Electrical conductivity is a material property that states how well a material conducts electric current, and it is the reciprocal of electrical resistivity. It is represented by the symbol σ (sigma) or κ (kappa). The unit of electrical conductivity is siemens per metre (S/m). According to the German Copper Institute, annealed copper reaches 57 MS/m, that is 57 million S/m, while a polyimide film such as Kapton HN reaches around 7 · 10⁻¹⁶ S/m according to DuPont's datasheet. Around 24 orders of magnitude separate the best conductor from the best insulating material. This article gives the table for metals, alloys and insulating materials with a source for every value, explains how to convert between S/m, % IACS and Ω·cm, and shows why copper conducts worse when hot while the insulation materials in the same winding become more conductive.
Formula and unit: from siemens per metre to % IACS
The basic equation links current density J and electric field strength E: J = σ · E. In practice the form with resistivity ρ is enough: σ = 1 / ρ. Measuring a conductor of length l and cross-section A, you first calculate the conductance G = I / U from current and voltage and then σ = G · l / A. The SI unit S/m is equivalent to 1 / (Ω · m).
Datasheets rarely state conductivity in plain S/m. Metal producers quote MS/m or the percentage of the International Annealed Copper Standard (IACS), water analysis works in µS/cm, and manufacturers of insulating materials give volume resistivity in Ω·cm. The table translates the common figures:
| Figure in the datasheet | Conversion to S/m | Typical for |
|---|---|---|
| 1 MS/m | 10⁶ S/m, numerically equal to 1 m/(Ω·mm²) | copper, aluminium, brass |
| 100 % IACS | 58 · 10⁶ S/m | reference value for annealed copper |
| 1 S/cm | 100 S/m | electrolytes, semiconductors |
| 1 µS/cm | 10⁻⁴ S/m | drinking water, process water |
| ρ in Ω·cm | σ = 100 / ρ | volume resistivity of films and papers |
| ρ in Ω·mm²/m | σ = 10⁶ / ρ | conductor materials, wires |
The last row explains why copper values are easy to remember: a copper wire 1 m long with a cross-section of 1 mm² has a resistance of 0.0175 Ω at 57 MS/m. The Copper Institute gives exactly this resistivity of 0.018 Ω·mm²/m for Cu-ETP. Besides σ and κ, older textbooks also use γ (gamma) as the symbol, and it always means the same quantity.
What is the difference between conductance and conductivity?
Conductance G describes a specific component, conductivity σ describes the material. G is given in siemens (S) and depends on geometry: a wire twice as long has half the conductance, a wire twice as thick has double. Conductivity is independent of that and is the same for every piece of Cu-ETP. The 1 mm² wire from the example above has a conductance of 57 S at 1 m length and only 5.7 S at 10 m, while its conductivity stays at 57 MS/m. Specific conductance and conductivity are two names for the same material property.
Electrical conductivity table: metals, alloys and insulating materials
The table puts conductors and insulating materials on one common scale, all values at 20 to 25 °C. For insulating materials, the volume resistivity from the manufacturer's datasheet has been converted to S/m using the formula above.
| Material | Conductivity in S/m | IACS share | Source |
|---|---|---|---|
| Silver | 61 to 63 · 10⁶ | around 106 % | Wikipedia, Studyflix |
| Copper Cu-ETP, annealed | 57 · 10⁶ (minimum) | around 98 % | German Copper Institute |
| Copper Cu-ETP, cold worked | 55 to 57 · 10⁶ | 95 to 98 % | German Copper Institute |
| Copper Cu-DHP (phosphorus deoxidised) | 47 · 10⁶ | 81 % | KME |
| Gold | 41 to 45 · 10⁶ | 71 to 78 % | Wikipedia, Studyflix |
| Aluminium EN AW-1350 | 35.5 to 37 · 10⁶ | 61 to 62 % | Aerumtec, Rapid Protos |
| Aluminium 6061-T6 | 23 to 25 · 10⁶ | 40 to 43 % | Rapid Protos |
| Brass CuZn37, soft | at least 16 · 10⁶ | at least 28 % | Aurubis |
| Iron | 10 · 10⁶ | around 17 % | Wikipedia, Formfedern |
| Stainless steel 1.4301 (304) | 1.4 · 10⁶ | around 2.4 % | Studyflix, Wikipedia |
| Graphite, parallel to the layers | 2 to 3 · 10⁵ | below 1 % | Wikipedia |
| Seawater | around 5 | Wikipedia | |
| Tap water | 5 · 10⁻³ to 5 · 10⁻² | Wikipedia | |
| Silicon, pure | 4 to 5 · 10⁻⁴ | Wikipedia | |
| Ultrapure water | 5.5 · 10⁻⁶ | Wikipedia | |
| Glass | 10⁻¹⁵ to 10⁻¹¹ | Wikipedia | |
| Aramid paper Nomex 410, 50 % relative humidity | 5 · 10⁻¹⁵ | Arclin (2 · 10¹⁶ Ω·cm) | |
| Polyimide film Kapton HN, 25 µm | 7 · 10⁻¹⁶ | DuPont (1.5 · 10¹⁷ Ω·cm) | |
| Polyester film Mylar (PET) | 10⁻¹⁶ | DuPont (10¹⁸ Ω·cm) | |
| PTFE | below 10⁻¹⁶ | Wikipedia |
Anyone comparing several tables finds copper values between 57 and 64.8 MS/m. The spread has three causes. First the reference temperature: Seilnacht measures at 0 °C and arrives at 64.8 MS/m, most sources refer to 20 °C. Second the temper: cold working lowers the conductivity of Cu-ETP to 55 to 57 MS/m. Third the grade: just 0.015 to 0.04 % phosphorus pushes Cu-DHP down to 47 MS/m. For an enquiry that means: state the material number and temper, not just the word copper.
