Selecting and applying wiring devices.
Receptacles, switches, plates, and cord connector bodies are available in a wide range of sizes, ratings, and styles with specific features and characteristics to meet most design/application requirements.
Grades of wiring devices
Manufacturer's catalogs use a variety of terms to indicate the quality or grade of wiring devices offered. For example, the term "economy," "competitive," "intermediate," or "residential" are sometimes used to indicate that the device is economically priced or designed for light-duty applications. Terms such as "specification" or "super-specification" would indicate devices that are of better quality, designed for greater reliability and usually higher priced. However, none of these terms has an official status with standardizing agencies such as Underwriters Laboratories (UL), Factory-Mutual (FM), Electrical Testing Laboratories (ETL), or the National Electrical Manufacturers Association (NEMA).
Presently, UL lists wiring devices for only two grades, standard and hospital grade. All devices, whether termed intermediate, economy, or specification must meet identical UL requirements (although as mentioned above, specification grade devices are of better quality construction).
Hospital-grade devices, intended for use in hospitals (in other than hazardous areas), may be used in other locations where high reliability is desired. These devices must pass UL tests that are more severe than those required for standard grade devices; they are identified by the marking "Hospital Only" or "Hospital Grade" and by a green dot on the device.
There are two basic types of snap switches - "AC General Use" and "AC-DC General Use." A "T"-rated AC-DC is also available for 125V, tungsten filament lamp loads.
The AC-DC type is designed with a quick-make/quick-break action requiring rugged springs and components to assure dependable operation. If T-rated, its contacts are designed to handle the high inrush current of incandescent lamps. The AC-only type has a somewhat slower make/break action.
Additional information concerning these switches is provided in the UL Electrical Construction Materials List (Green Book), including tables listing current ratings. For example, AC-DC general-use switches are rated 3A, 5A, 5A or 6A, 10A, 20A, 30A and 40A, and 60A at 125V; AC general-use switches are rated 15A, 20A, and 30A at 120V. Ratings for other voltages are also provided.
The NEC has a great deal of information that is helpful in the selection of wiring devices. For example, Article 100, Definitions and Article 380, Switches, provides the following data:
General-Use Snap Switch. A form of general-use switch so constructed that it can be installed in flush device boxes or on outlet box covers, or otherwise used in conjunction with wiring systems recognized by this Code.
AC general-use snap switch. A form of general use switch suitable only for use on alternating circuits for controlling the following:
(1) Resistive and inductive loads, including electric discharge lamps not exceeding the ampere rating at the voltage involved.
(2) Tungsten filament lamp loads not exceeding the ampere rating at 120 volts.
(3) Motor loads not exceeding 80% of the ampere rating of the switches at the rated voltage.
AC-DC General-Use Snap Switch. A form of general-use snap switch suitable for use on either ac or dc circuits for controlling the following:
(1) Resistive loads not exceeding the ampere rating of the switch at the voltage involved.
(2) Inductive loads not exceeding 50 percent of the ampere rating of the switch at the applied voltage.
(3) Tungsten-filament lamp loads not exceeding the ampere rating of the switch at the applied voltage if "T" rated.
All AC general-use switches are marked AC in addition to their electrical rating. AC-DC general-use switches usually are not marked AC-DC, but are always marked with their electrical rating.
Mercury switches have an AC rating of 125V, 15A or 20A, but they are also T-rated at 10A, 125VDC.
Flush snap switches are available in the following types: single-pole; double-pole; 3-way; 4-way; 2-circuit; maintained contact, SPDT, DPDT; and momentary contact SPDT, DPDT; SP normally open, SP normally closed; DP normally open and normally closed; and 3-way. Most of these switches can be obtained with key-lock design, and ratings vary among manufacturers.
Line terminals of 15A and 20A switches marked "CO/ALR" are for use with aluminum, copper, and copper-clad aluminum conductors. Terminals of switches rated 30A and above marked "AL/CU" also are suitable for aluminum, copper, and copper-clad aluminum conductors. Also, switches furnished only with screwless pressure terminal connectors are acceptable for use with copper and copper-clad aluminum conductors but are not suitable for use with aluminum conductors.
Switches provided with "push-in" screwless pressure terminal connectors have a particular advantage when they are ganged in the same box. Sec. 380-8(b) of the NE Code prohibits ganged snap switches with exposed live parts if the voltage between adjacent switches is over 300V. Because switches with pressure-type terminals have no exposed live parts, they can be used to satisfy this Code rule.
