An electrical "3G"
power cable
found commonly in modern European houses. The cable consists of 3 wires
(2 wires + 1 grounding in case if cable has "3G" name) and is
double-insulated.
To enable wires to be easily and safely identified, all common wiring
safety codes mandate a colour scheme for the insulation on power
conductors. In a typical
electrical code,
some colour-coding is mandatory, while some may be optional. Many local
rules and exceptions exist. Older installations vary in colour codes,
and colours may fade with insulation exposure to heat, light and ageing.
Many electrical codes now recognise (or even require) the use of wire
covered with green insulation, additionally marked with a prominent
yellow stripe, for safety earthing (grounding) connections. This growing
international standard was adopted for its distinctive appearance, to
reduce the likelihood of dangerous confusion of safety earthing
(grounding) wires with other electrical functions, especially by persons
affected by red-green
colour blindness.
The down side of the use of International colours is that, in the UK
for example, phases could be identified as being live by using coloured
indicator lights: red, yellow and blue. The new cable colours of brown,
black and grey do not lend themselves to coloured indicators. For this
reason, three-phase control panels will often use indicator lights of
the old colours.
[citation needed][clarification needed]
Standard wire colours for flexible cable
(e.g. Extension cords, power (line) cords and lamp cords) |
| Region or Country |
Phases |
Neutral |
Protective earth/ground |
| European Union (EU), Argentina, Australia, South Africa (IEC 60446) |
 |
 |
 |
| Australia, New Zealand (AS/NZS 3000:2007 3.8.3) |
,  |
 ,  |
|
| Brazil |
 , |
|
 |
| United States, Canada |
(brass) |
 (silver) |
(green) or (green/yellow) |
Standard wire colours for fixed cable
(e.g. In-, On- or Behind-the-wall wiring cables) |
| Region or Country |
Phases |
Neutral |
Protective earth/ground |
| Argentina |
, ,  |
|
|
| European Union (EU) (IEC 60446) including UK from 31 March 2004 (BS 7671) |
, , |
|
|
| UK prior to 31 March 2004 (BS 7671) |
, , |
|
(formerly)
 bare conductor, sleeved at terminations (formerly) |
| Australia, New Zealand (AS/NZS 3000:2007 clause 3.8.1, table 3.4) |
Any colours other than
Recommended for single phase:
Recommended for multiphase: |
or |
(since about 1980)
(since about 1980)
bare conductor, sleeved at terminations (formerly) |
| Brazil |
, , , |
|
|
| South Africa |
, or , |
|
bare conductor, sleeved at terminations |
| India, Pakistan |
, , |
|
(green) or (green/yellow) |
| United States |
, , (120/208/240 V)
(brass)
,  , (277/480 V) |
(120/208/240 V) (silver)
(277/480 V) |
(green)
bare conductor
(ground or isolated ground) |
| Canada |
, (120/208/240 V)
, , (600/347 V)
, (single phase isolated systems)
, , (three phase isolated systems) |
(120/208/240 V)
(600/347 V) |
(green)
bare conductor
(isolated ground) |
Notes:
Parenthesised colours in italics are used on metallic terminals. "Green/yellow" means green with yellow stripe. See illustrations nearby.
The colours in this table represent the most common and preferred
standard colours for wiring; however others may be in use, especially in
older installations.
Australian and New Zealand wiring standards allow both European and
Australian colour codes. Australian-standard phase colours conflict with
IEC 60446 colours, where IEC-60446 supported neutral colour (blue) is an allowed phase colour in the Australia/New Zealand standard. Care must be taken when determining system used in existing wiring.
Canadian and American wiring practices are very similar, with ongoing harmonisation efforts.
|
Wiring methods
Installing electrical wiring by "chasing" grooves into the masonry structure of the walls of a building
Materials for wiring interior electrical systems in buildings vary depending on:
- Intended use and amount of power demand on the circuit
- Type of occupancy and size of the building
- National and local regulations
- Environment in which the wiring must operate.
Wiring systems in a single family home or duplex, 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
requirements, such as very large currents and higher voltages, frequent
changes of equipment layout, corrosive, or wet or explosive atmospheres.
In facilities that handle flammable gases or liquids, special rules may
govern the installation and wiring of
electrical equipment in hazardous areas.
