Copper Material

What Copper Is and Why It Matters

Copper is a face-centred-cubic metal, element 29, with a melting point of 1084.62 degrees Celsius and a density of 8.96 grams per cubic centimetre (0.324 pounds per cubic inch). Its defining engineering virtue is conductivity. Annealed copper is the worldwide benchmark for electrical conductivity, fixed in 1913 as the International Annealed Copper Standard at a resistivity of 0.017241 ohm-square-millimetre-per-metre at 20 degrees Celsius, which is 100 percent IACS. Only silver beats it among common metals, and silver costs roughly one hundred times more. Copper also conducts heat exceptionally well, around 385 to 401 watts per metre-kelvin for the high-purity grades, which is why it dominates heat exchangers, cold plates, and bus connections.

Beyond conductivity, copper carries a balanced set of properties that explain its ubiquity. Its modulus of elasticity is about 117 gigapascals, its linear coefficient of thermal expansion is roughly 16.5 to 16.9 micrometres per metre-kelvin, and in the soft annealed state it is highly ductile, with elongation reaching 45 to 55 percent. That ductility lets copper be drawn into fine wire, deep-drawn into vessels, and rolled into thin foil. Copper is also naturally corrosion resistant in fresh water, in many atmospheres, and against a wide range of organic media, forming a protective patina rather than flaking rust the way bare steel does.

Copper has one further property that no other structural metal shares at scale: it is registered antimicrobial. In 2008 the United States Environmental Protection Agency registered antimicrobial copper alloys as capable of killing more than 99.9 percent of certain bacteria, including MRSA and Escherichia coli, on contact within hours, and the Copper Development Association has since registered several hundred qualifying alloys. This drives copper and copper-alloy touch surfaces in hospitals and food processing.

The history of industrial copper is the history of electrification. Copper has been worked for more than ten thousand years, but its modern role began with the telegraph and the dynamo in the nineteenth century, when copper wire became the nervous system of the electrical grid. The twentieth century added refined metallurgy: electrolytic refining produced tough-pitch copper of 99.9 percent purity, oxygen-free melting under reducing atmospheres pushed purity to 99.99 percent for electronics, and deoxidation with phosphorus solved the brazing-crack problem in plumbing tube. Today copper, aluminium, and their alloys carry essentially all of the world's electrical current.

Because the same element appears in so many forms, the engineering discipline is selection, not metallurgy. The questions that decide a copper purchase are how much oxygen the part can tolerate, how much strength it needs, what form it ships in, and which standard the customer or code requires. The remaining chapters answer each in turn.

Copper Grade Families

Copper grades are identified by the Unified Numbering System (UNS), where pure and high-copper materials occupy the C10000 to C19999 band. The first practical distinction is oxygen content, because oxygen left dissolved in copper as cuprous oxide is harmless during service but causes hydrogen embrittlement during brazing, welding, or reducing-atmosphere heating. The grades below cover the great majority of industrial copper procurement; the table compares the four most-specified pure grades plus the two leading high-strength copper alloys.

C11000 ETP (electrolytic tough pitch) is the workhorse. It is 99.90 percent minimum copper with up to roughly 0.04 percent oxygen present as finely dispersed cuprous oxide. That oxide is metallurgically harmless in normal use and even getters trace impurities, so ETP rates at or above 100 percent IACS, is cheap, and is the default for busbar, enclosed busway systems, grounding strap, building power cable, and power transformer windings. Its single weakness is hydrogen embrittlement above roughly 370 degrees Celsius in a reducing atmosphere, which rules it out of flame-brazed assemblies.

C10200 OF and C10100 OFE (oxygen-free grades) are melted under a reducing graphite cover so essentially no oxygen remains. C10200 holds oxygen near 0.001 percent at 99.95 percent purity, and C10100 reaches 99.99 percent purity with oxygen near 0.0005 percent. Removing the oxide eliminates hydrogen embrittlement and outgassing, which is why these grades are mandatory for vacuum electron tubes, brazed and welded conductors, cryogenic and superconductor leads, and high-reliability semiconductor components. They cost 10 to 30 percent more than ETP for no conductivity gain in ordinary service, so they are specified only when the oxide is genuinely a problem.

C12200 DHP (deoxidized high phosphorus) trades conductivity for weldability. It is deoxidized with a residual 0.015 to 0.040 percent phosphorus, which scavenges oxygen and prevents braze cracking, but the dissolved phosphorus scatters electrons and drops conductivity to about 85 percent IACS. Because plumbing and refrigeration joints are brazed, DHP is the standard material for water tube and air-conditioning tube, where corrosion resistance and joint integrity matter far more than conductivity.

High-strength copper alloys exist because pure copper cannot be hardened by heat treatment. Beyond the precipitation-hardening grades below, copper also forms large families of wrought alloys, including tin bronze for bearings and the copper-zinc brasses, which trade conductivity for strength and machinability. Chromium copper C18200 and chromium-zirconium copper C18150 are precipitation-hardening alloys that keep 80 to 85 percent IACS conductivity while resisting softening at the high temperatures of resistance-welding electrodes and switchgear contacts. Beryllium copper C17200 (Alloy 25), with roughly 1.8 to 2.0 percent beryllium, age-hardens to ultimate tensile strength above 1380 MPa and hardness near Rockwell C45, at the cost of conductivity falling to about 22 percent IACS, making it the standard for springs, the spring contacts inside electrical connectors and a terminal block, and non-sparking safety tools.

