Receptacles and Switches

Now that we have explored how electricity is distributed throughout a building, let's look at how it is controlled and accessed at the end point. Whether you are turning on a wall switch, plugging an appliance into a receptacle, or using a dimmer to dim the lights, you are using a wiring device installed in an outlet box. Even the attachment plugs (caps) and wall plates are considered to be wiring devices. Outlet boxes are also used where light fixtures are connected to the electrical system. Low voltage lighting control devices are considered wiring devices. Junction boxes are enclosures for housing and protecting electrical wires or cables that are joined together in connecting or branching electrical circuits. Commercial attachments that are not usually considered wiring devices include premise wiring, such as data and communications wiring and raceways.

Electrical equipment is given ratings for voltage and current. Voltage ratings indicate the maximum voltage that can be safely applied to the unit continuously. An ordinary electric wall receptacle is rated at 250V, but is only supplied with 120V in normal use. The type and quality of insulation used and the physical spacing between electrically energized parts determines the voltage rating. Current ratings are determined by the maximum operating temperature at which the equipment's components can operate at full load. This depends upon the type of insulation used. Wiring devices are usually rated at 300 A or less and frequently at 20 A. They can be mounted in a small wall box.

Manufacturers classify wiring devices to indicate their quality and expected use. Wiring devices are manufactured in three quality grades. However, their grades are not standard across manufacturers, so if you specify by grade without a manufacturer named, you have little control over what you may actually get. The highest quality is hospital grade, which has a green dot on the device face. Hospital grade wiring devices are built to withstand severe abuse while maintaining reliable operation, and must meet Underwriters' Laboratories (UL) requirements for their grade. Federal specification grade is roughly equivalent to industrial (premium) and commercial specification grades, and is less stringent than hospital grade. Industrial specification grade is used for industrial and high-grade commercial construction. Commercial specification grade is used for educational and good residential buildings, so it isn't limited to commercial work only. The UL general-purpose grade corresponds roughly to residential grade, and is the least demanding quality. Standard or residential grade is used in low-cost construction of all types, but not necessarily in all residential work.

Each electrical receptacle, lighting fixture, or switch

Octagonal junction box & cover

Figure 31-1 Junction boxes.

Square junction box & cover

Figure 31-1 Junction boxes.

is housed in a metal or plastic box that is fastened securely to the structure, supporting the device and protecting the box's contents. The cable or conduit serving the device is clamped tightly to the box where the wires enter. The bare neutral wire is connected to the box and to the frame of the device, ensuring that it will never cause shocks if the device becomes faulty. The black and white wires are stripped of insulation at their ends, and connected to the device with screws or clamps. The circuit is then tested for safe, satisfactory operation, and the device is screwed snugly to the box. A metal or plastic cover plate is attached to keep fingers out, keep electrical connections free of dust and dirt, and provide a neat appearance.

Outlet and device boxes are made of galvanized stamped sheet metal. Nonmetallic boxes may be used in some wiring installations. Cast-iron or cast aluminum boxes are used for outdoor work and in wet locations. Square and octagonal 10-cm (4-in.) boxes (Fig. 31-1) are used for fixtures, junctions, and electrical devices. Single switches and duplex outlets (one receptacle above another, as commonly found in homes) use a 102 by 57 mm (4 by 2X in.) box. Depths range from 38 to 76 mm (1}-3 in.).

Outlet boxes are wall- or floor-mounted for electrical receptacles. For lighting fixtures, they are wall- or ceiling-mounted. Floor boxes of cast metal are set directly into the floor slab. Switch boxes are typically mounted on the wall to control a lighting outlet box. They are usually mounted within the wall on a stud. Installation of an electrical box usually just penetrates the wall surface, and does not require firestopping. The opening in the wall can't allow a gap greater than 3 mm (8 in.) between the box and the gypsum wallboard.

The National Electrical Code (NEC) specifies the minimum number of electrical boxes allowed for some building types, especially dwelling units. Most electrical codes specify the maximum distance between receptacles in a room, so that a lamp or appliance with a standard length cord can be placed anywhere around the room perimeter without using an extension cord. Codes also specify the number of receptacles per room and the maximum number of receptacles per circuit, to prevent overloading wires with excessive flows of current. The NEC and model building codes typically specify that no more than 645 square cm (100 square in.) of electrical boxes can be installed per 9.3 square meters (100 square ft) of wall surface.

When an existing electrical box is not being used, it must either have a cover plate installed or be totally removed, including the box and all its wiring, with the wall opening properly patched. In fire-rated walls, when boxes are used on opposite sides of the same walls, they must be separated by 61 cm (24 in.) horizontally.

Poke-through fittings (Fig. 31-2) are often used in existing commercial spaces to meet expanded desktop power and data wiring needs. They are fed through the floor from within a hung ceiling below. Poke-through fittings allow wiring relocations in rental office spaces. The NEC requires that electrical penetrations in fire-

Power Data Floor Boxes

Power, phone, data and cable outlets

Power, phone, signal and data cables

Figure 31-2 Poke-through electrical fitting.

Power, phone, data and cable outlets

Power, phone, signal and data cables

Figure 31-2 Poke-through electrical fitting.

rated floors, walls, ceilings, and partitions maintain the fire ratings, so poke-through fittings must be properly sealed to preserve the fire rating.

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