The values by material and what they mean in practice
Silver
At 61 to 63 MS/m silver is the best metallic conductor, around 6 % better than copper. As a solid conductor it almost never pays off because of the price. It sits as a thin layer on contacts and as a coating on conductive fabrics used in EMC shielding.
Copper
The electrical conductivity of copper for the electrical grade Cu-ETP is at least 57 MS/m in the annealed state, and 100 % IACS corresponds to 58 MS/m. This value applies to electrolytically refined, oxygen-bearing copper with at least 99.9 % Cu. The phosphorus deoxidised grade Cu-DHP is easier to braze and weld but conducts only 47 MS/m. KME itself states that it is used where no high demands are placed on conductivity. For busbars, windings and copper foil, Cu-ETP is the standard choice.
Aluminium
Conductor-grade aluminium EN AW-1350 reaches 35.5 to 37 MS/m, that is 61 to 62 % of copper. For the same resistance an aluminium conductor therefore needs around 1.6 times the cross-section, but because of its density of 2.7 instead of 8.93 g/cm³ it weighs only about half as much. Alloys lose considerably: 6061-T6 sits at 23 to 25 MS/m. An anodised surface insulates, so contact faces must stay bare. What this means for motor and transformer windings is covered in the comparison of copper vs aluminium windings.
Brass and bronze
Zinc and tin sharply reduce the conductivity of copper. Aurubis gives at least 16 MS/m for CuZn37 in the soft state, around a quarter of pure copper. Brass is chosen for connectors and screw terminals because of strength and machinability. The current travels short distances there, so the lower conductivity costs little.
Iron and stainless steel
Pure iron conducts at around 10 MS/m, about one sixth as well as copper. Austenitic stainless steel 1.4301 drops to around 1.4 MS/m because chromium and nickel disturb the crystal lattice. Stainless steel is therefore the wrong choice for contact faces and busbars, even where it is electrically bonded as a housing material.
Water
The electrical conductivity of water depends almost entirely on the dissolved salts. Ultrapure water sits at 5.5 · 10⁻⁶ S/m, tap water at 5 · 10⁻³ to 5 · 10⁻² S/m and seawater at around 5 S/m. For insulating parts, water is a disturbance: even a film of moisture on the surface lowers the surface resistance of an insulating material by orders of magnitude.
Why copper conducts worse when hot and insulating materials better
In metals, free electrons carry the current. As temperature rises, the atoms in the lattice vibrate more strongly and slow the electrons down. The Copper Institute shows the curve for annealed Cu-ETP:
| Temperature | Conductivity Cu-ETP | compared with 20 °C |
|---|---|---|
| −100 °C | 110 MS/m | almost double |
| 20 °C | 57 MS/m | reference |
| 100 °C | 43 MS/m | a quarter less |
| 200 °C | 33 MS/m | 42 % less |
| 300 °C | 24 MS/m | 58 % less |
Between −100 and 200 °C this can be calculated with the temperature coefficient: ρ(T) = ρ₂₀ · (1 + α · (T − 20 °C)), with α = 0.00393 per kelvin for annealed and 0.00381 for cold worked copper. A motor winding designed at 20 °C and running at 100 °C has around 31 % more resistance and generates correspondingly more heat according to Joule's law.
Insulating materials behave the other way round, and that is the figure no school table shows. They have hardly any free charge carriers, and the small residual current comes from ions and impurities that become more mobile with heat. DuPont gives a volume resistivity of 10¹⁸ Ω·cm at 25 °C for Mylar polyester film and 10¹³ Ω·cm at 150 °C. Conductivity therefore rises by five orders of magnitude while copper loses a third over the same range. Kapton HN is specified at a minimum of 10¹² Ω·cm at 200 °C. Humidity acts the same way: according to Arclin, Nomex 410 falls from 6 · 10¹⁶ Ω·cm oven dry to 2 · 10¹⁴ Ω·cm at 96 % relative humidity.
One rule follows for design work: compare insulating materials at operating temperature and real humidity. The room temperature value from a table flatters every material. That is where testing measures the insulation resistance of a winding, and that is where the thermal class decides which material still has enough reserve.
