The electrical resistivity ($\rho$) of standard annealed copper at 20°C is 1.724 $\mu\Omega\cdot$cm (or $1.724 \times 10^{-8} \, \Omega\cdot$m), while standard aluminum is 2.65 $\mu\Omega\cdot$cm. These baseline values dictate everything from the voltage drop in your home branch circuits to the heat dissipation in a custom PCB trace. However, raw resistivity is only half the story; temperature derating, AC skin effect, and stranding geometry drastically alter real-world performance.
This reference guide provides the standard material data, explains how to apply the temperature coefficient to your specific installation environment, and details the physical limitations that a static chart cannot capture.
How to Read the Electrical Resistivity Chart
Before pulling numbers for a voltage drop calculation or a busbar design, you need to understand the three distinct columns in the data table below and which column applies to your installation.
- Resistivity ($\rho$): Measured in micro-ohm centimeters ($\mu\Omega\cdot$cm). This is an intrinsic material property. Use this column when calculating the DC resistance of a specific wire gauge and length using the formula $R = \rho(L/A)$. It tells you how strongly the material opposes current flow.
- Conductivity (% IACS): The International Annealed Copper Standard sets copper at exactly 100%. Use this column when comparing the relative efficiency of different busbar materials or when sizing aluminum feeders to match an existing copper infrastructure.
- Temperature Coefficient ($\alpha$): Measured in $^\circ$C$^{-1}$. This is your derating row. It dictates how much the base resistivity increases for every degree Celsius above the 20°C baseline. You must use this column to calculate hot-resistance for conductors operating near their NEC ampacity limits (e.g., 75°C or 90°C termination ratings).
Standard Resistivity and Conductivity Data Table
The following data is sourced from standard reference texts including the CRC Handbook of Chemistry and Physics and aligns with ASTM B193 standard test methods for resistivity of electrical conductor materials.
| Material | Resistivity at 20°C ($\mu\Omega\cdot$cm) | Conductivity (% IACS) | Temp Coefficient $\alpha$ ($^\circ$C$^{-1}$) |
|---|---|---|---|
| Silver (Annealed) | 1.59 | 108.4 | 0.0038 |
| Copper (Annealed) | 1.724 | 100.0 | 0.00393 |
| Gold | 2.44 | 70.7 | 0.0034 |
| Aluminum (1350 Alloy) | 2.65 | 61.0 | 0.00403 |
| Tungsten | 5.60 | 31.2 | 0.0045 |
| Zinc | 5.90 | 29.2 | 0.0037 |
| Nickel | 6.99 | 24.6 | 0.0060 |
| Iron (Pure) | 9.71 | 17.7 | 0.0050 |
| Platinum | 10.6 | 16.3 | 0.0039 |
| Nichrome (80/20) | 108.0 | 1.6 | 0.0004 |
| Carbon (Graphite) | ~780.0 | ~0.22 | -0.0005 (Negative) |
1. Copper (1.724 $\mu\Omega\cdot$cm): The universal baseline. Used for almost all indoor branch circuits, PCB traces, and motor windings.
2. Aluminum 1350 (2.65 $\mu\Omega\cdot$cm): The standard for utility transmission and large residential feeders (e.g., 2/0 or 4/0 SER cable). Requires upsizing by roughly two AWG sizes compared to copper for equivalent ampacity.
3. Nichrome (108.0 $\mu\Omega\cdot$cm): Extremely high resistivity and a near-zero temperature coefficient. Used exclusively for heating elements (toasters, 3D printer hotends) where resistance must remain stable as it glows red hot.
4. Tungsten (5.60 $\mu\Omega\cdot$cm): High melting point and moderate resistivity. Used in incandescent filaments and high-vacuum electronics.
What This Resistivity Chart Cannot Tell You
A static chart provides the DC baseline at room temperature. It does not account for the physical and electromagnetic realities of a live installation. Here is how derating rows modify the base value, and what the table leaves out.
