Why is copper used for wiring rather than silver?

Silver is 8% more conductive, but it costs roughly 70 times as much and tarnishes. Copper is the practical optimum: it is the second-best metallic conductor, it is abundant and workable, and its oxide is conductive enough not to ruin a connection. Silver is used only where the extra conductivity or the oxidation resistance justifies the cost — high-frequency contacts and switchgear.

Is aluminium a worse conductor than copper?

Per unit of cross-section, yes — aluminium is 61% as conductive, so it needs about 1.6 times the area. Per unit of weight aluminium wins: it is 30% of copper's density, so the same mass of aluminium carries roughly twice the current of the same mass of copper. That is why overhead transmission lines are aluminium, often with a steel core for strength, while building wiring is copper.

The International Annealed Copper Standard defines 100% IACS as a conductivity of 5.800 × 10⁷ S/m, which corresponds to a resistivity of 1.7241 × 10⁻⁸ Ω·m at 20 °C. It is a fixed reference value, not a rating of any particular copper product, and conductivity better than 100% is normal — pure silver reaches about 108% IACS, and oxygen-free high-conductivity copper slightly exceeds 100%.

Does bending or working a conductor change its resistance?

Yes, slightly. Cold working a metal increases its resistivity by disrupting the crystal lattice — hard-drawn copper is about 3% more resistive than annealed copper, which is why the two appear as separate rows in the table. Annealing restores the conductivity. The effect is small compared with temperature, but it matters in precision work and in sizing flexible conductors made of many fine strands.

Why does stainless steel make a poor conductor?

Its resistivity is about 42 times copper's, because chromium and nickel alloying elements scatter electrons heavily and the alloy's crystal structure is far from a pure metal's. That is exactly what makes it useful as a heating element or a resistance wire — high resistance converts current into heat — and exactly why it is never used for power distribution.

Electrical Resistivity Chart

Resistivity, conductivity as %IACS and temperature coefficient for 26 metals, alloys, electrolytes and insulators — ordered from the best conductor down.

Data verified 2026-09-29 · based on n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C

Quick Answer

Silver is the best conductor at 1.59 × 10⁻⁸ Ω·m, with annealed copper (100% IACS) a close second at 1.7241 × 10⁻⁸ Ω·m. Aluminium is 61% as conductive as copper but a third of the density, so an aluminium conductor of the same weight carries more current. Resistivity roughly doubles for every 10 °C rise in copper.

Find a material

Electrical Resistivity of Materials

Material^[3]	Class^[3]	Resistivity Ω·m^[3]	Resistivity Ω·mm²/m^[2]	Conductivity % IACS^[1]	Temp. coefficient /°C^[3]	Note^[3]
Silver #	Pure metal	1.59e-08	0.0159	108.4	0.0038	The best conductor of any metal; too costly for bulk wiring
Copper (annealed, 100% IACS) #	Pure metal	1.724e-08	0.01724	100	0.00393	The reference point of the IACS scale
Copper (hard-drawn) #	Pure metal	1.77e-08	0.0177	97.41	0.00393	About 3% more resistive than annealed, from work hardening
Gold #	Pure metal	2.44e-08	0.0244	70.66	0.0034	Used for contacts because it does not oxidise
Aluminium #	Pure metal	2.65e-08	0.0265	65.06	0.00429	61% of copper's conductivity, but 30% of its density
Aluminium alloy 6061-T6 #	Alloy	3.7e-08	0.037	46.6	0.0039	Alloying costs about 40% of pure aluminium's conductivity
Tungsten #	Pure metal	5.6e-08	0.056	30.79	0.0045	Used for lamp filaments because of its melting point, not its conductivity
Zinc #	Pure metal	5.9e-08	0.059	29.22	0.0037	Galvanising coating
Brass (70/30) #	Alloy	7e-08	0.07	24.63	0.0015	Alloying cuts conductivity sharply
Nickel #	Pure metal	6.99e-08	0.0699	24.67	0.006	Basis of resistance alloys
Iron (pure) #	Pure metal	9.71e-08	0.0971	17.76	0.0065	Much more resistive than copper
Carbon steel #	Alloy	1.43e-07	0.143	12.06	0.006	About 8× copper — why steel is not used for conductors
Stainless steel 304 #	Alloy	7.2e-07	0.72	2.395	0.00094	42× copper; used for heating elements and for its corrosion resistance
Nichrome (Ni-Cr 80/20) #	Alloy	1.1e-06	1.1	1.567	0.0004	64× copper and heat resistant — the standard heating element alloy
Kanthal A-1 (Fe-Cr-Al) #	Alloy	1.45e-06	1.45	1.189	5e-05	Very low temperature coefficient; furnace elements
Mercury #	Liquid metal	9.58e-07	0.958	1.8	0.0009	56× copper; the basis of older thermometers and relays
Graphite (bulk) #	Non-metal	7e-06	7	0.2463	-0.0005	Negative temperature coefficient — resistance falls as it heats
Silicon (pure, 20 °C) #	Semiconductor	2300	2.3e+09	7.496e-10	-0.07	Semiconductor; resistance falls steeply with temperature and rises with doping
Germanium (pure) #	Semiconductor	0.46	4.6e+05	3.748e-06	-0.048	Semiconductor
Sea water #	Electrolyte	0.2	2e+05	8.62e-06	—	Conductive because of dissolved salts
Drinking water #	Electrolyte	20	2e+07	8.62e-08	—	Varies by orders of magnitude with mineral content
Deionised water #	Electrolyte	1.8e+05	1.8e+11	9.578e-12	—	Thousands of times less conductive than drinking water
Glass (soda-lime) #	Insulator	1e+11	1e+17	1.724e-17	—	Insulator; drops sharply when hot
Rubber (hard) #	Insulator	1e+13	1e+19	1.724e-19	—	Classic insulator
PTFE #	Insulator	1e+22	1e+28	1.724e-28	—	One of the best solid insulators available
Air (dry, 20 °C) #	Insulator	1.6e+13	1.6e+19	1.078e-19	—	Insulating until the field exceeds about 3 kV/mm, when it ionises

