User:Enviromet/Subpage 5 - Copper in wire and cable

Copper has been used in electric wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 proved to be another early boon for copper wire.

Today, despite competition from other materials, copper remains the preferred electrical conductor in nearly all categories of electrical wiring. 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. Aside from electrical conductors, other important electrical applications for copper include electrical contacts and resistors.

Electrical wiring is the most important market for the copper industry. Roughly half of all copper mined is used to manufacture electrical wire and cable conductors.

Beneficial properties of copper for electrical wire and cable
Nearly all electrical devices rely on copper wiring because of its multitude of inherent beneficial properties. The most useful beneficial properties for electrical applications are summarized here.

Electrical Conductivity
Electrical conductivity is a measure of how well a material transports an electric charge. This is an essential property in electrical wiring systems. Copper has the highest electrical conductivity rating of all non-precious metals (electrical conductivity of copper = 101% IACS (International Annealed Copper Standard); electrical resistivity of copper = 16.78 nΩ•m at 20 °C).

The solid state theory of metals helps to explain the unusually high electrical conductivity of copper. In a copper atom, the outermost 4s energy zone, or conduction band, is only half filled, so many electrons are able to carry electric current. When an electric field is applied to a copper wire, the conduction of electrons accelerates towards the electropositive end, thereby creating a current. These electrons encounter resistance to their passage by colliding with impurity atoms, vacancies, lattice ions, and imperfections. The average distance travelled between collisions, defined as the “mean free path,” is inversely proportional to the resistivity of the metal. What is unique about copper is that it has a very long mean free path (approximately 100 atomic spacings at room temperature). Furthermore, this mean free path increases rapidly as copper is chilled.

Silver, an expensive precious metal, is the only metal with a higher electrical conductivity rating than copper (i.e., electrical conductivity of silver = 106% IACS, electrical resistivity of silver = 15.9 nΩ•m at 20°C). The high cost of silver combined with its low tensile strength limits its use to special applications, such as joint plating and sliding contact surfaces. With the exception of silver, copper conducts electricity with less resistance than any other metallic material.

Because of its superior conductivity, annealed copper became the international standard to which all other electrical conductors are compared. This took place back in 1913 when the International Electro-Technical Commission set the conductivity of copper in its International Annealed Copper Standard (IACS) to 100%. Today, copper conductors used in building wire often exceed the 100% IACS standard.

The main grade of copper used for electrical applications, such as building wire, motor windings, cables and busbars, is electrolytic tough pitch (ETP) copper (CW004A or ASTM designation C100140). This copper is at least 99.90% pure and has an electrical conductivity of at least 101% IACS. ETP copper contains a small percentage of oxygen (0.02 to 0.04%). If high conductivity copper needs to be welded or brazed or used in a reducing atmosphere, then oxygen-free high conductivity copper (CW008A or ASTM designation C10100) may be used.

Several electrically conductive metals are lighter than copper, but since they require larger cross sections to carry the same current, they are unacceptable when limited space is a major requirement.

Aluminum has 61% of the conductivity of copper. The cross sectional area of an aluminum conductor must be 56% larger than copper for the same current carrying capability. The need to increase the thickness of aluminum wire restricts its use in several applications, such as in small motors and automobiles.

Tensile Strength
Tensile strength measures the force required to pull an object such as rope, wire, or a structural beam to the point where it breaks. The tensile strength of a material is the maximum amount of tensile stress it can take before breaking.

Copper’s higher tensile strength (200-250 N/nm2 annealed) compared to aluminum is another reason why copper is used extensively in the building industry. Copper’s high strength resists stretching, neck-down, creep, nicks and breaks, and thereby also prevents failures and service interruptions.

In equipment installations and machinery using non-copper wiring, nicks and scratches due to vibration and flexing can deteriorate into large breaks in the wiring and lead to failure and long-term service interruptions. For example, when long runs of aluminum are pulled through conduit and cable trays, they can stretch and neck-down. These effects reduce current carrying capacity, wastes energy, and can cause overheating. Because of copper’s higher tensile strength, these problems are minimized in copper wire.

Ductility
Ductility is a material's ability to deform under tensile stress. This is often characterized by the material's ability to be stretched into a wire. Ductility is especially important in metalworking because materials that crack or break under stress cannot be hammered, rolled, or drawn (drawing is a process that uses tensile forces to stretch metal).