Applying the Temperature Derating Row
Conductors heat up under load. To find the true resistance at operating temperature, use the $\alpha$ column with this formula:
R_T = R_20 * [1 + \alpha * (T - 20)]
Worked Example: You have a copper wire with a measured DC resistance of 1.0 $\Omega$ at 20°C. It is installed in a conduit and reaches 75°C under full continuous load. Using the copper $\alpha$ of 0.00393:
R_75 = 1.0 * [1 + 0.00393 * (75 - 20)]
R_75 = 1.0 * [1 + 0.216] = 1.216 \Omega
Your voltage drop will be 21.6% higher at operating temperature than your bench measurement suggests. This is why NEC Chapter 9, Table 8 resistance values are often adjusted for terminal temperature ratings.
Hidden Variables: Stranding, Skin Effect, and Alloys
- Stranding Lay-Length Derating: The chart assumes a solid, perfectly straight cylinder of metal. In stranded wire (like THHN), the individual strands spiral around a core. This spiral increases the actual physical length of the metal by 2% to 5% compared to the jacket length, slightly increasing total resistance beyond the chart's prediction.
- AC Skin Effect: The chart lists DC resistivity. In AC circuits, especially at 60Hz for large conductors (typically 1/0 AWG and larger), current is pushed to the outer "skin" of the wire. This reduces the effective cross-sectional area ($A$), increasing AC resistance above the DC baseline. At high frequencies (like PWM signals from an ESP32 or VFD outputs), skin effect dominates, making the DC resistivity chart nearly irrelevant for impedance calculations.
- Alloy Variations: The chart lists pure Aluminum (1350 alloy). If you are using 6061 aluminum for a custom busbar, the resistivity jumps to roughly 3.7 $\mu\Omega\cdot$cm due to silicon and magnesium doping. Always verify the specific alloy temper for structural conductors.
For a deeper dive into how conductor geometry affects AC impedance, reference the All About Circuits guide on AC skin effect and the Georgia State University HyperPhysics resistivity modules.
Frequently Asked Questions
How does the resistivity of copper change at 75°C and 90°C?
Using the temperature coefficient formula, the resistivity of copper increases by approximately 21.6% at 75°C and 27.5% at 90°C compared to the 20°C baseline. At 90°C, the effective resistivity of copper is roughly 2.20 $\mu\Omega\cdot$cm. This is critical when calculating voltage drop for continuous loads where conductors operate near the 90°C insulation limit of THHN/THWN-2 wire.
Why is aluminum wire larger than copper for the same ampacity?
Aluminum has a resistivity of 2.65 $\mu\Omega\cdot$cm, which is about 54% higher than copper's 1.724 $\mu\Omega\cdot$cm. To achieve the same resistance (and therefore the same heat dissipation and ampacity) over the same distance, you must increase the cross-sectional area of the aluminum wire. In practice, NEC ampacity tables require aluminum conductors to be upsized by roughly two AWG sizes compared to copper (e.g., using 1/0 AL instead of 2 AWG CU for a 100A residential feeder).
Does the resistivity chart apply to AC circuits and high frequencies?
Only for DC or low-frequency AC in small conductors. The chart provides DC resistivity. In AC circuits, inductive reactance and the skin effect increase the total impedance. For standard 60Hz residential wiring on small gauges (14 to 6 AWG), DC resistivity is close enough for voltage drop calculations. However, for high-frequency applications like RF antennas, switching power supplies, or data lines, you must calculate AC resistance and impedance, which are significantly higher than the DC chart values.
What is the difference between resistivity and resistance?
Resistivity ($\rho$) is an intrinsic property of the material itself, regardless of its shape or size; it tells you how strongly copper or aluminum opposes current. Resistance ($R$) is the actual opposition of a specific, physical object (like a 50-foot spool of 12 AWG wire). You calculate resistance by multiplying the material's resistivity by the length of the wire, and dividing by its cross-sectional area ($R = \rho L / A$).
