Wiring to sockets or lighting points attached to the outside walls of a house can be part of the domestic circuit. Wiring to a point in the garden, greenhouse or separate garage, however, has to be treated as a different installation, with its own main switch and 20amp fuse unit close to the meter. It is now also recommended you fit an ELCB on this circuit.
Because wind and rough weather will cause wear and tear, sockets, switches and cables must be tough, weatherproof and protected from the possibility of accidental damage. Professional help with outside wiring is essential because electricity used outside is potentially more dangerous than inside.
Wiring a porch light, socket or switch onto the outside wall of a house is not difficult as long as you use weatherproof equipment. Sockets are usually in galvanized steel with covers and the switches are plastic. It is best to keep wiring on an outside wall to a minimum; it should be protected in plastic tubing or conduit and the connections, where the cable joins the switch or socket or light fitting, have to be water- and weatherproof. The wiring can be taken as a spur from the ring main, although a porch or outside light can be taken from a lighting point. In this case the cable goes through the wall as close to the light as possible. Study the wiring layout of the house to plan the shortest route from the new position to the existing wiring; this will give a neater installation.
To take a cable from the inside of a house to the outside, drill a hole through the wall using a masonry bit of up to 300mm (1 ft) long if it is a cavity wall. Insert a short length of plastic tube or conduit; this must be angled so the outside end is lower than the inside (to keep out rainwater) and cemented in place. Fit an elbow to the outside end.
With the overhead method of wiring, the PVC twin core and earth cable is supported by a galvanized steel cable called a catenary wire. The power cable is relieved of any stress or strain by being clipped and taped to the catenary wire which is itself suspended from permanent supports not less than 3.5m (lift 6in) above ground or 5.2m (17ft) if above a drive. To get the right height you may have to attach a weather-treated post to the greenhouse or garage and brace it to withstand strong winds. You must fit supporting vine eyes, one into a heavy duty plug on the house wall and the other into the side of the post near the top. The ends of the catenary wire are threaded through the eyes and twisted firmly round the main length of wire.
Since the catenary wire will be under strain for many years, it is vital to have a strong joint at each end. It is also a good idea to have an adjustable eye bolt fitting or a turnbuckle at one end of the catenary wire so it can be stretched tight. The catenary wire must be earthed using single core 6sq mm PVC-insulated earth wire connected to it by a corrosion-resistant screw-type connector and connected to the mains earth point in the house. Cable from the switch fuse should come through the wall using a tube or conduit as already described. The mains cable should be 2.5sq mm or 4sq mm and in one continuous length from the switch fuse to the new switch or socket.
A downward rainwater `drip loop’ of slack is usually left at each end and the supply cable is attached to the catenary wire by using slings or bitumen-impregnated insulating tape; this is turned two or three times between the cable and wire. Non-corrosive buckle-type cable clips are wrapped round the tape for strength.
Running cable underground is the least obtrusive method and is worth the extra trouble and expense – although this is reduced if the cable is laid during landscaping. You will need special cable, armoured PVC-insulated cable being recommended. It has two cores – red and black insulated – and an extruded covering of black PVC over the galvanized wire armour; the wire armour usually serves as the earth conductor. It is necessary to fit a metal screwed compression gland, secured by a lock nut and bush, over the wire armour at each end of the cable; this gland fits the conduit entry hole of a flush metal box, fixed inside the building or house, which is used as a junction box if the entry point of the armoured cable is some distance from the main switch or switch fuse. A terminal block inside the box is used to connect the armoured cable to ordinary twin core and earth PVC-sheathed cable. Alternatively the cable can be run to the switch mounting box. In both cases a short length of 4.0sq mm single core green/yellow PVC-insulated cable is used to connect the switch or terminal block earth terminal to the earth terminal on the box.
Some electricity companies may insist on the use of three core armoured cable, the yellow core being used for the earth and enclosed in green/yellow sleeving for identification.
A more expensive cable is the mineral-insulated, copper-clad type (MICC): this has two wires inside a protective copper tube which also serves as an earth connection. With mineral-insulated cable it is necessary to fit a seal at each end; if the cable runs directly into the switch and fuse unit, choose a seal which has an earth wire termination. A screwed gland can also be fitted with each seal if required. The cable runs from the main switch and fuse unit in the house to the control panel in the building outside, taking as direct a route as possible but avoiding all places likely to be disturbed in the future. The trench must be dug about 500mm (20in) deep and care taken not to damage any water or drainpipes and other cables you encounter. If there is a space below the ground floor of your house, it is easy to have a hole knocked through the wall; but be careful not to interfere with the damp proof course. The cable needs to be protected at points where it is exposed and securely fixed to the wall using special clips designed for the purpose. Additional protection can be given by galvanized steel channelling screwed to the brick or woodwork.
In an integral or attached garage the cable can run direct to sockets and light fittings; but ideally the cable should terminate in a control panel with a main switch. In the damper atmosphere of a greenhouse or garden shed, a control panel is strongly advisable for safety; switched points and socket outlets can then be connected to the switch. Permanent switches and fused connection units with red neon indicators are preferable, since most of the equipment will be permanently connected. With a plug and socket there is always a risk of damp working its way between the face of the plug and the socket surface, resulting in a current leakage. Provide at least one socket for connecting portable aids used in the garden and two for use in a garage.
Fused connection units, switches and sockets should be installed 1200mm (4ft) from the floor and wired with twin core and earth PVC-sheathed 2.5sq mm cable. The cable is taken from each in turn back to the outlet on the main switch. Fused connection units and sockets should be mounted on metal mounting boxes inset into a timber board, or on moulded plastic surface-mounted boxes, with cable holes drilled through the board.
In a greenhouse fit a strong frame to the back of the board to protect the cable and leave access for the mains cable. Once the wiring is complete, fit a back cover of weatherproof plywood and mount the board at chest level on a strong support. The wiring to the electrical equipment and heaters can be by PVC-sheathed three core flex secured at intervals to the greenhouse; there should be no trailing or loose cable or flex. With aluminium greenhouses you may have to drill small holes in the appropriate positions to allow buckle clips to be fixed with screws and nuts. In this case the PVC-sheathed cable will be in contact with metal and the greenhouse must be earthed by bolting an earth clamp to the frame and connecting a 6sq mm green/yellow PVC-insulated earth cable to this and to the earthing terminal of the main switch.