A double socket is a pair of sockets constructed in tandem, sometimes one atop the other and sometimes side-by-side. It’s usually installed on a wall but can also be found installed in floors and ceilings. A technological advance over single sockets, double sockets were introduced in the US around the middle of the 20th century, offering users the ability to connect two devices where before only one could be plugged in. This was a tremendous advantage at a time when rooms commonly had limited outlets and dangerous constructions of taps and extension cords were commonplace.
Generally, double sockets offered for use in the US must meet standards established by the National Electrical Manufacturers’ Association® (NEMA®). In modern household use, single sockets are generally restricted to specialty items like the 240-volt outlets used for electric dryers or ranges. Industrial facilities may also have more of these special outlets which are rarely configured in tandem. Double sockets in the US are configured one atop the other; side-by-side configurations are more prevalent in Europe and Asia.
In new construction, double sockets are generally installed in plastic junction boxes that are nailed to wall studs. They’re generally installed from 12 to 18 inches (30.48 – 45.74 cm) above the floor, but there are many cases where they’ll be found at other heights, especially above kitchen and bathroom counters. Electric cable is fed through the back of the box and clamped securely in place, and individual wires are connected to the double socket, which is then secured inside the junction box. Holes are cut in the walls prior to their installation to accommodate the junction boxes. Coverplates with openings for the two sockets are installed to mask the openings in the wall and to protect against accidental contact with exposed energized metal.
Installing double sockets in existing construction is more complicated. A hole must be cut in the wall adjacent to a stud and new cable run behind the wall using a special tool called a snake. Additional holes may be required to facilitate the snaking of cable between the location of the new outlet and the power source. A new junction box is installed in the first hole, after which installation of the double socket proceeds along the same lines as with new construction. Once the socket has been installed and tested, the coverplate can be installed and all damage done to the wall repaired.
In addition to providing additional outlets for connection of more electric devices, double sockets offer some wiring options that give the user some interesting capabilities. The two individual sockets are constructed with strips of metal connecting the two so that they’re part of the same circuit; when one socket is properly wired, both sockets are energized. This is the way most double sockets are configured and used. The strip can be deliberately broken, though, reconfiguring the duplex socket as two independent sockets.
The most common reason for configuring independent sockets in a double socket is to give control of one socket to a wall switch. One socket is connected in the conventional manner to the normal power supply, so that devices plugged into it are activated by their own power switches. The second socket is connected to a wall switch. When the wall switch is turned on, the socket or sockets to which it’s connected are energized.
Switched sockets are often found in larger rooms without overhead lighting. Lamps plugged into switched sockets are turned on and off by activating a single wall switch. When there is more than one switched socket in a room, the convention is to wire them identically for the sake of convenience; that is, the switched sockets all correspond to either the top socket or the bottom socket. As is the case whenever electrical work is done, power should be disconnected — and preferably locked out — at the source until the job is complete. In addition, some jurisdictions may regulate such work and may require permits.