A pattress or pattress box or fitting box (in the United States and Canada, electrical wall switch box, electrical wall outlet box, electrical ceiling box, switch box, outlet box, electrical box, etc.) is the container for the space behind electrical fittings such as power outlet sockets, light switches, or fixed light fixtures. Pattresses may be designed for either surface mounting (with cabling running along the wall surface) or for embedding in the wall or skirting board. Some electricians use the term "pattress box" to describe a surface-mounted box, although simply the term "pattress" suffices. The term "flush box" is used for a mounting box that goes inside the wall, although some use the term "wall box". Boxes for installation within timber or plasterboard walls are usually called "cavity boxes" or "plasterboard boxes". A ceiling-mounted pattress (most often used for light fixtures) is referred to as a "ceiling pattress" or "ceiling box". British English speakers also tend to say "pattress box" instead of just "pattress". Pattress is alternatively spelt patress. The word pattress, despite being attested from the late 19th century, is still rarely found in dictionaries. It is etymologically derived from pateras (Latin for bowls, saucers). The term is not used by electricians in the United States.
Pattresses Pattresses contain devices for input (switches) and output (sockets and fixtures), with transfer managed by junction boxes. A pattress may be made of metal or plastic. In the United Kingdom, surface-mounted boxes in particular are often made from urea-formaldehyde resin or alternatively PVC and usually white. Wall boxes are commonly made of thin galvanised metal. A pattress box is made to standard dimensions and may contain embedded bushings (in standard positions) for the attachment of wiring devices (switches and sockets). Internal pattress boxes themselves do not include the corresponding faceplates, since the devices to be contained in the box specify the required faceplate. External pattress boxes may often include corresponding faceplates, limiting the devices to be contained in the box. Although cables may be joined inside pattress boxes, due simply to their presence at convenient points in the wiring, their main purpose is to accommodate switches and sockets. They allow switches and sockets to be recessed into the wall for a better appearance. Enclosures primarily for joining wires are called junction boxes.
Types of outlet boxes Outlet boxes can be surface or sub-surface mounted. The latter type can be further divided by the type of wall it is intended for: sub-surface outlet boxes are available for mounting in drywall, in brick walls or in concrete walls.
North America In North America, outlet boxes are rectangular in shape and are available in different sizes, to accommodate varying numbers of switches. Boxes for drywall are commonly available in two types: new work and old work. New-work boxes are designed to be installed in a new installation. They are typically designed with nail or screw holes to attach directly to wall studs. Old-work boxes are designed to attach to already-installed wall material (usually drywall). The boxes will almost always have two or more parsellas (from Latin: small wing or part). The parsellas flip out when the box screws are screwed, securing the box to the wall with the help of the four or more tabs on the front of the box.
Europe In most of Europe, outlet boxes are round, with a standard diameter of 68 mm, to accommodate a single insert. This is often a single switch or wall socket, but inserts with two (sometimes even three) switches, a switch and an AC outlet, or two outlets (which protrudes from the wall slightly more than other inserts) are available. The round shape allows the corresponding holes to be simply drilled out with a hole saw rather than requiring a rectangular cavity to be cut out. This is an advantage especially when installing outlet boxes in brick or concrete walls, which are much more common in Europe than in North America. Boxes intended for drywall always have parsellas, similar to the North American old work type (the distinction between old and new work is not used in Europe), as the round shape of the boxes and prevalence of light-gauge steel framing in modern drywall make nailing to a stud impractical. Most outlet boxes can be connected to form a chain, limited in length only by the availability of faceplates for the inserts; depending on the product series, between 3 and 5 inserts can be combined in this manner. Some manufacturers also produce 2-gang, 3-gang or 4-gang boxes. The center distance between two inserts is always 71 mm. Even with those round-hole systems, the faceplates that cover them are mostly rectangular. Surface-mounted boxes are uncommon and surface-mounted switches or AC outlets are normally used instead.
Belgium Single gang boxes for installation in plasterboard are of the standard European type. Boxes for installation in brick walls are rectangular in shape and can be connected to form a chain, similar to their standard European counterparts. For plasterboard, 2-gang, 3-gang or 4-gang boxes are available instead. The center distance between inserts is 71 mm, as in most of the rest of Europe, for horizontal combinations; for vertical ones, it is 71, 60 or 57 mm. Single gang boxes, as well as multi-gang boxes or rows of boxes with a center distance of 71 mm, can accommodate standard European inserts.
British Isles In the UK, also in Ireland, outlet boxes are rectangular. Single gang boxes have roughly the same dimensions as the European box and can accommodate European inserts, but usually not vice versa. Larger boxes are also available, to accommodate a 2-gang outlet, as well as boxes to accommodate two inserts side by side. Metal boxes, uncommon in Europe, are available in the UK. The term "back box" (or "backbox") is also often used, sometimes to refer only to a recessed/sub-surface box as distinct from a pattress, but sometimes to refer to either.
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