Definitions

armored-cable

Power cable

A power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. Flexible power cables are used for portable devices, mobile tools and machinery.

History

Early telegraph systems used the first forms of electrical cabling, transmitting small amounts of power. Gutta-percha insulation used on the first Submarine cables was unsuitable for building wiring use since it deteriorated rapidly when exposed to air.

The first power distribution system developed by Thomas Edison in 1882 in New York City used copper rods, wrapped in jute and placed in rigid pipes filled with a bituminous compound. Although vulcanized rubber had been patented by Charles Goodyear in 1844, it was not applied to cable insulation until the 1880s, when it was used for lighting circuits. Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls power project. Oil-impregnated paper-insulated high voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.

Construction

Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses. Large single insulated conductors are also sometimes called power cables in the industry.

Cables consist of three major components: conductors, insulation, protective jacket. The makeup of individual cables varies according to application. The construction and material are determined by three main factors:

  • Working voltage, determining the thickness of the insulation;
  • Current-carrying capacity, determining the cross-sectional size;
  • Environmental conditions such as temperature, water, chemical or sunlight exposure, and mechanical impact, determining the form and composition of the outer cable jacket.

Cables for direct burial or for exposed installations may also include metal armor in the form of wires spiralled around the cable, or a corrugated tape wrapped around it. The armor may be made of steel or aluminum, and although connected to earth ground is not intended to carry current during normal operation.

Power cables use stranded copper or aluminum conductors, although small power cables may use solid conductors. The cable may include uninsulated conductors used for the circuit neutral or for ground (earth) connection.

The overall assembly may be round or flat. Non-conducting filler strands may be added to the assembly to maintain its shape. Special purpose power cables for overhead or vertical use may have additional elements such as steel or Kevlar structural supports.

For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916; the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at the ends of the cable, and at locations along the lenght if voltage rise during faults would be dangerous.

Some power cables for outdoor overhead use may have no overall sheath. Other cables may have a plastic or metal sheath enclosing all the conductors. The materials for the sheath will be selected for resistance to water, oil, sunlight, underground conditions, chemical vapors, impact, or high temperatures. In nuclear industry applications the cable may have special requirements for ionizing radiation resistance. Cable materials may be specified not to produce large amounts of smoke if burned. Cables intended for underground use or direct burial in earth will have heavy plastic or metal, most often lead sheaths, or may require special direct-buried construction. When cables must run where exposed to mechanical impact damage, they may protected with flexible steel tape or wire armor, which may also be covered by a water resistant jacket.

Cables for power distribution of 8kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath. The oil may be kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. Newer cables use polymers or polyethylene, including (XLPE) for insulation.

Most multiconductor cables today have a bare or insulated grounding or bonding wire which is for connection to earth ground. The grounding conductor connectes equipment enclosures to ground for protection from electric shock.

Electrical power cables are often installed in raceways, including electrical conduit and cable trays, which may contain one or more conductors.

A hybrid cable can include conductors for control signals or may also include optical fibers for data.

Named cable types

Common types of general-purpose cables used by electricians are regulated by national or international codes, which define the various wire alloys that may make up a cable, its insulation type and characteristics, including its resistance to chemicals and sunlight.

These general purpose cables are known by the National Electrical Code's 310-12 'Type Letter'. The USA's NEC Type Letter, refers to the insulation type and thickness coating each working wire along with the outer covering [jacket] material, if any. The cables used by utility companies for power distribution do not have these ratings.

'NM (Non-Metallic'') often referred to as "Romex"TM, is a cable made of solid copper wires with a nonmetallic jacket containing a waxed paper wrapped inner group of at least a pair of 600 volt insulated wires and maybe a bare grounding wire. Now the most common Romex/NM-B (NM-variant B) cable thus has three wires: the neutral wire, colored white), the wire providing power to the load, which is nearly always black, and the bare grounding wire. NM-B cables are available in different wire gauges, providing a range of current-carrying capacities at expected maximal ambient temperatures in a cable run.

RomexTM wire
label type
count of
insulated wires
Colors
(normal)
total
wires
wire gauge
rating
12-2 NMB 2 WH + BK three AWG 12
14-2 NMB 2 WH + BK three AWG 14
12-3 NMB 3 WH + BK + RD four AWG 12
14-3 NMB 3 WH + BK + RD four AWG 14
"AWG" means American Wire Gauge, see table here for current capacities derated for various maximum expected temperatures within a cable run or conduit. Other commonly used wire gauges for common electric services distribution range from AWG-4 through AWG-10.

The next most common "Romex" variant (sometimes called a "Three way" cable, after its application in lighting) has the neutral, identified by white coloring and two phase wires: the first conductor is always black and the second conductor is usually colored red (sometimes yellow or blue, but only rarely—such colors must be specially ordered and are more expensive) and an uninsulated copper grounding wire for safety. This type is generally used for multiple switching locations of a common or shared lighting need, such as for switches located at either end of a hallway, or on both upper and lower floors for stairway lighting, and are called a "three-way circuit": a well-entrenched misnomer in North America. Similarly, three or more switches directly controlling a light require one or more "four-way switches" plus a pair of "three-way switches"). Type NM-B cables with three insulated conductors are also suitable for larger loads like clothes dryers, heavy duty air conditioners and electric heaters that require both phases (hot legs) for 220+ volts.

Thus for NEC NM-B series cables, at least two THWN insulated wires plus the bare copper grounding wire are always present. Older NM cables do not have the grounding wire; and have thicker THW or RHW insulation. THWN and THHN have thinner insulation because they also have a Nylon jacket. In general, NEC NM-B wire is not enclosed with other wiring in conduits.

Another common nickname is type UF (underground feeder) which has a nonmetallic protective sheath but uses a moisture- and sunlight-resistant construction suitable for direct burial in the earth, where exposed to sunlight, or in unusually wet, unusually dry, or corrosive locations. "UF" uses a sheath that is a flexible solid about the insulated wires instead of the waxed paper wrapping, is generally heavier, less flexible than NM-B, and is rated against breakdown from harmful ultraviolet light wavelengths commonly encountered in direct sunlight.

Another common type is "AC", a fabricated assembly of insulated conductors in flexible metallic armor, formed by wrapping an interlocking metal strip around the conductors. This is a descendant of type "BX", an early genericized trademark of the General Electric company, used from before and during World War II, designating a particular design of armored cable. The "BX" name is commonly applied loosely to all flexible metal armored cables including styles with individual wires running through a flexible metal conduit of similar appearance.

In Canada, type TECK cable, with a flexible aluminum or steel armor and overall flame-retardant PVC jacket, is used in industry for wet or dry locations, run in trays or attached to building structure, above grade or buried in earth. A similar type of cable is designated type "MC" in the United States.

Mineral-insulated copper-clad cable (type MI) is a fire-resistant cable using magnesium oxide as an insulator. It is used in demanding applications requiring superior heat resistance such as fire alarms and oil refineries.

Flexible cables

All electrical cables are somewhat flexible, allowing them to be shipped to installation sites wound on reels or drums. Where applications require a cable to be moved repeatedly, such as for portable equipment, more flexible cables called "cords" or "flex" are used. Flexible cords contain fine stranded conductors, not solid core conductors, and have insulation and sheaths to withstand the forces of repeated flexing and abrasion. Heavy duty flexible power cords such as those feeding a mine face cutting machine are carefully engineered—their life is measured in weeks. Very flexible power cables are used in automated machinery, robotics, and machine tools. See power cord and extension cable for further description of flexible power cables. Other types of flexible cable include twisted pair, extensible, coaxial, shielded, and communication cable.

See also

References

External links

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