Copper has a higher ductility than alternate metal conductors with the exception of gold and silver, both expensive precious metals reserved for highly specialized wiring applications. Because of copper’s high ductility, it is easy to draw down to diameters with very close tolerances.

Strength and ductility combination
Usually, the stronger a metal is, the less pliable it is. This is not the case with copper. A unique combination of high strength and high ductility makes copper ideal for wiring systems. At junction boxes and at terminations, for example, copper can be bent, twisted, and pulled without stretching or breaking.

Creep resistance
Creep is the gradual deformation of a material from constant expansions and contractions under “load, no-load” conditions. This process has adverse effects on electrical systems: terminations can become loose, causing connections to heat up or create dangerous arcing. Copper does not creep or loosen at its connections. For other metal conductors that creep, extra maintenance is required to check terminals periodically and ensure that screws remain tightened to prevent arcing and overheating. These extra measures can be avoided with the use of copper wire.

Corrosion resistance
Corrosion is the unwanted breakdown and weakening of a material due to chemical reactions. Copper resists corrosion from moisture, humidity, industrial pollution, and other atmospheric influences. However, any corrosion oxides, chlorides, and sulfides that do form on copper are conductive, not resistive. Therefore, copper connections and terminations will not overheat from corrosion. Aluminum corrosion products, on the other hand, are resistive and therefore can cause unwanted heat. To prevent corrosion and protect joints, special surface preparations or oxide-inhibiting pastes are applied to aluminum. Copper connections do not require these preparations and their associated additional costs.

Electrolytic tough pitch (ETP) copper, which is used in building wire, is a noble metal. It is not subject to galvanic corrosion when connected to other, less noble metals and alloys.

Coefficient of thermal expansion
Metals and other solid materials expand upon heating and contract upon cooling. This is an undesirable occurrence in electrical systems. Copper has a low coefficient of thermal expansion for an electrical conducting material. Aluminum, an alternate common conductor, expands nearly one third more than copper under increasing temperatures. This higher degree of expansion, along with aluminum’s lower ductility can cause electrical problems when bolted connections are improperly installed. By using proper hardware, such as spring pressure connections and cupped or split washers at the joint, it may be possible to create aluminum joints that compare in quality to copper joints.

Thermal conductivity
Thermal conductivity is the ability of a material to conduct heat. In electrical systems, high thermal conductivity is important for dissipating waste heat, particularly at terminations and connections. It is also important for reducing energy consumption due to the generation of waste heat.

Copper has a 60% better thermal conductivity rating than aluminum, so it is better able to reduce thermal hot spots in electrical wiring systems.

Solderability
Soldering is a process whereby two or more metals are joined together by a heating process. This is a desirable property in electrical systems. Some electrical codes require soldered joints. Copper is readily soldered to make durable connections when necessary.

Resistance to electrical overloads
Electrical overloads are sudden unexpected surges that produce unwanted I2r (current2 x resistance) heating. These surges can melt conducting materials with low melting points and create operational or safety problems in electrical systems. Therefore, to withstand electrical overloads, it is important that an electrical conductor have a high melting point.

Copper has a melting point of 1083°C. Because of its high melting point, copper conductors can take heavy overloads or current surges without melting. Of all wiring materials, copper is best able to withstand electrical overloads. Also, due to copper’s creep resistance as described previously, electrical overloads will not loosen copper joints.

Compatibility with electrical insulators
Electrical insulators reduce the flow of electricity. Hence, an insulator on an electric cord prevents electricity from injuring a person who touches the cord. Copper is compatible with all common insulation materials, such as PVC and polyethylene.

Ease of installation
The inherent strength, hardness, and flexibility of copper building wire make it very easy to work with. Copper wiring can be installed simply and easily with no special tools, washers, pigtails, or joint compounds. Its flexibility makes it easy to join, while its hardness helps keep connections securely in place. It has good strength for pulling wire through tight places (“pull-through”), including conduits. It can be bent or twisted easily without breaking. It can be stripped and terminated during installation or service with far less danger of nicks or breaks. And it can be connected without the use of special lugs and fittings. The combination of all of these factors makes it easy for electricians to install copper wire.

Meets electrical codes
Copper wiring, the industry standard, complies with codes, ordinances, and regulations for electrical conductors.

