Annealed copper defines 100% IACS, corresponding to electrical conductivity of 58.108 MS/m and resistivity of 1.7241 × 10⁻⁸ Ω·m at 20°C [S2][S3][S7].
The International Annealed Copper Standard gives engineers one comparative scale for copper, aluminum, brass, steel, and other conductive materials, with higher %IACS meaning lower resistivity [S1][S5].
What the IACS Rating Measures
The International Annealed Copper Standard is an empirical conductivity reference established from commercial copper samples analyzed around 1913 and published by the US Department of Commerce on October 1, 1914 [S1]. Its reference value lets engineers specify conductivity without relying on inconsistent conductivity units or test conditions.
The reference resistance was 0.15292 Ω for a 1 m length of uniform copper wire with a mass of 1 g at 20°C [S1]. A German modification, 0.15328 Ω for the same geometry and mass, was adopted by the International Electrotechnical Commission in 1913 and corresponds to exactly 58 MS/m at 20°C [S1].
Modern reference tables express the baseline as 58.108 MS/m at 20°C [S3]. The value must be read with temperature in mind because 100% IACS is defined specifically at 20°C, not as a temperature-independent material constant [S2][S5].
Converting %IACS to Conductivity and Resistivity
Conductivity can be calculated by multiplying the percentage rating by 0.58108 MS/m. Thus, 100% IACS is 58.108 MS/m, 80% IACS is 46.486 MS/m, 50% IACS is 29.054 MS/m, and 20% IACS is 11.6216 MS/m [S2][S3].
For resistivity, the IHI Connectors table gives the calculation “IACS % = 172.41/resistivity,” with resistivity expressed in micro-ohms per centimeter [S3]. The equivalent reference resistivity is 1.7241 × 10⁻⁸ Ω·m, or 1.7241 micro-ohms per centimeter, at 20°C [S3][S7].
Those conversions make %IACS suitable for procurement specifications. A buyer can require conductivity as a minimum percentage, convert it to MS/m for thermal or ampacity calculations, and compare it with the corresponding resistance limit. Copper at 100% IACS has one resistivity, while 50% IACS material has twice the reference resistivity under equivalent test conditions [S2][S3].
Copper and Conductive Alloy Ratings Compared
Published comparative values show why %IACS is useful across conductor materials: copper is rated 100% IACS, silver 85%, gold 76%, beryllium copper 38%, brass 28%, nickel 24%, nickel silver 10%, and stainless steel 2.7% [S5].
Within the high-copper group, electrolytic (ETP), silver-bearing, and oxygen-free (OF) copper are each listed at 101% IACS, phosphorized DLP copper at 98%, phosphorized DHP copper at 85%, and free-cutting copper at 90–98% IACS [S3]. Chromium copper is lower at 80–90% IACS, while 1% cadmium copper also falls in the 80–90% range [S3].
Aluminum grades differ substantially. Alloy 1100 is rated at 59% IACS, 1350 at 62%, 2024 at 45.5–49%, 6101 at 57%, 6061-T6 at 43%, and 6005 or 6105 at 46–47% [S3].
Where %IACS Fits Material Selection
%IACS is appropriate when the design requires comparative electrical conductivity, especially for busbars, winding wire, cable conductors, electrical contacts, heat-spreading components, and conductivity inspection of finished metal parts [S1][S5]. The rating should be coupled to mechanical, thermal, corrosion, joining, and dimensional requirements rather than used as the sole selection criterion [S5].
For copper buswork, conductor cross-section and temperature remain necessary design inputs because conductivity does not, by itself, determine current capacity. How Copper Busbar Thickness Sets Current Capacity provides the related thickness and ampacity framework, while the IACS value establishes the material’s resistance baseline.
For aluminum conductors, aluminum alloy selection must account for the specified temper and alloy rather than a generic “aluminum” value. The 45.5–49% IACS range for 2024 and 43% for 6061-T6 demonstrate that composition and heat treatment materially affect the comparison [S1][S3].
For structural or corrosion-resistant applications, alloy steel, carbon steel, and cast iron may be selected for mechanical duty even though their conductivity is far below copper. In that case, %IACS serves as a comparative electrical property, not the controlling selection criterion.
Inspection, Heat Treatment, and Acceptance Limits
Temperature must be controlled during comparison because the 100% IACS reference is defined at 20°C [S2][S5]. Conductivity also varies with alloy chemistry, temper, surface condition, and plating thickness, so a bulk-material rating should not be substituted for a finished-part requirement [S1][S5].
Limits of a Single Conductivity Figure
A %IACS value does not establish suitability by itself. Copper with a 100% IACS rating can still require constraints for strength, fatigue, creep, joining temperature, corrosion, and thermal expansion, while copper material grades can vary substantially despite sharing the same nominal conductivity class. [S5]
Surface plating introduces another layer of comparison. Published conductivity values are 85% IACS for silver, 76% for gold, 24% for electrolytic nickel, and 2.2% for electroless nickel [S5]. Plating thickness affects the finished component’s performance, so the base material and surface finish must be specified separately when both carry electrical current [S5].
For conductors, thermal effects, insulation, geometry, and frequency determine real circuit performance in addition to the IACS number. For inspection, the test method, temperature correction, calibration, part geometry, and applicable material specification determine acceptance. The percent IACS rating is therefore best treated as a controlled comparison and procurement value, not a complete engineering model.
For a defensible specification, state the alloy and temper, minimum %IACS, reference temperature, test method, and required part condition; for example, annealed copper at 100% IACS equals 58.108 MS/m at 20°C [S2][S3].