Electrical wiring
Electrical wiring in general refers to conductors used to carry electricity, and their accessories. This article describes general aspects of electrical wiring as used to provide power in or to buildings and structures, commonly referred to as building wiring. Electrical wiring practices vary greatly by locality. This article is intended to describe common features of electrical wiring that that should apply worldwide.
Wiring Safety Codes
In the U.S., U.K, Canada and other industrialized countries installation of wiring is governed by national or local regulations. Often a national technical standards-setting organization will produce a model electrical code, which is then adopted,perhaps with local amendments, by state/provincial or city regulations. The intention of wiring safety codes is to provide technical, performance and material standards that will allow efficient distribution of electrical energy and communication signals, at the same time protecting persons in the building from electric shock and preventing fire or explosion. Electrical codes arose in the 1880's with the early commercial introduction of electrcal power, since many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations.
The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. The U.S. National Fire Protection Association, a private non-profit association, produced the first draft of the U.S. National Electrical Code in 1885.
Since 1927, the Canadian Standards Association has produced the Canadian Safety Standard for Electrical Installations, which is the basis for provincial electrical codes.
In the United Kingdom wiring installations are regulated by the produced by the IEERequirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2001 which is now in its 16th edition.
Although these three national standards all deal with the same physical phenomena and broadly similar objectives, they differ significantly in technical detail. As part of the NAFTA program, US and Canadian standards are slowly converging towards each other, in a process known as harmonization. Small countries, with relatively small technical societies, may adopt one of these three standards as their national standard, and concentrate on developing local regulatory amendments instead of redeveloping the basic requirements of a national code.
In European countries, an attempt has been made to harmonize national wiring standards in an IEC standard, IEC 60364 Electrical Installations for Buildings. However, this standard is not written in such language that it can readily be adapted as a national wiring code. Neither is it designed for field use by electrical tradesmen and inspectors for verification of compliance to national wiring standards. National codes, such as the NEC or CSA C22.2, exemplify the common objectives of IEC 60364, and provide rules in a form that allows for guidance of persons installing and inspecting electrical systems.
National codes are often amended by regional or municipal authorities. These amendments may be to "grandfather" existing practices and make them acceptable for local use, or may be to incorporate local requirements not addressed by the national code.
On or Off The Grid
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Electrical wiring may be "on the grid", meaning that it is continuous with common electrical utility supplies. In North America, this means alternating current (AC) at 60 Hz, with 120 volts nominal. In European countries, this means AC at 50 Hz with 230 volts nominal.
"Off the grid" electrical wiring is often configured to carry direct current (DC). DC systems often run at a lower voltage than AC systems and so heavier wiring may be needed. This however is often offset by the fact that off grid systems are usually configured to minimize electricity use.
Wiring Methods
Materials for wiring interior electrical systems in buildings vary depending on:
- Rating of the circuit
- Type of occupancy of the building
- Type of electrical system
- National and local regulations
- Conditions which the wiring must operate.
Wiring systems in a home, for example, are simple, with relatively low power requirements, infrequent changes to the building structure and layout, usually with dry, moderate temperature, and non-corrosive environmental conditions. In a light commercial environment, more frequent wiring changes can be expected, large apparatus may be installed, and special conditions of heat or moisture may apply. Heavy industries have more demanding wiring requirments, such as very large currents and power ratings, frequent changes of equipment layout, corrosive, wet or explosive atmospheres.
Early wiring methods
The very first interior power wiring systems used conductors that were bare or covered with cloth, which were secured by staples to the framing of the building or on running boards. Where conductors went through walls, they were protected with cloth tape. Splices were done similarly to telegraph connections, and soldered for security. Underground conductors were insulated with wrappings of cloth tape soaked in pitch, and laid in wooden troughs which were then buried. Such wiring systems were unsatisfactory due to the danger of electrocution and fire, and due to the high labor cost for installation.
Knob and tube
The earliest standardized method of wiring in buildings, from about 1880 to the 1940's, was single insulated copper conductors run across interior walls or within ceiling cavities, passing through holes in porcelain insulating tubes, and supported along their length on porcelain insulators. This system is known as "knob-and-tube" from the insulators used. Where conductors entered a wiring device such as a lamp or switch, they were protected by flexible insulating sleeving. Wire splices in such installations were typically soldered and wrapped with cloth tape, or made inside metal junction boxes.
Knob and tube wiring at a museum display While a knob-and-tube wiring system can be safe and reliable when in good condition, it is not used in modern building construction. The installation is costly due to the high labor content, and originally knob-and-tube installations did not include a safety ground connection.
Older homes may have knob-and-tube wiring for all or part of their electrical system. Such wiring systems may require replacement or upgrade. Wiring in such buildings may be inadequate for modern levels of power use. Wiring may have been damaged by renovations done in the building. Insulation covering the wires may be brittle due to age or may be damaged by rodents or carelessness (for example, hanging objects off wiring running in accessible areas like basements).
Other historic wiring methods
Other methods of securing wiring that are now obsolete include:
- Re-use of existing gas pipes for electric lighting. Insulated conductors were pulled into the pipes feeding gas lamps.
- Wood moldings with grooves cut for wires. These were eventually prohibited in North American electrical codes by the 1930's, but may still be permitted in other regions.
Cables
The first cables for building wiring were introduced in 1922. These were two or more solid copper wires, with woven cloth and paper insulation, sometimes impregnated with tar as a protection from small amounts of moisture. The advantages were that the conductors were insulated, and since they were paired, less labor was required in installation. A cable could be "fished" (pulled) into existing wall cavities or between roof joists, without opening the space as would be required for knob-and-tube installation.