Single wire
Single-strand copper wire, also called solid wire or solid-core wire, consists of one piece of copper metal wire surrounded by an insulator. Single-strand copper conductors are typically used as magnet wire in motors and transformers. They are relatively rigid, do not bend easily, and are typically installed in permanent, infrequently handled, and low flex applications. Single strand copper wires also provide mechanical ruggedness and good protection against the environment.

Stranded wire
A stranded copper wire refers to a group of copper wires that are braided or twisted together. Examples include alternating current line cords for appliances, musical instrument cables, computer mouse cables, connections between circuit boards in multi-printed circuit-board devices, welding electrode cables, control cables connecting moving machine parts, mining machine cables, and trailing machine cables.

A stranded copper wire is more flexible and easier to install than a single-strand copper wire. Stranding also improves wire longevity for applications with moderate to high vibration. A particular cross-section of a stranded conductor gives it essentially the same resistance characteristics as a single-strand conductor, but with added flexibility.

Cable
A copper cable is comprised of two or more copper wires running side by side and bonded, twisted or braided together to form a single assembly. Electrical cables may be made more flexible by stranding the wires.

Copper wires in a cable may be bare or they may be plated to reduce oxidation with a thin layer of another metal, most often tin but sometimes gold or silver. Plating may lengthen wire life and makes soldering easier.

Cables can be made with one or two different types of wire. For example, all-copper cables are used in a wide range of applications, including telecommunications and power distribution. Combination conductor cables, such as copper and steel, are used when increased strength with high conductivity is required over long distances (e.g., several hundred-meter spans), such as for telephone cables or for thin hookups, such as CATV cable.

Some cables are designed to be multi-functional, such as those installed in residences to carry power, telephone, video, and control/communications signals. They are usually made from copper.

Current-carrying cables radiate an electromagnetic field. Cables also pick up energy from any existing electromagnetic fields that are around it. These effects are often undesirable, in the first case amounting to unwanted transmission of energy which may adversely affect nearby equipment or other parts of the same piece of equipment; and in the second case, unwanted pickup of noise which may mask the desired signal being carried by the cable, or, if the cable is carrying power supply or control voltages, cause equipment malfunctions.

Three principal cable designs (shielding, twisted-pair geometry, and coaxial geometry) help to minimize electromagnetic pickup and transmission.

Shielding cables
Shielding cables are encased in foil or wire mesh. The wires inside the shielding are mostly decoupled from external electric fields. Simple shielding is not too effective against low-frequency magnetic fields, resulting, for example, in a magnetic "hum" from a nearby power transformer.

Twisted pair cables
Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together to cancel out electromagnetic interference (EMI) from external sources and reduce signal loss. This is why twisted pairs have been used in telephone communications for many decades.

Coaxial cables
Coaxial cables reduce low-frequency magnetic transmission and pickup. They consist of two or more wires that are wrapped concentrically and separated by a dielectric insulation material. The term, coaxial, was coined because the center conductor and the outer conductor, or shield, form concentric cylinders (i.e., co-axial). This causes voltages induced by a magnetic field between the shield and the core conductor to consist of two nearly equal magnitudes which cancel out each other. The center conductor of a coaxial cable may be a single strand or it may be stranded.

Common conductor materials used in coaxial cables include copper, tinned or silver plated copper, copper clad steel, and copper clad aluminum. Less frequently, aluminum is used as an alternate inner conductor. The outer conductor is typically made from a woven copper wire mesh braid shield layer, or less frequently, aluminum foil. This layer also gives the wire protection from interference. The cables are insulated with a flexible, tubular insulating layer made from polyethylene (PE), polypropylene (PP), fluorinated ethylene propylene (FEP) or polytetrafluoroethylene (PTFE). This is surrounded by a tubular conducting shield (i.e., the dielectric, normally made from foam).

The advantage of coaxial design is that the electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Conversely, electric and magnetic fields outside the cable are largely kept from causing interference to signals inside the cable. This property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the environment or for higher power signals that must not be allowed to radiate or couple into adjacent structures or circuits.

Applications for copper wire and cable
Electrolytic tough pitch (ETP) copper, a high-purity copper that contains oxygen as an alloying agent, represents the bulk of electrical conductor applications because of its high electrical conductivity and improved annealability. ETP copper is used for power transmission, power distribution, and telecommunications. Common applications include building wire, motor windings, cables, and busbars. Oxygen-free coppers are used to resist hydrogen embrittlement when extensive amounts of cold work is needed, and for applications requiring higher ductility (e.g., telecommunications cable). When hydrogen embrittlement is a concern and low electrical resistivity is not required, phosphorous may be added to copper.