Wires and cables are rated by the circuit voltage, temperature rating
and environmental conditions (moisture, sunlight, oil, chemicals) in
which they can be used. A wire or cable has a voltage (to neutral)
rating and a maximum conductor surface temperature rating. The amount of
current a cable or wire can safely carry depends on the installation
conditions.
Early wiring methods
The 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 because of the danger of
electrocution and fire, plus the high labour cost for such
installations.
Knob and tube
Knob-and-tube wiring (the orange cable is an unrelated extension cord)
The earliest standardised method of wiring in buildings, in common use in North America from about 1880 to the 1930s, was
knob and tube
(K&T) wiring: single conductors were run through cavities between
the structural members in walls and ceilings, with ceramic tubes forming
protective channels through joists and ceramic knobs attached to the
structural members to provide air between the wire and the lumber and to
support the wires. Since air was free to circulate over the wires,
smaller conductors could be used than required in cables. By arranging
wires on opposite sides of building structural members, some protection
was afforded against short-circuits that can be caused by driving a nail
into both conductors simultaneously.
By the 1940s, the labour cost of installing two conductors rather
than one cable resulted in a decline in new knob-and-tube installations.
However, the US code still allows new K&T wiring installations in
special situations (some rural and industrial applications).
Metal-sheathed wires
In the United Kingdom, an early form of insulated cable,
[1]
introduced in 1896, consisted of two impregnated-paper-insulated
conductors in an overall lead sheath. Joints were soldered, and special
fittings were used for lamp holders and switches. These cables were
similar to underground telegraph and telephone cables of the time.
Paper-insulated cables proved unsuitable for interior wiring
installations because very careful workmanship was required on the lead
sheaths to ensure moisture did not affect the insulation.
A system later invented in the UK in 1908 employed vulcanised-rubber
insulated wire enclosed in a strip metal sheath. The metal sheath was
bonded to each metal wiring device to ensure earthing continuity.
A system developed in Germany called "Kuhlo wire" used one, two, or
three rubber-insulated wires in a brass or lead-coated iron sheet tube,
with a crimped seam. The enclosure could also be used as a return
conductor. Kuhlo wire could be run exposed on surfaces and painted, or
embedded in plaster. Special outlet and junction boxes were made for
lamps and switches, made either of porcelain or sheet steel. The crimped
seam was not considered as watertight as the
Stannos wire used in England, which had a soldered sheath.
[2]
A somewhat similar system called "concentric wiring" was introduced
in the United States around 1905. In this system, an insulated
electrical wire was wrapped with copper tape which was then soldered,
forming the grounded (return) conductor of the wiring system. The bare
metal sheath, at earth potential, was considered safe to touch. While
companies such as
General Electric
manufactured fittings for the system and a few buildings were wired
with it, it was never adopted into the US National Electrical Code.
Drawbacks of the system were that special fittings were required, and
that any defect in the connection of the sheath would result in the
sheath becoming energised.
[3]
Other historical wiring methods
Other methods of securing wiring that are now obsolete include:
- Re-use of existing gas pipes when converting gas light
installations to electric lighting. Insulated conductors were pulled
through the pipes that had formerly supplied the gas lamps. Although
used occasionally, this method risked insulation damage from sharp edges
inside the pipe at each joint.
- Wood mouldings
with grooves cut for single conductor wires, covered by a wooden cap
strip. These were prohibited in North American electrical codes by 1928.
Wooden moulding was also used to some degree in England, but was never
permitted by German and Austrian rules.[4]
- A system of flexible twin cords supported by glass or porcelain
buttons was used near the turn of the 20th century in Europe, but was
soon replaced by other methods.[5]
- During the first years of the 20th century, various patented forms
of wiring system such as Bergman and Peschel tubing were used to protect
wiring; these used very thin fiber tubes, or metal tubes which were
also used as return conductors.[6]
- In Austria, wires were concealed by embedding a rubber tube in a groove in the wall, plastering over it, then removing the tube and pulling wires through the cavity.[7]
Metal moulding systems, with a flattened oval section consisting of a
base strip and a snap-on cap channel, were more costly than open wiring
or wooden moulding, but could be easily run on wall surfaces. Similar
surface mounted raceway wiring systems are still available today.
