The electrical resistivity of annealed copper at 20°C is exactly 1.7241 × 10⁻⁸ Ω·m (or 1.7241 μΩ·cm), establishing the 100% IACS (International Annealed Copper Standard) baseline. Whether you are sizing a DC busbar, calculating PCB trace voltage drop, or selecting winding wire for a custom transformer, this metal resistivity chart provides the exact baseline values and temperature coefficients for the most common conductive metals used in electrical engineering.
How to Read the Metal Resistivity Chart (and Which Column Matters)
Before jumping to the numbers, you need to know which column applies to your specific installation. Resistivity data is typically presented in three formats, each serving a different engineering purpose:
- Resistivity (ρ) in Ω·m: Use this column for physics calculations, voltage drop formulas (R = ρL/A), and PCB trace width calculators (like IPC-2221). It is the absolute measure of how strongly a material opposes current flow.
- Conductivity (% IACS): Use this column for power transmission and bulk wire purchasing. The IACS standard sets annealed copper at 100%. If you are comparing aluminum transmission lines to copper, % IACS tells you the relative efficiency at a glance.
- Temperature Coefficient (α): Use this column to calculate how much the resistance will increase when the conductor heats up under load. It is expressed per degree Celsius (°C⁻¹).
Bookmark-Friendly Quick-Jump Index
For rapid reference on the workbench, here are the most queried metals in the chart below:
- Silver: Lowest absolute resistivity, used in high-end RF contacts and aerospace relays.
- Copper (Annealed): The universal baseline for branch circuits, motor windings, and PCB traces.
- Aluminum (EC Grade): The standard for utility transmission and heavy feeder cables.
- Iron/Steel: High resistivity, primarily relevant for calculating ground rod resistance or structural bonding.
Complete Metal Resistivity Reference Table
The following data is standardized at 20°C (68°F). Values are sourced from the Georgia State University HyperPhysics database and align with the All About Circuits conductor reference standards for pure elemental metals.
| Metal (Pure) | Resistivity (ρ) at 20°C [Ω·m] | Resistivity [μΩ·cm] | Conductivity [% IACS] | Temp Coeff (α) [°C⁻¹] |
|---|---|---|---|---|
| Silver (Ag) | 1.59 × 10⁻⁸ | 1.59 | 108.4% | 0.00380 |
| Copper (Cu, Annealed) | 1.72 × 10⁻⁸ | 1.72 | 100.0% | 0.00393 |
| Gold (Au) | 2.44 × 10⁻⁸ | 2.44 | 70.7% | 0.00340 |
| Aluminum (Al, EC Grade) | 2.65 × 10⁻⁸ | 2.65 | 64.9% | 0.00429 |
| Zinc (Zn) | 5.90 × 10⁻⁸ | 5.90 | 29.2% | 0.00370 |
| Nickel (Ni) | 6.99 × 10⁻⁸ | 6.99 | 24.6% | 0.00600 |
| Iron (Fe) | 9.71 × 10⁻⁸ | 9.71 | 17.7% | 0.00651 |
| Platinum (Pt) | 1.06 × 10⁻⁷ | 10.6 | 16.2% | 0.00392 |
| Tin (Sn) | 1.15 × 10⁻⁷ | 11.5 | 15.0% | 0.00450 |
| Lead (Pb) | 2.20 × 10⁻⁷ | 22.0 | 7.8% | 0.00390 |
Temperature Derating: How Heat Modifies Base Resistivity
The values in the chart above are strictly for a 20°C ambient environment. In the real world, conductors heat up due to I²R losses and environmental ambient temperatures. To find the actual resistivity at your operating temperature, you must apply the temperature coefficient (α) using this linear approximation formula:
ρ(T) = ρ₀ [1 + α(T - T₀)]
Where:
• ρ(T) = Resistivity at the target temperature
• ρ₀ = Base resistivity at 20°C (from the chart)
• α = Temperature coefficient (from the chart)
• T = Target operating temperature in °C
• T₀ = Reference temperature (20°C)
Worked Numeric Example: PCB Trace at 85°C
Imagine you are routing a 1 oz copper trace (ρ₀ = 1.72 × 10⁻⁸ Ω·m) on a PCB inside an enclosure that reaches 85°C under load. Copper's α is 0.00393.