For certain applications, copper alloy conductors are preferred instead of pure copper, especially when higher strengths and/or improved abrasion and corrosion resistance properties are required. An example of a copper alloy conductor is cadmium copper trolley wire, which is used for railroad electrification in North America. However, relative to pure copper, the higher strength and corrosion resistance benefits that are offered by copper alloys are offset by their lower electrical conductivities. Design engineers weigh the advantages and disadvantages of the various types of copper and copper alloy conductors when determining which type to specify for a specific electrical application.

Some of the major application markets for copper wire and cable are summarized below.

Building wire
Building wire distributes electric power inside residential, commercial, or industrial buildings, mobile homes, recreational vehicles, boats, and substations at voltages up to 600V. The thickness of the wire is based on ampacity requirements in conjunction with safe operating temperatures. Solid wire is used for smaller diameters; thicker diameters are stranded to provide flexibility. Conductor types include non-metallic/non-metallic corrosion-resistant cable (two or more insulated conductors with a nonmetallic outer sheath), armored or BX cable (cables are surrounded by a flexible metal enclosure), metal clad cable, service entrance cable, underground feeder cable, TC cable, fire resistant cable, and mineral insulated cable.

Copper is considered the material of choice for building wire because of its conductivity, strength, and reliability. Over the life of a building wire system, copper can also be the most economical conductor.

Copper used in building wire has a conductivity rating of 100% IACS or better. Copper building wire requires less insulation and can be installed in smaller conduits than when lower-conductivity conductors are used. Also, comparatively, more copper wire can fit in a given conduit than conductors with lower conductivities. This greater “wire fill” is a special advantage when a system is rewired or expanded.

Copper building wire is compatible with brass and quality plated screws. The wire provides connections that will not corrode or creep. It is not, however, compatible with aluminum wire or connectors. If the two metals are joined, a galvanic reaction can occur. Anodic corrosion during the reaction can disintegrate the aluminum. This is why most appliance and electrical equipment manufacturers use copper lead wires for connections to building wiring systems.

"All-copper" building wiring is a term that refers to homes where the inside electrical service is carried exclusively over copper wiring. In all-copper homes, copper conductors are used in circuit breaker panels, branch circuit wiring (to outlets, switches, lighting fixtures and the like), and in dedicated branches serving heavy-load appliances (such as ranges, ovens, clothes dryers and air conditioners).

Attempts to replace copper with aluminum in building wire were curtailed in most countries when it was found that aluminum connections gradually loosened due to their inherent slow relaxation process (i.e., creep), combined with the fact that aluminum oxidation products are resistive (i.e., generate heat). Spring-loaded contacts have largely alleviated this problem with aluminum conductors in building wire, but some building codes still forbid the use of aluminum.

For branch-circuit sizes, virtually all basic wiring for lights, outlets and switches is made from copper. The market for aluminum building wire today is mostly confined to larger gauge sizes used in supply circuits.

Twisted pair cable
Since the dawn of telephony, telephones have relied on wire pairs. Twisted pair cabling, the most popular network cable today, is often used in data networks for short and medium length connections (up to 100 meters or 328 feet). This is due to its relatively lower costs compared to optical fiber and coaxial cable.

Twisted pair cables can be shielded or unshielded.

Shielded twisted pair (STP) cables are made from copper braid, aluminum braid, or aluminum foil surrounding the conductors, giving it greater resistance to electrical interference than the unshielded twisted pair cable and enabling it to be used in much longer lengths. However, since STP is much heavier than unshielded twisted pair (UTP), is harder to work with, and has a different impedance rating, it is not used for the Ethernet.

Unshielded twisted pair (UTP) cables are the primary cable type for telephone usage. In recent years, UTPs have emerged as the most common cable in computer networking Category cables, especially as patch cables or temporary network connections. They are increasingly used in video applications, primarily in security cameras.

UTP plenum cables (i.e., cables that run above ceilings and inside walls) use a solid copper core for each conductor, which enables the cable to hold its shape when bent. Patch cables, which connect computers to wall plates, use stranded copper wire because they are expected to be flexed during their lifetimes.

UTPs are the best balanced line wires (i.e., they reject noise) available. However, they are also the most prone to interference and the easiest to tap into. When interference and security are concerns, shielded cable or fiber optic cable is often considered.

