Correctly installed, an exterior socket is a very useful facility to have. It means you no longer have to run extension leads from inside the house, with the potential for tripping over them! It enables you to run electric garden equipment such as a lawn mower, hedge trimmer and various other items including power tools, in a far safer way. In fact it is wise to have one at the back and one at the front of the property, which can be used for car washing equipment, power washers and other items such as vacuum cleaners.
Sighting of the exterior socket will be influenced by factors such as the size of your garden or drive, a firm supporting wall or other structure to support the socket and the type of equipment you wish to use, running from it.
There are many different types of exterior socket but with regard to safety, the best type is one that incorporates a residual current device (RCD), which will trip and cut off the electrical supply if any fault occurs, such as a cut cable whilst hedge trimming or mowing. Although more expensive, the socket design, as well as incorporating an RCD, will have a front cover which will encapsulate the 13 amp plug and start of the cable, when the cover is clicked shut. This prevents accidental tugging. Exterior sockets of this type include a housing in plastic, with seals providing a minimum ingress protection (IP) rating of 56, see the DIY Doctor project on IP ratings. The first number given is, in lay terms, affording protection against dust. The second number in similar terms is for protection against the wet. A top class exterior socket of the type described would most likely have an IP rating of 66 or even possibly 68.
Assuming that the main consumer unit does not incorporate an RCD, an exterior socket must be connected with an RCD somewhere in the installation.
You could run cable from the consumer unit through a fused connection unit (FCU) incorporating a 13 amp fuse and then to an RCD. The combined cost of doing this is possibly more than running cable to a dedicated exterior socket, with an integral RCD.
The choice of cable running to the exterior socket should be, as a minimum, 2.5 mm2 twin and earth (TWE) housed in conduit, which could be the mini trunking type or a round design. Another method, especially suitable if the run of cable can in any way be accidentally damaged, by garden equipment including spades, shovels or forks, requires that the cable should be steel wire armoured (SWA). This type of cable is available with at least two main wires (live and neutral), where the steel wires are used as an earth. For the sake of a small extra cost, it is better to purchase 3 core cable, which includes a separate earth lead.
SWA cable is connected and secured to the exterior socket box by glands. The glands are commonly brass, with a retaining washer and a securing nut. The gland is tapered to allow for the steel wires of the cable to be secured to the gland via a brass nut. A 3 core cable of the 2.5mm2 sort would be 18mm outside diameter (O/D), so you would then need to use 20mm connecting glands. The exterior socket boxes usually have round knock out openings. Having chosen where the connection will go to - side, top or bottom - the process is to tap out the appropriate plug and connect the gland.
Glands from electrical suppliers are usually sold in pairs, one for each end of the cable and also come with a weather-proof sheath, a tapering conical item that is cut to suit the cable size and then the wire fed through this hole. Slide the sheath further down the cable out of the way temporarily, then slide on the external nut which will secure the cable to the gland. Connect the male threaded part of the gland into the knocked out hole in the exterior box. Fit the retaining washer, from the inside and secure the gland to the box with the brass securing nut. Tighten to secure without over stressing the connection.
The next stage is to calculate the length of the inner insulated wires which will fit to the terminals within the box. A minimum of 250mm (10 inches) is required. Next cut an appropriate amount of the outer covering of the cable, using a trimming knife. Peel away the outer covering, which will expose the many strands of steel wires. Beneath these steel wires is the inner cable, which has a similar protective covering. Using a pair of good quality side cutter pliers, or nippers as they are sometimes called, from the point where the outer covering has been cut, judge 20mm (3/4 inch) and cut each strand of the outer steel wires. The inner exposed wire will be around 250mm long at this stage.
From the point where the cut steel wires are, measure a point 50mm along the inner wires which has the protective covering. With great care, so as not to cut into the insulated current carrying wires within, remove the outer covering to expose the insulated wires. Feed the insulated wires through the gland into the box. The conical end of the gland should now meet with the steel wires which are approx 20mm long. Hold the wires with one hand from inside the box, slide along the brass exterior retaining nut to fit over the steel wires, to the treaded part of the gland exposed and tighten up, hand tight at this stage. The 3 inner current carrying wires should be connected to the terminals, live, neutral and earth of the socket, where indicated by the manufacturer.
Ensuring the inner wires are not kinked, secure the front part of the socket box to the wall mounting and using the retaining screws, fix positively home. Using a spanner or crescent wrench of the appropriate size, tighten the exterior gland nut and slide the protective sheath over the gland to provide a good seal against the elements. Make sure that all the armoured cable is kept securely in place with cable clips.
Steel wire armoured cable or twin and earth cable run in conduit or trunking should be secured to the wall or fence in such as way as to prevent damage.
Plastic conduit is usually fixed with saddle clips, SWA cable can be fixed with special cable clips which are fitted with screws and rawlplugs. The run to the consumer unit inside the house should be as short as possible, bearing in mind that out of reach is the best policy. Make the cable run as high as practicable and try to avoid sharp bends where possible. It is unlikely that you will be able to fix steel wired armoured cable gland directly to the consumer unit housing. It is good practice to run the exterior cable and/or conduit to an adaptable box usually sighted next to the consumer unit with screws and plugs and using a gland to secure the cable or conduit to it.
Building regulation approval is required to any work within the grounds of dwellings or buildings sharing their supply with dwellings. This would also include a pond pump and garden lighting systems. This means that it is illegal to do this work yourself unless you are in a position, as a competent installer, to self certify your work. If you cannot and you do this work yourself you are breaking the law.