ρ(85) = 1.72 × 10⁻⁸ [1 + 0.00393(85 - 20)]
ρ(85) = 1.72 × 10⁻⁸ [1 + 0.00393(65)]
ρ(85) = 1.72 × 10⁻⁸ [1 + 0.25545]
ρ(85) = 2.16 × 10⁻⁸ Ω·m
The resistivity has increased by roughly 25.5%. If you sized your trace for a 50mV drop at room temperature, you will actually see a 62mV drop at operating temperature. This derating step is where most hobbyist and junior engineering designs fail thermal validation.
What This Chart Cannot Tell You (And Where to Look Next)
While this metal resistivity chart is definitive for pure elemental metals, it has strict limitations in practical electrical work. Here is what the table hides:
- Alloy Variations: The chart lists pure aluminum and pure copper. However, structural aluminum busbars are often 6061-T6 alloy, which has a significantly higher resistivity (approx. 3.5 × 10⁻⁸ Ω·m) than the 1350-H19 EC (Electrical Conductor) grade used in wire. Similarly, beryllium copper or phosphor bronze used in spring contacts have much higher resistivity than pure copper.
- The Skin Effect: At high frequencies (RF, switching power supplies above 100kHz), current travels only on the outer surface of the conductor. The DC resistivity chart does not account for AC skin depth, which drastically increases effective impedance.
- Contact Resistance: A gold-plated connector has excellent bulk resistivity, but the actual voltage drop across a mated connector pair is dominated by microscopic contact resistance and surface oxidation, not the bulk metal chart value.
Frequently Asked Questions About Metal Resistivity
Why is aluminum used for transmission lines if copper has lower resistivity?
It comes down to the conductivity-to-weight ratio. While aluminum has only 64.9% of the conductivity of copper by volume, it is roughly 30% of the weight. For a given weight of metal, aluminum can actually carry more current than copper over long spans. This allows transmission towers to be spaced further apart, saving massive amounts of structural steel and concrete costs, which easily offsets the need for a physically thicker aluminum cable.
How does the metal resistivity chart apply to PCB trace width calculations?
PCB trace calculators (based on IPC-2221 or IPC-2152 standards) use the resistivity of copper (1.72 × 10⁻⁸ Ω·m) to determine the cross-sectional area required to carry a specific current without exceeding a target temperature rise (usually 10°C or 20°C). If you are designing a board with heavy copper pours (e.g., 2 oz or 3 oz), the calculator relies directly on this baseline resistivity value, adjusted for the dielectric thermal resistance of the FR4 material.
Does the resistivity of solder change significantly after reflow?
Standard SAC305 (Tin/Silver/Copper) lead-free solder has a bulk resistivity of roughly 1.4 × 10⁻⁷ Ω·m, making it about 8 times more resistive than pure copper. While reflow creates intermetallic compounds at the pad boundary that slightly alter local resistance, the bulk resistivity remains dominated by the tin content. In high-current paths, solder should never be relied upon as the primary conductor; it is strictly for mechanical and electrical joining.
What is the difference between resistivity and resistance in wire sizing?
Resistivity (ρ) is an intrinsic material property—it is the same for a microscopic flake of copper and a massive copper busbar. Resistance (R), measured in Ohms, is the physical property of a specific object, calculated by multiplying the material's resistivity by the length of the wire and dividing by its cross-sectional area (R = ρL/A). You use the metal resistivity chart to find ρ, then apply your wire gauge (AWG) and length to find the actual resistance R of your installation.