UTP cables include: Category 3 cable, now the minimum requirement by the FCC (USA) for every telephone connection; Category 5e cable, 100-MHz enhanced pairs for running Gigabit Ethernet (1000Base-T); and Category 6 cable, where each pair runs 250 MHz for improved 1000Base-T performance.

The most modern practice today is to wire buildings with Category 5e UTP cables, which are suitable for transmission up to 100Mbps.

Some Category cables use bonded pairs where the two wires in each pair are joined without glue. This dramatically improves impedance variations, especially after installation, where bending and twisting the cable tend to make a pair open. Cables with individual pairs in their own channel allow tight control of spacing and crosstalk between pairs. Most Category cables require a minimum bend radius of "four times the diameter" when installed.

Coaxial cable
Coaxial cables were extensively used in mainframe computer systems and were the first type of major cable used for Local Area Networks (LAN). Common applications for coaxial cable today include computer network (Internet) and instrumentation data connections, video and CATV distribution, RF and microwave transmission, and feedlines connecting radio transmitters and receivers with their antennas.

Most coaxial cables have a characteristic impedance of 50 Ω, 52 Ω, 75 Ω, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly-used coaxial cable for home use, and the majority of connections outside Europe are by F connectors.

Coaxial cables differ from other shielded cables used for carrying lower frequency signals, such as audio signals, in that the dimensions of the cables are controlled to give a precise, constant conductor spacing, which is needed to function efficiently as a radio frequency transmission line.

Coaxial cables operate at much higher frequencies than UTPs. While 600 MHz might be cutting edge for a twisted pair, a coaxial cable can handle this frequency easily.

While coaxial cables can go longer distances and have better protection from EMI than twisted pairs, coaxial cables are harder to work with and more difficult to run from offices to the wiring closet. For these reasons, it is now generally being replaced with lesser expensive UTP cables for long distances (less than 100 meters or 328 feet) or by fiber optic cables for more capacity.

Limitations in the performance of copper cables are dependent upon size of the cable (e.g., smaller coaxials work better at high frequencies) and size of the wires (e.g., smaller wires have higher losses). Therefore, there are theoretical limitations to both coaxial cables and twisted pairs. Coaxial performance is being approached with new developments in twisted pairs.

Today, many CATV companies still use high performance coaxial cables into homes. These cables, however, are increasingly connected to a fiber optic data communications system outside of the home. Debates ensue regarding copper vs. fiber vs. wireless on premises cabling. This is especially true for Local Area Network (LAN) cabling where both fiber and copper may be viable options.

Most building management systems use proprietary copper cabling (e.g., thermostat wiring), as do paging/audio speaker systems. Security monitoring and entry systems, certainly the lower cost ones, still depend on copper, although higher-security facilities, like government and military installations, use fiber, which is more expensive and more secure.

Structured Wiring (for voice, audio, video, data, controls, and security in homes)
Virtually all telephone lines in modern societies now share voice and data simultaneously. Pre-digital quad telephone wiring in homes is unable to handle communications needs for multiple phone lines, Internet service, video communications, data transmission, fax machines, and security services. Crosstalk, static interference, inaudible signals, and interrupted service are common problems with outdated wiring. Computers connected to old-fashioned communications wiring often experience poor Internet performance.

“Structured wiring” is the general term for today's high-capacity telephone, video, data-transmission, security, control, and entertainment wiring systems. Installations usually include a central distribution panel where all connections are made, as well as outlets with dedicated connections for phone, data, TV and audio jacks.

Structured wiring enables computers to communicate with each other error-free and at high speeds while simultaneously resisting interference among various electrical sources, such as household appliances (e.g., microwave ovens, vacuum cleaners, etc.), fluorescent lights, power tools, and external communications signals. Networked computers are able to share high-speed Internet connections simultaneously. Structured wiring can also connect computers with printers, scanners, telephones, fax machines, and even home security systems and home entertainment equipment.

Copper Category 5 unshielded twisted pair (UTP) wires (i.e., four twisted wire pairs, or eight color coded wires) is the standard for carrying Internet, computer communications, and telephone signals. Category 5 has an approved bandwidth (i.e., information-carrying capacity) of 100 MHz (megahertz), which is many orders of magnitude greater than a modern 56 Kb (kilobits) per second modem. Category 5 is increasingly being supplanted by a higher-speed version, known as Category 5e ("e" for enhanced). Category 6, which will likely accommodate at least twice the bandwidth of Category 5, will be able to carry at least 1 gigabit (billion bits) per second. This equates to about 50,000 pages of text per second.