Later, thermoplastic insulation was introduced which improved the flexibility and durability of the wiring system. After World War II, the cost and other advantages of cable resulted in a decline in new knob-and-tube installations.
The simplest form of cable is two insulated conductors twisted together to form a unit; such unjacketed cables with two or three conductors are still commonly used for low-voltage signal and control applications such as doorbell wiring, or for interconnecting the thermostats of a central heating and air conditioning plant.
Modern building wiring cables have thermoplastic insulation and an overall thermoplastic jacket. Where more protection of the cable is desired, such as in commercial buildings or even in exposed areas of residential construction, a layer of steel wires or corrugated steel armor is wound over the cable. Generally building wire in small sizes is solid wire, since the wiring is not required to be very flexible. However, conductors much larger than #10 AWG (or about 6 square millimetres) are usually stranded, for flexibility and ease of installation.
Industrial cables for power and control may contain many insulated conductors in an overall jacket, with helical tape steel or aluminum armor, or steel wire armor, and perhaps as well an overall PVC or lead jacket for protection from moisture and physical damage. Signal cables, such as Ethernet cables, that must be run in air-handling spaces (plenums) of office buildings may be required by local electrical codes to be of plenum rating, meaning the jacket is fireproof, made of Teflon or other material that will not burn.
For industrial uses in steel mills and similar hot environments, no organic material gives satisfactory service. Cables insulated with compressed mica flakes are sometimes used. Another form of high-temperature cable is a mineral insulated cable, with individual conductors placed within a copper tube, and the space filled with magnesium oxide powder. The whole assembly is drawn down to smaller sizes, which compresses the powder. Such cables are fireproof and can be used up to 200 C, but are costly to purchase and install, and have little flexibility.
Mineral insulated cables at a panel board Because conductors in a cable are in contact and so cannot dissipate heat as easily as single insulated conductors, they usually are rated at a lower current carrying capacity. Tables in electrical safety codes give the maximum allowable current for a particular size of conductor and for a temperature rating of the cable insulation. conductors. The allowable working temperature of the cables is limited by the ratings of the insulation, and the same cable may have different temperature ratings in wet or dry applications.
Cables usually are secured by special fittings where they enter electrical apparatus; this may be a simple screw clamp for jacketed cables in a dry location, or a rubber-gasketed cable connector that mechanically engages the armor of an armored cable and provides a water-resistant connection. Special cable fittings may be applied to prevent explosive gases from flowing in the interior of jacketed cables, where they cable pass through areas where flammable gases are present. To prevent loosening of the connections of individual conductors of a cable, cables must be supported near their entrance to devices and at regular intervale through their length. In tall buildings special cable designs ar required to support the conductors of vertical runs of cable.
Special cable constructions and termination techniques are required for cables installed in ocean-going vessels; in addition to electrical safety and fire safety, such cables may also be required to be pressure-resistant where they penetrate bulkheads of a ship.
Conduits, ducts, wire ways, cable trays
Insulated wires may be run in one of several forms of tube between electrical devices. This may be a rigid steel or aluminum pipe, called a conduit, or in one of several varieties of metal or non-metallic tubing. Wires run underground, for example, may be run in plastic tubing encased on concrete. Wiring in exposed areas, for example factory floors, may be run in tubing for protection from mechanical damage and to prevent ignition of flammable gasses that may be present. Special fittings are used to mount wiring devices in conduit run or tubing, and to mechanically connect the tubing with equipment enclosures. Depending on the type of metallic pipe or tube, and local regulations, the metal pipe may form all or part of the grounding (earthing) conductor for the equipment.
In Edison's first commercial distribution system, conductors were wrapped in cloth tape, coated with a bituminous compound, and placed in steel pipes buried below ground. Alternatively, the conductors were placed in buried wooden troughs.
Cable trays are used in industrial areas where many insulated cables are run together. Where wiring regulations allow it, individual cables can exit the tray at any point, simplifying the wiring installation and reducing the labor cost for installing new cables.
Since wires run in conduits or underground cannot dissipate heat as easily as in open air, wiring regulations give rules to establish the current capacity of enclosed wiring based on the insulation temperature rating and the number of conductors in the enclosure.
Bus bars, bus duct
See also main article on Bus bars
For very heavy currents in electrical apparatus, and for heavy currents distributed through a building, bus bars can be used. Each live conductor of such a system is a rigid piece of copper or aluminum, usually in flat bars (but sometimes as tubing or other shapes). Open bus bars are never used in publically- accessed areas but are sometimes applied in electrical switchrooms.
In industrial applications, conductor bars are assembled with insulators in grounded enclosures. This assembly, known as bus duct, can be used for connections to large switchgear or for bringing the main power feed into a building. A form of bus duct known as plug-in bus is used to distribute power down the length of a building; it is constructed to allow tap-off switches or motor controllers to be installed at definite places along the bus.
Bus duct may have all phase conductors in the same enclosure (non-isolated bus), or may have each conductor separated by a grounded barrier from the adjacent phases (segregated bus). For very large currents in generating stations or substations, where it is difficult to provide circuit protection, isolated-phase bus is used. Each phase of the circuit is run in a separate metal enclosure. The current induced in the enclosure essentially cancels all magnetic field outside the enclosure. A fault in any phase cannot jump between phases. This type of bus can be rated up to 50,000 amperes and up to hundreds of kilovolts, but is not used for building wiring in the conventional sense.
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