Combination switches serve particular design needs. They are available as two switches, a switch and receptacle, a switch and pilot light, and similar configurations. The units are integral assemblies and have separate or common terminals, as desired. Other specialty types of switches include quiet operation, locking feature type, door switches, ceiling pull switches, and pedestal switches.
Timer switches serve as energy-saving devices in small, infrequently used rooms or spaces. In addition to controlling room lighting, typical applications are the operation of heat lamps and exhaust fans, generally in bathrooms.
Wall-mounted occupancy sensors can replace standard wall switches, and many of these models incorporate a switch mechanism for override capability. The area they can cover depends on several factors, including the mounting location and height, room configuration, furniture, the sensors' sensitivity setting, and type of motion detection. They are suited to small infrequently used rooms, conference rooms, private offices, and storage rooms.
Electronic enhanced switches provide a number of features that are increasingly being used in residential and institutional occupancies. For example, fully automated lighting controls allow any indoor or outdoor light to be remotely controlled from virtually any point within the system, which consists of controllers and receivers. Configured as wall switches and receptacles, the receivers accept command signals sent through the existing AC branch circuits. In addition, a wall-mounted programmer can be used or a pocket-sized, handheld controller (a wireless transmitter) can be carried by a homeowner.
The handheld controller sends radio frequency signals to a plug-in transceiver that injects the controller's signals into the AC wiring. Because it is battery-powered, the controller/transmitter can be carried or mounted near the bedside, or on the arm of a wheelchair. The signals, which travel through walls and floors, can be received up to 50 ft away.
Another example is a three-function security wall switch with standard on/off operation and a special illuminated button marked FLASH. When the FLASH button is activated, the "911 switch" causes the front door and exterior lights to flash on and off at one-sec intervals, alerting neighbors and helping emergency vehicles to locate the house quickly.
These specialized wiring devices help young children, older people, and people with a wide variety of disabilities to perform everyday activities in the home and to maintain a greater sense of security
Low-voltage, remote control switching uses solenoid relays to operate lighting circuits. The standard switching device, generally a SPDT switch wired into a Class 2 remote control circuit takes the place of a standard flush switch. The operating portion of the circuit is a single-pole relay that provides on/offcontrol for a complete lighting branch circuit or an individual luminaire (fixture). The relay contacts are normally rated for a 20A filament load and are mechanically latching. Only a momentary 24V (rectified AC) switch circuit pulse is needed to open or close the contacts of the branch circuit ungrounded (or phase) conductor.
While remote control relays may be inserted through one-half-in. knockouts of standard wiring boxes, the use of a relay center box is a preferred method.
Receptacle ratings range from 10A to 400A, with some sizes available at 125, 250, 277, 480, and 600V. Most popular are the 15A and 20A flush type, rated at 125V and 250V. These are available with several different slot configurations in grounding and nongrounding type. NE Code rules require that receptacles installed for the attachment of portable cords shall be rated at not less than 15A, 125V or 10A 250V. Sec. 210-7 states that the receptacles on 15A or 20A branch circuits must be of the grounding type.
Receptacles located outdoors but protected from direct contact with rain by a roof or overhang are considered to be damp areas. These receptacles must be protected by a closable cover when the plug cap is not being used.
In wet areas, the receptacles must be protected by a cover that will not be affected by rain when the plug cap is installed. Any cover acceptable for use in wet locations (outdoors without protection of roof or cover) also is acceptable in damp areas.
NEMA has developed standard configurations for all common ratings of straight-blade and locking-type receptacle and plug caps. Each current and voltage rating has a single noninterchangable configuration so that differently rated devices cannot fit together. This data, presented in a convenient chart, is useful when selecting receptacles.
Grounding-type receptacles are available with special grounding means designed into their box-attachment screws that automatically ground the grounding terminal to the box when the receptacle is installed.
The grounded terminal of a grounding-type receptacle must be identified by a green hex-headed screw or nut, a green pressure wire connection device or something similar green connection device in the case of an adapter.
Split-bus receptacles are available in flush duplex types with ratings of 15A or 20A, 125V or 250V. Typical units consist of duplex assemblies with one parallel-blade receptacle (125V) and one tandem blade receptacle (250V), or two parallel-blade receptacles. Split bus receptacles make it possible to wire separate circuits or controls to each set of line terminals.