Quad-shielded RG-6 can service a large number of TV channels at the same time. A star wiring pattern, where the wiring to each jack extends to a central distribution device, facilitates flexibility of services, problem identification, and better signal quality. This pattern has advantages to daisy chain loops. Installation tools, tips, and techniques for networked wiring systems using twisted pairs, coaxial cables, and connectors for each are available.

Structured wiring competes with wireless systems (e.g., wireless telephones), single-channel video transmitters, and wireless computer networks in homes. While wireless systems certainly have convenience advantages, they also have drawbacks over copper-wired systems: the higher bandwidth of systems using Category 5e wiring typically support more than ten times the speeds of wireless systems for faster data applications and more channels for video applications. Alternatively, wireless systems are a security risk as they can easily and unintentionally transmit sensitive information to unintended users over similar receiver devices. Also, wireless systems are more susceptible to interference from other devices and systems, which can compromise performance. And certain geographic areas and some buildings may be unsuitable for wireless installations. Incompatibilities exist among wireless systems, thereby making managing these systems more complicated.

Power distribution
Power distribution is the final stage in the delivery of electricity for an end use. A power distribution system carries electricity from the transmission system to consumers.

Power cables are used for the transmission and distribution of electric power, either outdoors or inside buildings. Details on the various types of power cables are available.

Copper is the preferred conductor material for underground transmission lines operating at high and ultra-high voltages (64V to 400 kV). The predominance of copper underground systems stems from its higher volumetric electrical and thermal conductivities compared to other conductors. These beneficial properties for copper conductors conserve space, minimize power loss, and maintain lower cable temperatures.

Aluminum may be used for medium-voltage underground transmission cables, but these usually contain as much as 50% copper. Copper continues to dominate low-voltage lines in mines and underwater applications, as well as in electric railroads, hoists, and other outdoor services.

Aluminum is the preferred conductor for overhead transmission lines in many countries due to its lighter weight. Copper is, however, used for overhead transmission lines in some regions. Medium- and low-voltage overhead systems in some countries are being converted to copper underground cables.

Appliance wire
Appliance wire for domestic applications and instruments is manufactured from bunch-stranded soft wire, which may be tinned for soldering or phase identification. Depending upon loads, insulation can be PVC, neoprene, ethylene propylene, polypropylene filler, or cotton.

Automotive wire and cable
Automotive wire and cable requires insulation that is resistant to elevated temperatures, petroleum products, humidity, fire, and chemicals. PVC, neoprene, and polyethylene are the most common insulators. Potentials range from 12V for electrical systems to between 300V-15,000V for instruments, lighting, and ignition systems.

Magnet wire (Winding wire)
Magnet wire is used in windings of electric motors, transformers, inductors, generators, headphones, loudspeaker coils, hard drive head positioners, potentiometers, electromagnets, and other devices.

The most suitable materials for magnet wire applications are unalloyed pure metals, particularly copper and aluminum. When factors such as chemical, physical, and mechanical property requirements are considered, copper is considered the first choice conductor for magnet wire.

Most often, magnetic wire is composed of fully-annealed, electrolytically-refined copper to allow closer winding when making electromagnetic coils. The wire is coated with a range of polymeric insulations, including varnish, rather than the thicker plastic or other types of insulation commonly used on electrical wire.

Aluminum magnet wire is sometimes used as an alternative for large transformers and motors. Because of its lower electrical conductivity, aluminum wire requires a 1.6-times larger cross sectional area than a copper wire to achieve comparable DC resistance.

High-purity oxygen-free copper grades are used for high-temperature applications in reducing atmospheres or in motors or generators cooled by hydrogen gas.

Magnet wire in electric motors
Electric motors convert electrical energy into mechanical motion, usually through the interaction of magnetic fields and current-carrying conductors. Electric motors are found in numerous diverse applications, such as fans, blowers, pumps, machines, household appliances, power tools, and disk drives. The very largest electric motors with ratings in the thousands of kilowatts are used in such applications as the propulsion of large ships. The smallest motors move the dials in electric wristwatches.