Cables
Main article:
Power cable
Wiring for extremely wet conditions
Armoured
cables
with two rubber-insulated conductors in a flexible metal sheath were
used as early as 1906, and were considered at the time a better method
than open knob-and-tube wiring, although much more expensive.
The first rubber-insulated
cables for building wiring were introduced in 1922 with
US patent 1458803, Burley, Harry & Rooney, Henry, "Insulated electric wire", issued 1923-06-12, assigned to Boston Insulated Wire And Cable.
[citation needed]
These were two or more solid copper electrical wires with rubber
insulation, plus woven cotton cloth over each conductor for protection
of the insulation, with an overall woven jacket, usually impregnated
with tar as a protection from moisture. Waxed paper was used as a filler
and separator.
Over time, rubber-insulated
cables
become brittle because of exposure to atmospheric oxygen, so they must
be handled with care and are usually replaced during renovations. When
switches, socket outlets or light fixtures are replaced, the mere act of
tightening connections may cause hardened insulation to flake off the
conductors. Rubber insulation further inside the cable often is in
better condition than the insulation exposed at connections, due to
reduced exposure to oxygen.
The sulphur in vulcanised rubber insulation attacked bare copper wire
so the conductors were tinned to prevent this. The conductors reverted
to being bare when rubber ceased to be used.
Diagram of a simple electrical cable with three insulated conductors
About 1950,
PVC
insulation and jackets were introduced, especially for residential
wiring. About the same time, single conductors with a thinner PVC
insulation and a thin nylon jacket (e.g. US Type THN, THHN, etc.) became
common.
The simplest form of
cable has two insulated conductors twisted together to form a unit. Such un-jacketed
cables with two (or more) conductors are used only for
extra low voltage signal and control applications such as doorbell wiring.
US single-phase residential power distribution transformer, showing the
two insulated "Line" conductors and the bare "Neutral" conductor
(derived from the earthed center-tap of the transformer). The
distribution supporting centenaries are also shown.
In North American practice, an overhead
cable
from a transformer on a power pole to a residential electrical service
usually consists of three twisted (triplexed) conductors, with one being
a bare protective neutral/earth/ground contuctor (which may be made of
copper), with the other two being the insulated conductors for the both
of the two 180 degree out of phase 120 V line voltages normally
supplied.
[8] However, the earthed/grounded conductor is often be a
catenary
cable (made of steel wire), which is used to support the insulated Line
conductors. For additional safety, the ground conductor may be formed
into a stranded co-axial layer completely surrounding the phase/line
conductors, so that the outermost conductor is grounded.
Copper conductors
Electrical devices often contain copper conductors because of their multiple beneficial properties, including their high
electrical conductivity,
tensile strength,
ductility,
creep resistance,
corrosion resistance,
thermal conductivity,
coefficient of thermal expansion,
solderability, resistance to
electrical overloads, compatibility with
electrical insulators and ease of installation.
Despite competition from other materials, copper remains the preferred
electrical conductor in nearly all categories of electrical wiring.
[9][10] For example, copper is used to conduct electricity in high, medium and low
voltage power networks, including
power generation,
power transmission,
power distribution,
telecommunications,
electronics circuitry,
data processing,
instrumentation,
appliances, entertainment systems,
motors,
transformers, heavy
industrial machinery and countless other types of
electrical equipment.
[11]
Aluminium conductors
Terminal blocks for joining aluminium and copper conductors. The terminal blocks may be mounted on a
DIN rail.
Aluminium wire
was common in North American residential wiring from the late 1960s to
mid-1970s due to the rising cost of copper. Because of its greater
resistivity, aluminium wiring requires larger conductors than copper. For instance, instead of 14 AWG (
American wire gauge)
for most lighting circuits, aluminium wiring would be 12 AWG on a
typical 15 ampere circuit, though local building codes may vary.
Aluminium conductors were originally indiscriminately used with
wiring devices intended for copper conductors. This practice was found
to cause defective connections unless the aluminium was one of a special
alloy, or all devices—breakers, switches, receptacles, splice
connectors,
wire nuts,
etc.—were specially designed for the purpose. These special designs
address problems with junctions between dissimilar metals, oxidation on
metal surfaces and mechanical effects that occur as different metals
expand at different rates with increases in temperature.
