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Ampacity is a portmanteau for ampere capacity defined by National Electrical Codes, in some North American countries. Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Also described as current-carrying capacity.

A portmanteau or portmanteau word is a linguistic blend of words, in which parts of multiple words or their phones (sounds) are combined into a new word, as in smog, coined by blending smoke and fog, or motel, from motor and hotel. In linguistics, a portmanteau is defined as a single morph that represents two or more morphemes.

National Electrical Code

The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA), a private trade association. Despite the use of the term "national", it is not a federal law. It is typically adopted by states and municipalities in an effort to standardize their enforcement of safe electrical practices. In some cases, the NEC is amended, altered and may even be rejected in lieu of regional regulations as voted on by local governing bodies.


The ampacity of a conductor depends on its ability to dissipate heat without damage to the conductor or its insulation. This is a function of the insulation temperature rating, the electrical resistance of the conductor material, the ambient temperature, and the ability of the insulated conductor to dissipate heat to the surrounds.

Thermal insulation technique used to minimize the heating demand of buildings

Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.

All common electrical conductors have some resistance to the flow of electricity. Electric current flowing through them causes voltage drop and power dissipation, which heats conductors. Copper or aluminum can conduct a large amount of current without damage, but long before conductor damage, insulation would, typically, be damaged by the resultant heat.

Joule heating, also known as Ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat.

Copper Chemical element with atomic number 29

Copper is a chemical element with symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Heat energy transfer process, or its amount (and direction), that is associated with a temperature difference

In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The mechanisms include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or isochoric mechanical work done by the surroundings on the system of interest; or Joule heating by an electric current driven through the system of interest by an external system; or a combination of these. When there is a suitable path between two systems with different temperatures, heat transfer occurs necessarily, immediately, and spontaneously from the hotter to the colder system. Thermal conduction occurs by the stochastic (random) motion of microscopic particles. In contrast, thermodynamic work is defined by mechanisms that act macroscopically and directly on the system's whole-body state variables; for example, change of the system's volume through a piston's motion with externally measurable force; or change of the system's internal electric polarization through an externally measurable change in electric field. The definition of heat transfer does not require that the process be in any sense smooth. For example, a bolt of lightning may transfer heat to a body.

The ampacity for a conductor is based on physical and electrical properties of the material and construction of the conductor and of its insulation, ambient temperature, and environmental conditions adjacent to the conductor. Having a large overall surface area can dissipate heat well if the environment can absorb the heat.

In electrical cables different conditions govern, and installation regulations normally specify that the most severe condition along the run will govern each cable conductor's rating. Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation. Derating is necessary for multiple cables in close proximity. When multiple cables are in close proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. The overall ampacity of insulated cable conductors in a bundle of more than three cables must also be derated, whether in a raceway or cable. Usually the derating factor is tabulated in a nation's wiring regulations.

Electrical cable two or more wires running side by side and bonded, twisted, or braided together to form a single assembly

An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current.

An oil is any nonpolar chemical substance that is a viscous liquid at ambient temperatures and is both hydrophobic and lipophilic. Oils have a high carbon and hydrogen content and are usually flammable and surface active.

Electrical conduit

An electrical conduit is a tube used to protect and route electrical wiring in a building or structure. Electrical conduit may be made of metal, plastic, fiber, or fired clay. Most conduit is rigid, but flexible conduit is used for some purposes.

Depending on the type of insulating material, common maximum allowable temperatures at the surface of the conductor are 60, 75, and 90 °C, often with an ambient air temperature of 30 °C. In the United States, 105 °C is allowed with ambient of 40 °C, for larger power cables, especially those operating at more than 2 kV. Likewise, specific insulations are rated 150, 200, or 250 °C.

United States federal republic in North America

The United States of America (USA), commonly known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.

The allowed current in a conductor generally needs to be decreased (derated) when conductors are in a grouping or cable, enclosed in conduit, or an enclosure restricting heat dissipation. e.g. The United States National Electrical Code, Table 310.15(B)(16), specifies that up to three 8 AWG copper wires having a common insulating material (THWN) in a raceway, cable, or direct burial has an ampacity of 50  A when the ambient air is 30 °C, the conductor surface temperature allowed to be 75 °C. A single insulated conductor in free air has 70 A rating.

Ampacity rating is normally for continuous current, and short periods of overcurrent occur without harm in most cabling systems. Electrical code rules will give ratings for wiring where short-term loads are present, for example, in a hoisting motor. For systems such as underground power transmission cables, evaluation of the short-term over-load capacity of the cable system requires a detailed analysis of the cable's thermal environment and an evaluation of the commercial value of the lost service life due to excess temperature rise.