One of the essential components of electric motors is the coil, which is made from a wound conductor. Electrical conductivity is a key operating parameter in determining which type of material to use in a motor’s coil. Wires made from better electrical conductors result in more efficient transfers of electrical energy into mechanical energy. Poorer conductors generate more waste heat when transferring electrical energy into kinetic energy.

Because of its high electrical conductivity, copper is commonly used in coil windings, bearings, collectors, brushes, and connectors of motors, including the highest quality motors. Copper’s greater conductivity versus other materials enhances the electrical energy efficiency of motors. For example, to reduce load losses in continuous-use induction-type motors above 1 horsepower, manufacturers invariably use copper as the conducting material in windings. Aluminum is an alternate material in smaller horsepower motors, especially when motors are not used continuously.

One of the design elements of premium motors is the reduction of heat losses due to the electrical resistance of conductors. To improve the electrical energy efficiencies of induction-type motors, load loss can be reduced by increasing the cross section of copper coils. A high efficiency motor will usually have 20% more copper in the stator winding than its standard counterpart.

Early developments in motor efficiency focused on reducing electrical losses by increasing the packing weight of stator windings. This made sense since electrical losses typically account for more than half of all energy losses, and stator losses account for approximately two‐thirds of electrical losses.

There are, however, disadvantages in increasing the electrical efficiency of motors through higher packing densities. This invariably results in an increase in motor size which may not be desirable in certain applications, especially in consumer appliances and in the automotive market.

Magnet wire in transformers
A transformer is a device that transfers electrical energy from one circuit to another through its coils (windings). The properties needed for motor windings are similar to those needed for transformers, but with the additional requirement to withstand mechanical vibration and centrifugal forces at operating temperatures.

Transformer windings are normally made from electrolytic tough pitch (ETP) copper but aluminum is a suitable competitor where weight and first cost are decisive factors.

In North America, aluminum is the predominant choice of winding material for low-voltage, dry-type transformers larger than 15 kilovolt-amperes (kVA). In most other areas of the world, copper is the predominant winding material. Purchasing decisions are generally a function of loss valuations expressed in currency per kilowatt.

Copper used for the manufacture of transformer windings is in the form of wire for small products and strip for larger equipment. For small products, the wire must be strong enough to be wound without breakage, yet flexible enough to provide close-packed windings. Strip products must be of good surface quality so that insulating enamels do not break down under voltage. Good ductility is essential for the strip to be formed and packed while good strength is needed to withstand the high electro-mechanical stresses set up under occasional short-circuit conditions. Copper winding wires in transformers are compatible with all modern insulation materials, such as lacquer and enamel. Lacquers permit the close spacing of windings to give best efficiency in the coils.

A major engineering reason to choose copper windings over aluminum is space considerations. This is because a copper-wound transformer can be made smaller than aluminum transformers. To obtain equal ratings in aluminum transformers, a 66% larger cross-sectional area is required than for copper conductors. However, the use of larger-sized conductors results in aluminum winding strength nearly equivalent to copper windings.

Connectivity is another important benefit of copper-wound transformers. Cleaning and brushing with a quality joint compound to prevent oxidation is not necessary with copper.

Magnet wire in generators
The trend in modern generators is to operate at higher temperatures and higher electrical conductivities with oxygen-free copper for field bars and magnetic wire in place of formerly used deoxidized copper.

Some future trends
Copper will continue to be the predominant material in most electrical wire applications, especially where space considerations are important. The automotive industry for decades has considered the use of smaller-diameter wires in certain applications. This might become more of a reality with copper wire in the future.

Due to the need to increase the transmission of high-speed voice and data signals, the surface quality of copper wire is expected to continue to improve. Demands for better drawability and movement towards “zero” defects in copper conductors are expected to continue.

A minimum strength requirement for magnet wire may evolve in order to improve formability and prevent excessive stretching of wire during high speed coiling operations.

It does not seem likely that standards for copper wire will increase beyond the current minimum value of 101% IACS. Although 6-nines copper (99.9999% pure) has been produced in small quantities, it is extremely expensive and probably unnecessary for most commercial applications such as magnet, telecommunications, and building wire. The electrical conductivity of 6-nines copper and 4-nines copper (99.99% pure) is nearly the same at ambient temperature, although the higher-purity copper has a higher conductivity at cryogenic temperatures. Therefore, for non-cryogenic temperatures, 4-nines copper will probably remain the dominant material for most commercial wire applications.