Tempers and Mechanical Conditions

A copper grade fixes chemistry but not strength. Strength comes from temper, the degree of cold work or heat treatment, coded under ASTM B601. Before 1974 tempers were named soft, quarter hard, half hard, full hard, extra hard, and spring; ASTM B601 replaced these with alphanumeric codes that appear on every modern mill certificate. Specifying a grade without a temper is incomplete: the same C11000 strip can ship soft and formable or hard and self-supporting, and the difference in tensile strength is roughly a factor of two.

O-series annealed tempers are produced by heating the metal until cold-work stresses recrystallise and disappear. O60 (soft) is the most ductile standard condition, with elongation of 45 percent or more, and it is the right choice whenever the part must be bent, deep-drawn, swaged, or formed after delivery. Annealed copper is also the IACS reference state, so it carries the highest conductivity. The penalty is low yield strength: a soft busbar sags under its own weight and must be supported.

H-series cold-worked tempers are produced by rolling, drawing, or stretching the metal after a final anneal. In the ASTM B601 code, the digits indicate increasing reduction: H01 quarter hard, H02 half hard, H03 three-quarter hard, H04 hard, H06 extra hard, and H08 spring. Each step raises tensile and yield strength while cutting elongation and, very slightly, conductivity. Hard tempers are chosen when the part must hold its shape under mechanical or electromagnetic load, such as a self-supporting busbar between switchgear sections inside a motor control center, or a stamped contact that must resist bending.

Heat-treated tempers (T-series) apply only to the precipitation-hardening alloys, chiefly beryllium copper and chromium copper. The TB00 condition is solution-annealed and soft for forming, after which an aging treatment (TF00 and related codes) precipitates fine strengthening particles and develops the full strength. This sequence is what lets beryllium copper springs be formed soft and then hardened to near-spring-steel strength while retaining usable conductivity. Pure copper grades cannot be heat-treated to higher strength; their only path to strength is cold work.

One practical caution: temper is not permanent. Any cold-worked copper that is later heated above its recrystallisation range, by brazing, welding, or service temperature, will anneal back toward soft and lose its strength locally. A hard busbar brazed at a joint will be soft for some distance around the joint. When a part sees both heat and load, account for the softened zone in the mechanical design rather than assuming the as-delivered temper survives fabrication.

Product Forms and Standards

Copper ships as bar, plate, sheet, strip, rod, wire, tube, and foil, and each form has its own dimensional standard. A complete purchase order names both the grade and the product-form specification, because the same C11000 can arrive as a busbar to ASTM B187 or as a sheet to ASTM B152, with entirely different tolerances, edge conditions, and tests. The table below maps the common forms to their governing ASTM and EN standards.

Bar and busbar for electrical conductors are bought to ASTM B187 in the ASTM world and EN 13601 in Europe. These standards fix cross-section tolerance, corner radius, and the electrical and mechanical property minimums for the grade and temper. Busbar is most often C11000 ETP because conductivity is the point and the bar is bolted, not brazed, so hydrogen embrittlement is irrelevant. Where the bar will be brazed into an assembly, the specifier switches to oxygen-free C10200 to B187.

Sheet, strip, and plate are bought to ASTM B152, which covers copper sheet, strip, plate, and rolled bar across the same grades. Sheet and strip feed stamping, roofing, gaskets, EMI shielding, and the deep-drawing operations that demand soft O60 temper, while plate feeds machined bus connections and ground plates. The European equivalents are EN 1652 for general purposes and EN 13599 for electrical sheet and strip.

Tube splits cleanly by service. Seamless copper bus pipe and tube for electrical current goes to ASTM B188 in hard or annealed temper. Plumbing water tube goes to ASTM B88 in three wall thicknesses, type K (thickest), type L (medium), and type M (thinnest), all in deoxidized C12200 because the joints are soldered or brazed. Air-conditioning and refrigeration tube goes to ASTM B280, also in C12200, cleaned and capped for refrigerant service. Specifying ETP for any brazed tube is an error that produces cracked joints.

Across all forms, the grade-to-grade mapping between systems is worth memorising for cross-region sourcing: C11000 is Cu-ETP (CW004A), C10200 is Cu-OF (CW008A), C10100 is Cu-OFE (CW009A), and C12200 is Cu-DHP (CW024A). A datasheet that lists only the EN designation can be translated directly to the UNS grade and the matching ASTM product standard, and vice versa, which prevents the common mistake of cross-ordering an incompatible grade.

Key Specification Parameters

Reading a copper mill certificate or datasheet is a core procurement skill. Across grades, six parameters drive the selection decision: electrical conductivity, chemistry and oxygen content, temper and mechanical strength, thermal conductivity, corrosion behaviour, and dimensional tolerance. Each is explained below, with the physical reason it matters.