Design of an electrical system will normally include consideration of the current carrying capacity of all conductors of the system.

Some devices are limited by power rating, and when this power rating occurs below their current limit, it is not necessary to know the current limit to design a system. A common example of this is lightbulb holders.

Current rating

For electronic components (such as transistors, voltage regulators, and the like), the term current rating is more-commonly used than ampacity, but the considerations are broadly similar. However the tolerance of short-term overcurrent is near zero for semiconductor devices, as their thermal capacities are extremely small.

See also

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Electrical conductor object or material which permits the flow of electricity

In physics and electrical engineering, a conductor is an object or type of material that allows the flow of an electrical current in one or more directions. Materials made of metal are common electrical conductors. Electrical current is generated by the flow of negatively charged electrons, positively charged holes, and positive or negative ions in some cases.

American wire gauge (AWG), also known as the Brown & Sharpe wire gauge, is a logarithmic stepped standardized wire gauge system used since 1857, predominantly in North America, for the diameters of round, solid, nonferrous, electrically conducting wire. Dimensions of the wires are given in ASTM standard B 258. The cross-sectional area of each gauge is an important factor for determining its current-carrying ampacity.

Electrical wiring in North America follows regulations and standards for installation of building wiring which ultimately provides mains electricity.

Fuse (electrical) type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, and it must be replaced or rewired, depending on type.

In electrical engineering and mechanical engineering, the power rating of equipment is the highest power input allowed to flow through particular equipment. According to the particular discipline, the term "power" may refer to the electrical or mechanical power. A power rating can also involve average and maximum power, which may vary depending on the kind of equipment and its application.

Electrical wiring electrical installation of cabling and associated devices such a switches

Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets and light fittings in a structure.

Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission of electrical power to them.

A power cable is an electrical cable, an assembly of one 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.

In electronics, derating is the operation of a device at less than its rated maximum capability in order to prolong its life. Typical examples include operation below the maximum power rating, current rating, or voltage rating.

Trace heating

Electric heat tracing, heat tape or surface heating, is a system used to maintain or raise the temperature of pipes and vessels. Trace heating takes the form of an electrical heating element run in physical contact along the length of a pipe. The pipe is usually covered with thermal insulation to retain heat losses from the pipe. Heat generated by the element then maintains the temperature of the pipe. Trace heating may be used to protect pipes from freezing, to maintain a constant flow temperature in hot water systems, or to maintain process temperatures for piping that must transport substances that solidify at ambient temperatures. Electric trace heating cables are an alternative to steam trace heating where steam is unavailable or unwanted.

Mineral-insulated copper-clad cable

Mineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro. A similar product sheathed with metals other than copper is called mineral insulated metal sheathed (MIMS) cable.

Thermoplastic-sheathed cable (TPS) consists of an outer toughened sheath of polyvinyl chloride (PVC) covering one or more individual cables which are PVC insulated annealed copper conductors. It is a commonly used type of wiring for residential and light commercial construction in many countries. The flat version of the cable with two insulated conductors and an uninsulated earth conductor all within the outer sheath is referred to as twin and earth. In mainland Europe, a round equivalent is more common.

Knob-and-tube wiring style of electrical wiring

Knob-and-tube wiring is an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the early 1940s. It consisted of single-insulated copper conductors run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators. Where conductors entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom. The first insulation was asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted together for good mechanical strength, then soldered and wrapped with rubber insulating tape and friction tape, or made inside metal junction boxes.

Circuit integrity

Circuit integrity refers to the operability of electrical circuits during a fire. It is a form of fire-resistance rating. Circuit integrity is achieved via passive fire protection means, which are subject to stringent listing and approval use and compliance.

In an electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short-circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.

Magnet wire

Magnet wire or enameled wire is a copper or aluminium wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, speakers, hard disk head actuators, electromagnets, and other applications that require tight coils of insulated wire.

Tinsel wire is a type of electrical wire used for applications that require high mechanical flexibility but low current-carrying capacity. Tinsel wire is commonly used in cords of telephones, handsets, headphones, and small electrical appliances. It is far more resistant to metal fatigue failure than either stranded wire or solid wire.

In electrical engineering, Neher–McGrath is a method of estimating the steady-state temperature of electrical power cables for some commonly encountered configurations. By estimating the temperature of the cables, the safe long-term current-carrying capacity of the cables can be calculated.

Copper conductor

Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.