Values are for 20 °C unless the row says otherwise. Resistivity is strongly temperature-dependent — for pure metals it rises with temperature (positive coefficient), while semiconductors and graphite fall. %IACS (International Annealed Copper Standard) is conductivity as a percentage of annealed copper, where 100% IACS = 1.7241 × 10⁻⁸ Ω·m = 58 MS/m. The Ω·mm²/m column is numerically equal to µΩ·m and is the unit European cable and heating-element datasheets usually use.

Resistivity, Resistance and Conductivity

Resistivity is a property of the material; resistance is a property of the object. The relationship is R = ρL/A — resistance is resistivity times length divided by cross-sectional area. That is why a long thin wire has more resistance than a short fat one of the same metal, and why the two must not be used interchangeably.

Resistivity is quoted in Ω·m in SI, and in Ω·mm²/m in most European cable and heating-element datasheets. The two differ by exactly a factor of 10⁶, so Ω·mm²/m is numerically identical to µΩ·m — which is why copper's 1.7241 × 10⁻⁸ Ω·m is written as 0.017241 Ω·mm²/m, or 17.241 nΩ·m. Mixing the two units without converting introduces a million-fold error.

Conductivity is simply the reciprocal of resistivity, in S/m. The %IACS scale is a convenient normalisation for metals: it expresses conductivity as a percentage of annealed copper, so pure silver is 108% IACS, aluminium is 61%, and stainless steel 304 is around 2.4%. Anything above 100% IACS conducts better than the copper it is measured against.

Temperature Coefficient and Why It Matters

The temperature coefficient in the table is the fractional change in resistivity per degree Celsius at 20 °C, so a coefficient of 0.00393 means resistivity rises by 0.393% for each degree. Over a 55 °C rise from 20 °C to a 75 °C conductor rating, copper's resistance rises about 22% — which is why cable ampacity tables are published against a conductor temperature, not an ambient one.

Two practical consequences follow. First, voltage drop calculated at 20 °C understates the drop in a loaded conductor; use the resistance at the expected operating temperature. Second, the positive coefficient is what makes a conductor fail safely: as current rises the conductor heats, its resistance rises, and the current is limited — the opposite of thermal runaway.

The exceptions are instructive. Graphite and semiconductors have negative coefficients — their resistance falls as they heat, which is why a graphite brush or a semiconductor device can run away thermally if it is not current-limited. Nichrome and Kanthal have very small coefficients — around 0.0004 and 0.00005 per °C — which is exactly why they are used for heating elements: the element's resistance barely changes as it heats, so the power output is predictable.