Electrical conductivity is quoted in percent IACS or, equivalently, in MS/m or as resistivity in nano-ohm-metres. The high-purity grades C10100 and C10200 reach 101 percent IACS, C11000 holds at or above 100 percent, deoxidized C12200 falls to about 85 percent, chromium copper C18200 runs 80 to 85 percent, and aged beryllium copper C17200 is only about 22 percent. Conductivity sets conductor cross-section and resistive heating directly, so a half-percent difference rarely matters but a drop from 100 to 22 percent IACS changes the entire conductor design.

Chemistry and oxygen content determine fabrication behaviour more than service behaviour. The minimum copper percentage (99.90 for ETP and DHP, 99.95 for OF, 99.99 for OFE) and the oxygen level decide whether the part can be brazed or welded without hydrogen embrittlement. A certificate that lists copper plus silver content and an oxygen maximum, or a residual phosphorus figure for DHP, tells you immediately which fabrication processes are safe.

Temper and mechanical strength are read together. The temper code (O60, H02, H04, and so on) maps to a tensile-strength and elongation band, and the certificate should list the measured tensile, yield, elongation, and often hardness. Soft tempers offer high elongation for forming; hard tempers offer high yield strength for load-bearing parts. For precipitation-hardening alloys, the certificate also states the heat-treatment condition, which dominates the properties.

Thermal conductivity tracks electrical conductivity through the Wiedemann-Franz relationship, so high-purity copper reaches 385 to 401 watts per metre-kelvin, while alloying lowers it in proportion to the conductivity drop. This parameter governs heat-exchanger, cold-plate, and bus-joint thermal design. The linear thermal expansion coefficient, about 16.5 to 16.9 micrometres per metre-kelvin, matters for assemblies that bolt copper to dissimilar metals, where differential expansion can loosen joints over thermal cycles.

Corrosion behaviour and dimensional tolerance close out the list. Copper resists fresh water, steam, many atmospheres, and a wide range of organics, forming a protective patina, but it is attacked by oxidising acids, ammonia (which causes stress-corrosion cracking), and some sulphur compounds, so for those media a more resistant material such as stainless steel is preferred. Dimensional tolerance is set by the product-form standard: thickness, width, straightness, and for busbar the corner radius and edge condition. A part that meets the chemistry and temper but misses the tolerance band will still be rejected at incoming inspection, so the product-form standard is as load-bearing as the grade.

Selection Decision Factors

To turn the preceding chapters into a specific order, follow the decision sequence below. Most copper selection mistakes are not exotic; they are ordering ETP where oxygen-free or DHP was required, or omitting the temper, or naming a grade without a product-form standard. The seven steps below can serve as a fixed RFQ template.

1. Function first, conductor or structure: Decide whether the part carries current and heat (favour conductivity, choose C11000 or oxygen-free) or carries mechanical load (favour strength, consider chromium or beryllium copper). This single split eliminates most of the grade catalogue immediately.
2. Fabrication method: If the part will be flame-brazed, welded, or heated in a reducing atmosphere, do not use ETP. Choose oxygen-free C10200 for conductors or deoxidized C12200 for tube. If the part is only bolted, machined, or stamped, ETP is fine and cheaper.
3. Conductivity target: Set the minimum IACS the design needs. If 100 percent IACS is required, you are limited to pure grades; if 80 to 85 percent is acceptable in exchange for strength, the chromium-copper family opens up; if conductivity is secondary to spring force, beryllium copper becomes viable at about 22 percent IACS.
4. Strength and temper: Choose the temper from ASTM B601 that meets the mechanical requirement. Soft O60 for parts formed after delivery, hard H04 or spring H08 for self-supporting or load-bearing parts, and a solution-plus-aged condition for the precipitation-hardening alloys.
5. Product form and standard: Name the form (bar, sheet, strip, rod, wire, tube, foil) and cite the governing standard: ASTM B187 or EN 13601 for busbar, ASTM B152 or EN 1652 for sheet, ASTM B88 for water tube, ASTM B280 for ACR tube. The standard fixes dimensions, tolerances, and tests.
6. Corrosion and service environment: Verify the media against copper's limits. Avoid ammonia and oxidising acids, confirm water chemistry for plumbing tube, and where surface hygiene matters consider EPA-registered antimicrobial copper alloys for touch surfaces.
7. Total cost of ownership: Weigh the copper premium against alternatives. Copper costs more per kilogram than aluminium alloy, but its higher conductivity reduces conductor cross-section and joint resistance, and its longer service life and lower running loss often make it cheaper over the lifecycle of a busbar or winding.

One last dimension is commonly overlooked: traceability and supply. A copper order should carry a mill test certificate to EN 10204 3.1 stating the heat number, chemistry, temper, and measured properties, so the grade and condition can be proven at incoming inspection and audited later. For long-running production it also matters whether the supplier can hold consistent temper and tolerance lot to lot, and whether a second source exists for the same grade and standard, because a mid-program substitution of ETP for an oxygen-free part can pass a paperwork check yet crack at the braze line.