Unlike copper, aluminium has a tendency to
cold-flow
under pressure, so screw clamped connections may get loose over time.
This can be mitigated by using spring-loaded connectors that apply
constant pressure, applying high pressure cold joints in splices and
termination fittings, or using a bolted mechanical type clamp wire
connector and tightening it to a specified torque.
Also unlike copper, aluminium forms an insulating oxide layer on the
surface. This is sometimes addressed by coating aluminium conductors
with an
antioxidant paste at joints, or by applying a mechanical termination designed to break through the oxide layer during installation.
Because of improper design and installation, some junctions to wiring
devices would overheat under heavy current load, and cause fires.
Revised standards for wiring devices (such as the
CO/ALR
"copper-aluminium-revised" designation) were developed to reduce these
problems. Nonetheless, aluminium wiring for residential use has acquired
a poor reputation and has fallen out of favour.
Aluminium conductors are still used for bulk power distribution and
large feeder circuits, because they cost less than copper wiring, and
weigh less, especially in the large sizes needed for heavy current
loads. Aluminium conductors must be installed with compatible
connectors.
Modern wiring materials
Modern non-metallic sheathed cables, such as (US and Canadian) Types NMB and NMC, consist of two to four wires covered with
thermoplastic
insulation, plus a bare wire for grounding (bonding), surrounded by a
flexible plastic jacket. Some versions wrap the individual conductors in
paper before the plastic jacket is applied.
Special versions of non-metallic sheathed cables, such as US Type UF,
are designed for direct underground burial (often with separate
mechanical protection) or exterior use where exposure to
ultraviolet radiation
(UV) is a possibility. These cables differ in having a
moisture-resistant construction, lacking paper or other absorbent
fillers, and being formulated for UV resistance.
Rubber-like synthetic polymer insulation is used in industrial cables
and power cables installed underground because of its superior moisture
resistance.
Insulated cables are rated by their allowable operating voltage and their maximum
operating temperature
at the conductor surface. A cable may carry multiple usage ratings for
applications, for example, one rating for dry installations and another
when exposed to moisture or oil.
Generally, single conductor building wire in small sizes is solid
wire, since the wiring is not required to be very flexible. Building
wire conductors larger than 10
AWG (or about 6 mm²) are stranded for flexibility during installation, but are not sufficiently pliable to use as appliance cord.
Cables
for industrial, commercial and apartment buildings may contain many
insulated conductors in an overall jacket, with helical tape steel or
aluminium armour, or steel wire armour, and perhaps as well an overall
PVC or lead jacket for protection from moisture and physical damage.
Cables intended for very flexible service or in marine applications may
be protected by woven bronze wires. Power or communications cables
(e.g., computer networking) that are routed in or through air-handling
spaces (plenums) of office buildings are required under the model
building code to be either encased in metal conduit, or rated for low
flame and smoke production.
Mineral insulated cables at a panel board
For some 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, thereby compressing the powder. Such cables have a
certified
fire resistance rating and are more costly than non-fire rated cable.
They have little flexibility and behave more like rigid conduit rather
than flexible cables.
Because multiple conductors bundled in a cable cannot dissipate heat
as easily as single insulated conductors, those circuits are always
rated at a lower "
ampacity".
Tables in electrical safety codes give the maximum allowable current
for a particular size of conductor, for the voltage and temperature
rating at the surface of the conductor for a given physical environment,
including the insulation type and thickness. The allowable current will
be different for wet or dry, for hot (attic) or cool (underground)
locations. In a run of cable through several areas, the most severe area
will determine the appropriate rating of the overall run.
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 polymer-gasketed cable connector that
mechanically engages the armour of an armoured 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 the cable passes through areas where inflammable 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 intervals 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
unless the fitting is otherwise rated.
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. Resistance
to
corrosion caused by
salt water or
salt spray is also required.
Raceways
Insulated wires may be run in one of several forms of a raceway
between electrical devices. This may be a specialised bendable pipe,
called a
conduit, or one of several varieties of metal (rigid steel or aluminium) or non-metallic (
PVC or
HDPE)
tubing. Rectangular cross-section metal or PVC wire troughs (North
America) or trunking (UK) may be used if many circuits are required.