Many standard parallel-blade duplex receptacles contain a break-off feature that permits the regular connection of both receptacles on a singe circuit, or a jumper can be removed on the line terminal bus to permit two-circuit operation.
Locking-type receptacles are available for applications where it is important that cord attachment caps be firmly held in place.
Flush or surface receptacles are made in 3- or 4-pole types with ratings of 30, 50 or 60A. Three-pole 30 and 50A receptacles are commonly used for residential electric dryers and ranges where the grounded neutral of 230/115V circuits can be used to ground such appliances. Four-pole receptacles are designed for commercial and industrial applications to supply 3-phase or 230/115V equipment.
Heavy-duty receptacles are mounted in integral cast-metal (or plastic) assemblies. Combination units of this type have conduit hubs and receptacle sizes ranging up to 400A at 250V and 200A at 600V. The enclosures are available in general-purpose or raintight types. Other receptacle and enclosure assemblies are designed for use in corrosive or hazardous locations.
These heavy-duty receptacles are available with a variety of features that enhance safety and satisfy OSHA lockout requirements at industrial facilities. For example, one safety enclosure integrates a receptacle, a switch with padlockable handle and a hinged liftcover in a low-profile nonmetallic housing. This locking plug design encourages industrial users to deenergize the switch before disconnecting the plug and receptacle to prevent possible arcing.
Receptacle-type ground fault circuit interrupters (GFCI) can, in many instances, be used in place of circuit-breaker type GFCIs to provide protection from shock hazard. A receptacle-type GFCI may be wired as a terminal device or as a feed-through unit to protect additional downstream receptacles. Such receptacles are designed to provide protection against the shock hazard of low-level, line-to-ground faults. When properly installed, the device automatically shuts off power when it detects current leakage to ground in milliamperes, preventing serious injury or electrocution.
These GFCI devices are available with a number of installer-friendly features. For example, one manufacturer offers a device with two backwire holes per termination to allow for a multiple of wiring options without having to pigtail and use wire connectors. It also has easy-to-read Line/Load markings.
The NE Code specifies those areas where GFCIs are required. However, GFCI protection should be provided for any type of circuit where there is a danger to personnel from ground faults. Installers must be particularly careful to wire these devices correctly.
Isolated grounding receptacles are for use where electronic equipment can be adversely affected by pickup of transient signals present on the equipment grounding pathway that causes malfunction in the sensitive circuits. This is especially true of medical and communication equipment that may receive undesired currents of very low magnitude commonly present in conventional grounding systems.
Rather than relying on a mechanical ground path through the metallic housing of the raceway system, these devices use an insulated grounding conductor in the raceway to provide a separate "pure" grounding path. This separate grounding conductor is run with the circuit conductors in the same raceway and is connected with the normal equipment grounding circuit only at the service equipment ground terminal.
Flat wiring systems, an alternative method of wiring in office space, use specialized receptacles. A flat wiring system is composed of several main components: transition boxes, tap and splice assemblies, and outlets. The outlet, which is also called a service fitting, a pedestal, or a monument, encloses the receptacle. There are two basic methods of connecting a duplex receptacle to a flat cable. In the first, a transition connector pierces the flat cables insulating jacket and contacts the conductors; then connectors (round conductors or pigtails leads) are attached to the receptacle, as in conventional wiring. The second method eliminates the pigtail wiring by having a receptacle device pierce the insulation of the flat cable and make direct contact with the conductors.
Layout of wiring devices
Sec. 210-25(b) requires receptacles in rooms to be installed so that no point along the floor line in any wall space 2 or more ft wide is more than 6 ft, measured horizontally, from an outlet.
In residential occupancies the receptacles preferably should be located near the end of wall space rather than near the center, reducing the likelihood of being concealed behind large pieces of furniture. Generally, outlets are located 12-in. above the floor line.
Also, in residential occupancies, wall switches should normally be located at the latch side of doors or at the traffic side of arches and within the room or area to which control is applicable. Some exceptions to this practice are the control of exterior lights from indoors, the control of stairway lights from adjoining areas, when stairs are closed off by doors at head or foot and the control of lights from the access space adjoining infrequently used storage areas. Wall switches are normally mounted at a 48-in. height.