for connecting the immersion heater to the isolating switch (two lengths for a two-element heater), cable clips, wood screws and wall plugs.
If there is no spare fuseway in the consumer unit it is necessary to install a main switch and fuse unit. This must be fitted with a fuse or MCB of 15A rating.
Before you start, check there is a boss provided in the cylinder to take an immersion element. Turn off the water supply to the cylinder and then drain it. If the boss is at the top you need only drain a few pints. Remove the blanking cap and screw in the immersion heater.
Run the circuit cable from the fuseway in the consumer unit, or from the switch and fuse unit if there is no spare fuseway, to the position of the isolating switch following the most convenient route for the cable and avoiding hot pipes.
Fix the mounting box of the isolating switch unit to the wall in the airing cupboard as close as possible to the immersion heater terminal head.
Connect the red and the black wires of the circuit cable to the mains L and N terminals respectively of the isolating switch, and the brown and blue wires of the heat-resisting flex to the corresponding load terminals of the switch. Connect the earth conductors to the earth terminal of the box. For a two-element immersion heater, the L load terminal of the isolating switch is connected to the common terminal of the changeover switch using a short length of 2.5mm2 red PVC insulated cable. The brown wire of one flexible cord is connected to the L1 terminal of the changeover switch, the brown wire of the other flex is connected to the L2 terminal. The two blue flex wires are connected to the load N terminal of the isolating switch and all earth wires are connected to the E terminal in the box.
At the immersion heater, the brown wires are connected to the L terminals respectively, the two blue wires to the N terminal and the earth wires to the E terminal.