The electrical conductivity chart is the definitive benchmark for comparing how efficiently different metals and alloys transport electrical current. The universal baseline for this chart is the International Annealed Copper Standard (IACS), established in 1913. Under this standard, 100% IACS is defined as a volume resistivity of exactly 1.7241 µΩ·cm at 20°C. Every other conductive material is ranked as a percentage of this baseline. If you are sizing busbars, selecting relay contacts, or calculating voltage drop for long feeder runs, this chart provides the raw physics required before applying NEC ampacity tables.
The Master Electrical Conductivity Chart (IACS & Resistivity)
How to read this table: The %IACS column gives you a rapid relative comparison (higher is better). The Volume Resistivity column is the critical metric for physical sizing—use it to calculate voltage drop and I²R heating in fixed installations. The Temperature Coefficient (α) is your derating multiplier; it dictates how much resistance increases (and conductivity drops) as the metal heats up under load. All values are referenced to 20°C (68°F) per ASTM B193 and IEC 60028 testing standards.
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Bookmark Quick-Jumps:
- Jump to Copper (Annealed & Hard-Drawn)
- Jump to Aluminum (EC Grade 1350 & Structural Alloys)
- Jump to Steel & Stainless (Grounding & Structural)
| Material / Alloy | %IACS | Volume Resistivity (µΩ·cm @ 20°C) | Temp Coefficient α (/°C) |
|---|---|---|---|
| Silver (Pure, Annealed) | 105.0% | 1.59 | 0.0038 |
| Copper (Annealed, IACS Base) | 100.0% | 1.724 | 0.00393 |
| Copper (Hard-Drawn) | 97.0% | 1.777 | 0.00393 |
| Beryllium Copper (2% Be) | 22.0% | 7.80 | 0.0015 |
| Gold (Pure, Annealed) | 70.0% | 2.44 | 0.0034 |
| Aluminum (EC Grade 1350-O) | 61.0% | 2.82 | 0.00403 |
| Aluminum (6061-T6 Structural) | 43.0% | 4.00 | 0.0040 |
| Brass (70/30 Cartridge) | 28.0% | 6.20 | 0.0020 |
| Nickel (Pure) | 25.0% | 6.90 | 0.0060 |
| Iron (Pure, Annealed) | 17.0% | 10.0 | 0.0050 |
| Stainless Steel (304) | 2.5% | 72.0 | 0.0009 |
Applying the Chart: Temperature, Derating, and Installation Columns
When engineers and electricians pull data from a conductivity chart, the most common mistake is treating the 20°C baseline as a real-world constant. Metals heat up under load, and as they heat, their crystal lattice vibrates more violently, scattering electrons and increasing resistance. This is where the Temperature Coefficient (α) column dictates how derating modifies your base value.
Which Column Applies to Your Installation?
If you are sizing branch circuit wiring, panel busbars, or transformer windings, Volume Resistivity is your governing column. It tells you the resistance per unit of cross-sectional area and length. However, if you are designing aerospace wiring, drone power distribution, or moving cable carriers where weight is a premium, you must look at Mass Resistivity (resistance per unit of mass). Aluminum wins the mass resistivity battle: while it requires a larger physical volume than copper to carry the same current, an aluminum conductor will weigh roughly half as much as the equivalent copper conductor.
How Derating Rows Modify the Base Value
To find the true resistance of a conductor at its operating temperature, apply the linear approximation formula using the α column:
R_T = R_20 × [1 + α × (T - 20)]
Worked Numeric Example: You are designing a 100A DC busbar using EC Grade 1350 Aluminum. At 20°C, your calculated baseline resistance for the busbar length is 0.100 mΩ. Under full load, the busbar stabilizes at 75°C (a standard NEC termination limit). What is the derated resistance and power loss?
- Step 1: Identify α for Aluminum 1350 from the chart:
0.00403. - Step 2: Calculate the temperature delta:
75 - 20 = 55°C. - Step 3: Apply the formula:
R_75 = 0.100 × [1 + (0.00403 × 55)]. - Step 4:
R_75 = 0.100 × [1 + 0.22165] = 0.122 mΩ.
Your resistance increased by 22%. Consequently, your I²R heat dissipation jumped from 1.0 Watt at room temperature to 1.22 Watts at operating temperature. In high-current, tightly packed switchgear, this 22% derating factor is exactly why thermal runaway occurs if ventilation is undersized.
What the Conductivity Chart Cannot Tell You (And How to Fill the Gaps)
A pure physics chart assumes ideal, clean, bulk materials. It completely ignores the mechanical, chemical, and high-frequency realities of jobsite installations. Here is what the chart leaves out, and how to engineer around those blind spots.
1. Surface Oxidation and Contact Resistance
The chart ranks Aluminum 1350 at a highly respectable 61% IACS. What it omits is that aluminum instantly forms a layer of aluminum oxide (alumina) when exposed to air. Alumina is an aggressive electrical insulator. If you torque an aluminum wire into a copper lug without breaking this oxide layer and applying an antioxidant compound (like Noalox), your contact resistance will dwarf the bulk resistance of the wire, leading to a melted termination. Conversely, silver is ranked #1 at 105% IACS, but in high-sulfur environments, silver sulfide tarnish can increase surface contact resistance, which is why gold (70% IACS) is preferred for low-voltage, low-current signal contacts—it simply does not oxidize.
2. The Skin Effect at High Frequencies
The volume resistivity values in the chart are strictly for DC or low-frequency AC (50/60Hz). If you are building high-frequency inverters, induction heaters, or RF amplifiers operating above 10 kHz, current stops flowing through the core of the wire and migrates to the outer surface (the skin effect). At 100 kHz, the skin depth in copper is roughly 0.21 mm. A solid 4 AWG copper wire will have an effective AC resistance vastly higher than the chart suggests. To bypass this limitation, high-frequency builders must use Litz wire (many individually insulated thin strands) or silver-plated copper wire, leveraging silver's slightly higher conductivity strictly on the surface where the high-frequency current actually travels.
3. Galvanic Corrosion and Bimetallic Junctions
The chart ranks materials in isolation. It does not warn you that connecting the #3 ranked metal (Gold) or the #5 ranked metal (Aluminum) directly to Copper in the presence of ambient moisture creates a galvanic cell. Aluminum acts as the anode and will literally corrode away over time, increasing resistance and causing arc faults. When the conductivity chart dictates using aluminum for long, cost-sensitive feeder runs, you must bridge the gap to copper equipment using listed bimetallic lugs or aluminum-to-copper transition pigtails to prevent galvanic degradation.