Electrical conductivity of plastics and insulating materials
Hardly any manufacturer states the conductivity of plastics directly. The datasheet gives volume resistivity, measured to ASTM D257 on a specimen between two electrodes with a guard ring, and separately the surface resistance in Ω. To classify antistatic and conductive materials, the ESD standard IEC 61340 uses three resistance ranges:
- conductive below 10⁵ Ω, such as metals and conductive polymers
- dissipative from 10⁵ to 10¹¹ Ω, the classic ESD range for packaging and mats
- insulating above 10¹¹ Ω, meaning all standard plastics, glass and ceramics
According to DuPont, Mylar film reaches a surface resistance of 10¹⁶ Ω at 30 % relative humidity and 10¹² Ω at 80 %. It therefore stays above the limit even when damp, but with only one order of magnitude to spare. Why non-polar plastics such as PE and PTFE insulate better than polar ones such as polyamide, and how strongly absorbed water lowers resistance, is covered in the article on electrically insulating plastics.
How is electrical conductivity measured?
The method depends on which end of the scale the material sits at, because resistance changes by a factor of 10²⁴ between copper and Kapton.
- Metals and metal foils are measured with the four-wire method: two contacts feed the current in, two separate contacts pick up the voltage, so the contact resistance of the clamps does not distort the result.
- Thin layers and semiconductor wafers are measured by the van der Pauw method with four contacts on the edge of a specimen of any shape.
- Insulating materials are measured as volume resistivity with DC voltage and a guard ring electrode to ASTM D257 and converted to Ω·cm from the area and thickness of the specimen.
- Water and solutions are measured with a conductivity cell in µS/cm and corrected to the reference temperature of 25 °C, because electrolytes conduct better as temperature rises.
Conductivity and foil thickness: the skin effect in copper foil
With alternating current, the conductor's own magnetic field pushes the current towards the surface. The skin depth, at which current density has fallen to 37 %, decreases with frequency. For copper, ADT-Audio gives 9.4 mm at 50 Hz, 2.1 mm at 1 kHz, 0.21 mm at 100 kHz, 66 µm at 1 MHz and 21 µm at 10 MHz. Because skin depth falls with the square root of frequency, the calculated value at 100 MHz is around 6.6 µm.
That is the yardstick for choosing shielding foils. A 35 µm copper foil is thinner than one skin depth at 1 MHz but five times as thick at 100 MHz. Sheet resistance, the resistance of a square of foil of any size, follows from 1 / (σ · thickness): at 57 MS/m that is 0.5 mΩ for 35 µm and 2.9 mΩ for 6 µm. GOBA slits copper foil from 6 to 200 µm for EMC shielding, and the question of the right thickness always starts with the frequency to be shielded against.
Thermal and electrical conductivity are linked in metals
In metals, the same free electrons carry current and heat. The Wiedemann-Franz law puts this into one number: the ratio of thermal conductivity λ to electrical conductivity σ, divided by temperature, gives the Lorenz number of 2.1 to 2.9 · 10⁻⁸ W·Ω/K². For Cu-ETP with 394 W/(m·K) and 57 MS/m at 293 K the result is 2.4 · 10⁻⁸, right within the band. Knowing the electrical conductivity of a metal therefore gives a good estimate of its thermal conductivity.
Insulating materials break this rule. Kapton HN still conducts heat measurably at 0.20 W/(m·K), while electrically it sits at 10⁻¹⁶ S/m. In plastics, lattice vibrations carry the heat, and there are hardly any free electrons. An insulating film can therefore pass heat on and still block the voltage.
Both ends of the scale on one slitting line
GOBA processes materials from both ends of the table, often for the same component. In battery film slitting, current collector foils of copper at 6 to 20 µm and aluminium at 10 to 25 µm run alongside the insulating films that separate cells and modules. In EMC shielding film slitting, copper, aluminium and nickel foils and copper-nickel laminates are added. Slitting widths range from 5 to 1,600 mm with a tolerance of plus minus 0.1 mm, wound onto cores of 25, 55, 76 or 152 mm.
With conductive foils, the edge decides. A burr that would be harmless on a polyester film can later trigger a short circuit on copper foil or disturb the contact of a shield, so we run thin metal foils at controlled web tension. One objection we often hear: conductivity only matters for conductors. For insulating parts it matters just as much, only at the other end of the scale. Anyone designing slot insulation or a cell separator should therefore look for the volume resistivity at operating temperature and humidity in the datasheet.
GOBA takeaway
Electrical conductivity separates conductors and insulating materials by 24 orders of magnitude, and the two sides react in opposite ways to heat: copper loses a quarter from 20 to 100 °C, while PET film becomes a hundred thousand times more conductive between 25 and 150 °C. Anyone taking values from tables needs grade, temper and reference temperature, otherwise the figures scatter by more than 10 %.
Send us material, thickness and operating conditions, and we will slit copper and aluminium foil to size through contract slitting or produce insulating parts from our insulation materials, with the datasheet values that fit your application.
