Conduit and Fittings
Webpages concerning "Conduit and Fittings"
|
The contents has been generating using technology developed by scientec.
Wikipedia-Article "Conduit"
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 should apply worldwide.
Contents |
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 formed by insurance companies, 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 IEE Requirements 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 the United States local governing bodies such as counties or cities often include the National Electrical Code in their local building codes by reference along with any local differences. Because of copyright law one most obtain both the local codes and the National Electrical Code separately.
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.
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 1940s, 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.
Two advantage of knob- and- tube wiring are better air cooling, e.g. a #14awg copper wire has the same ampacity of #12awg wire in a Romex cable and resistance to short circuiting across conductors, e.g. a nail going through one conductor cannot reach the other conductor.
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 knob- and- tube installations did not include a safety grounding connection between metal enclosures.
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 1930s, 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 moisture wicking. The advantage was that cables require less labor.
Just before World War II, the cost and other advantages of cable resulted in a decline in new knob-and-tube installations.
Next, the insulation was rubber; through mid-1960 in the US. 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 in the smaller gauges for low-voltage signal and control applications such as doorbell wiring however today even cable sold for low voltages has a thin plastic sheath.
Modern non-metalic sheathed cable like the brand name Romex consists of plastic coated wires and uncoated wires (usually one for ground) and paper wadding surrounded by a flexible plastic sheathing. The most common cable consists of two plastic coated wires and one bare ground wire forming a flat cable.
Now, rubber- like polymer insulation is used in premimum cables such as for nuclear power plants and large power cables installed underground because of its superior moisture resistance. The common cables used in house and light commercial buildings have thermoplastic insulation.
For the 300- 600 volt rated cables, mechanical strength is the dominating characteristic because of the installation techniques used. An insulated cable has two ratings: voltage to ground, temperature at the conductor surface.
Where more protection of the cable is desired, the insulated conductors are overwrapped with bare wires. In residential and light commercial, this is the neutral conductor. In manufacturing plants, power plants, large commercial buildings, a layer of steel wires or corrugated steel armor is sometimes wound over the cable.
Generally single conductor building wire in small sizes is solid wire, since the wiring is not required to be very flexible. And, solid wire is used for grounding connections since it has the least surface area, which resists corrosion.
Building wire conductors larger than #10 AWG (or about 6 square millimetres) are stranded for flexibility during installation. The wires used in raceways, most commercial and industrial building is stranded. And those used in control wiring have more individual strands for more flexibility.
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 materials are fire resistant, made of Teflon or other material that produce little toxic fumes or smoke.
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.
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, "ampacity". Tables in electrical safety codes give the maximum allowable current for a particular size of conductor, for the voltage and temperature rating of the insulation, and for a given physical environment. The allowable ampacity will be different for wet or dry, for hot ( attic ) or underground (cool). HOWEVER, the limiting factor used must be the most stringent.
The common item for trouble is at the ends of an insulated wire. In better installations, mechanical connections are torqued; pressure fittings are used. The better pressure fittings achieve a cold weld between the fitting and the conductor.
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 designs are required to support the conductors of vertical runs of cable. Usually, only one cable per fitting is allowed.
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 raceway 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 in concrete, but metal elbows may be used in severe pulls. Wiring in exposed areas, for example factory floors, may be run in raceways or cable trays. For protection from flame spread, fire stopping material of silicone may be used. However, the thickness of such material along the cable must be held to a minimum, else this becomes a heat limit. Special fittings are used for wiring in potentially explosive atmospheres.
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. However, overcrowding can lead to fires, even with control wiring.
Since wires run in conduits or underground cannot dissipate heat as easily as in open air, and adjacent circuits contribute induced currents, wiring regulations give rules to establish the ampacity.
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 used in manufacturing plants and power company switch yards to gain the benefit of air cooling.
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. The big advantage of this scheme is the ability to remove or add a branch circuit without removing voltage from the whole duct.
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). Likewise, for conducting large currents between devices, cable bus is used. 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 grounded metal enclosure. A fault in any phase jumps to ground. 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.
External links
See also
- Electric power transmission
- Electricity distribution
- Electrical wiring (U.S.)
- Electrical wiring (UK)
References
- Insuring a home with knob and tube insulation
- History of Residential Wiring Methods - U.S.
- [1] NEMA comparison of IEC 60364 with the US NEC