Wires run underground 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 cable trays or rectangular
raceways having lids.
Where wiring, or raceways that hold the wiring, must traverse
fire-resistance rated walls and floors, the openings are required by local
building codes to be
firestopped. In cases where safety-critical wiring must be kept operational during an accidental fire,
fireproofing must be applied to maintain
circuit integrity in a manner to comply with a product's
certification listing. The nature and thickness of any
passive fire protection
materials used in conjunction with wiring and raceways has a
quantifiable impact upon the ampacity derating, because the thermal
insulation properties needed for fire resistance also inhibit air
cooling of power conductors.
Cable trays
are used in industrial areas where many insulated cables are run
together. Individual cables can exit the tray at any point, simplifying
the wiring installation and reducing the labour cost for installing new
cables. Power cables may have fittings in the tray to maintain clearance
between the conductors, but small control wiring is often installed
without any intentional spacing between cables.
Since wires run in conduits or underground cannot dissipate heat as
easily as in open air, and since adjacent circuits contribute induced
currents, wiring regulations give rules to establish the current
capacity (ampacity).
Special sealed fittings are used for wiring routed through potentially explosive atmospheres.
Bus bars, bus duct, cable bus
For very high currents in electrical apparatus, and for high currents
distributed through a building, bus bars can be used. (The term "bus"
is a contraction of the Latin
omnibus – meaning "for all".) Each
live conductor of such a system is a rigid piece of copper or aluminium,
usually in flat bars (but sometimes as tubing or other shapes). Open
bus bars are never used in publicly accessible areas, although they are
used in manufacturing plants and power company switch yards to gain the
benefit of air cooling. A variation is to use heavy cables, especially
where it is desirable to transpose or "roll" phases.
In industrial applications, conductor bars are often pre-assembled
with insulators in grounded enclosures. This assembly, known as bus duct
or busway, 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 designated 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.
Busbars for distributing
PE (ground)
Bus ducts 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 conducting large
currents between devices, a cable bus is used.
[further explanation needed]
For very large currents in generating stations or substations, where it is difficult to provide circuit protection, an
isolated-phase bus
is used. Each phase of the circuit is run in a separate grounded metal
enclosure. The only fault possible is a phase-to-ground fault, since the
enclosures are separated. This type of bus can be rated up to 50,000
amperes and up to hundreds of kilovolts (during normal service, not just
for faults), but is not used for building wiring in the conventional
sense.
Electrical panels
Electrical panels are easily accessible
junction boxes used to reroute and switch
electrical services. The term is often used to refer to
circuit breaker panels or fuseboxes.
Degradation by pests
Rasberry crazy ants have been known to consume the insides of electrical wiring installations, preferring
DC over
AC currents. This behaviour is not well understood by scientists.
[12]
Squirrels, rats and other rodents may gnaw on unprotected wiring, causing fire and shock hazards.
[13][14]
See also
References
Robert M. Black, The History of Electric Wires and Cable, Peter Pergrinus Ltd. London, 1983 ISBN 0-86341-001-4, pp. 155–158
Croft
Schneider, Norman H., Wiring houses for the electric light; together with special references to low voltage battery systems, Spon and Chamberlain, New York 1916, pp. 93–98
Croft, p. 142
Croft, p. 143
Croft, p. 136
Croft, p. 137
http://science.howstuffworks.com/environmental/energy/power7.htm
Pops, Horace (June 2008). "Processing of wire from antiquity to the future". Wire Journal International: 58–66.
The Metallurgy of Copper Wire. litz-wire.com
Joseph,
Günter, 1999, Copper: Its Trade, Manufacture, Use, and Environmental
Status, Kundig, Konrad J.A. (ed.), ASM International, ISBN 0871706563, pp. 141–192, 331–375
Andrew R Hickey (15 May 2008). "'Crazy' Ant Invasion Frying Computer Equipment".
"Guide to Safe Removal". Squirrels in the Attic. Retrieved 19 April 2012.
Bibliography
Further reading
External links
The Pros and Cons of Gas Vs. Electric Ovens
How to Install a Garbage Disposal
How to Help Your Dishwasher Run Better
Childproofing the Kitchen
How to Repair a Single Handle Kitchen Faucet