Frequently Asked Questions

Why is copper used for wiring rather than silver?: Silver is 8% more conductive, but it costs roughly 70 times as much and tarnishes. Copper is the practical optimum: it is the second-best metallic conductor, it is abundant and workable, and its oxide is conductive enough not to ruin a connection. Silver is used only where the extra conductivity or the oxidation resistance justifies the cost — high-frequency contacts and switchgear.
Is aluminium a worse conductor than copper?: Per unit of cross-section, yes — aluminium is 61% as conductive, so it needs about 1.6 times the area. Per unit of weight aluminium wins: it is 30% of copper's density, so the same mass of aluminium carries roughly twice the current of the same mass of copper. That is why overhead transmission lines are aluminium, often with a steel core for strength, while building wiring is copper.
What is 100% IACS?: The International Annealed Copper Standard defines 100% IACS as a conductivity of 5.800 × 10⁷ S/m, which corresponds to a resistivity of 1.7241 × 10⁻⁸ Ω·m at 20 °C. It is a fixed reference value, not a rating of any particular copper product, and conductivity better than 100% is normal — pure silver reaches about 108% IACS, and oxygen-free high-conductivity copper slightly exceeds 100%.
Does bending or working a conductor change its resistance?: Yes, slightly. Cold working a metal increases its resistivity by disrupting the crystal lattice — hard-drawn copper is about 3% more resistive than annealed copper, which is why the two appear as separate rows in the table. Annealing restores the conductivity. The effect is small compared with temperature, but it matters in precision work and in sizing flexible conductors made of many fine strands.
Why does stainless steel make a poor conductor?: Its resistivity is about 42 times copper's, because chromium and nickel alloying elements scatter electrons heavily and the alloy's crystal structure is far from a pure metal's. That is exactly what makes it useful as a heating element or a resistance wire — high resistance converts current into heat — and exactly why it is never used for power distribution.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	Conductivity expressed as % IACS	derived	% IACS = (1.7241 × 10⁻⁸ Ω·m ÷ resistivity) × 100, using the IACS reference resistivity. Values above 100% are valid and expected — silver exceeds it.
[2]	Resistivity converted to Ω·mm²/m	derived	Ω·mm²/m = Ω·m × 10⁶, exact. Numerically equal to µΩ·m. Checked at build time for every row.
[3]	Published electrical resistivity values for conductors, alloys and insulators	standard	compilations as published 2024–2026

Data Sources

Standard	Revision	What it covers on this page
International Annealed Copper Standard (IACS)	n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C	the %IACS column and the conductivity reference
IEC 60028 — International standard of resistance for copper	IEC 60028:1925 (withdrawn but still the source of the 1.7241 value)	the copper resistivities
Published resistivity tables for metals, alloys and insulators	compilations as published 2024–2026	every value in the table

Cross-checked against:

Copper and aluminium values checked against cable-standard resistivity figures
Nichrome and Kanthal values checked against heating-element manufacturer datasheets
%IACS values recomputed from resistivity and compared against published conductivity figures
Isolator values checked for order of magnitude rather than precise value, since they vary widely by grade and moisture

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Resistivity in Ω·mm²/m	Ω·m × 10⁶, exact. Rechecked at build time for every row to within 1%.
Conductivity as % IACS	1.7241 × 10⁻⁸ ÷ resistivity × 100. Copper is held as a known-value check at exactly 100% to confirm the reference is being applied correctly.

Values are for 20 °C and for the purest form of each material. Alloying, cold work, impurities and temperature all move resistivity substantially — hard-drawn copper is 3% more resistive than annealed, and a 55 °C temperature rise adds about 22%. Insulator resistivity varies enormously with moisture, temperature and grade, so treat those rows as order-of-magnitude figures only.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Electrical Resistivity Chart." https://refmanual.com/charts/electrical-resistivity/ (accessed 2026-09-29). Data from International Annealed Copper Standard (IACS), n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C.

BibTeX

@misc{electrical-resistivity,
  title        = {Electrical Resistivity Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/electrical-resistivity/}},
  note         = {Accessed: 2026-09-29},
  source       = {International Annealed Copper Standard (IACS), n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Electrical Resistivity Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/electrical-resistivity/
PY  - 2026
DA  - 2026-09-29
N1  - Source: International Annealed Copper Standard (IACS), n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C
ER  -

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