Electrical resistivity ($\rho$) is the fundamental material property that quantifies how strongly a specific metal opposes the flow of electric current. Unlike resistance, which changes based on a wire's length and gauge, resistivity is an intrinsic constant of the material itself. For DC circuits, precise voltage drop calculations, and custom busbar fabrication, knowing the exact baseline resistivity of your conductor is mandatory. The universal baseline for these measurements is 20°C (68°F), and the industry standard for comparison is the International Annealed Copper Standard (IACS), where pure annealed copper is defined as exactly 100% conductivity.
Below is the definitive reference data you need to calculate voltage drop, select busbar materials, or troubleshoot unexpected heating in high-current DC systems. We will cover the raw material properties, how to mathematically derate them for real-world operating temperatures, and the critical installation limits this chart cannot show you.
The Master Resistivity Chart of Metals
How to read this table: The Resistivity at 20°C column provides the baseline opposition to current flow in micro-ohm centimeters ($\mu\Omega\cdot$cm). Lower numbers mean better conductivity. The Conductivity (% IACS) column benchmarks the metal against the ASTM B193 standard for 100% annealed copper. The Temp Coefficient ($\alpha$) column is the fractional change in resistivity per degree Celsius, which you will use in the derating formula below. All values assume standard atmospheric pressure and pure elemental compositions or standard electrical alloys.
| Metal / Alloy (Standard Grade) | Resistivity at 20°C ($\mu\Omega\cdot$cm) | Conductivity (% IACS) | Temp Coefficient $\alpha$ (per °C) |
|---|---|---|---|
| Silver (Pure, Annealed) | 1.59 | 105.0% | 0.00380 |
| Copper (ETP C11000, Annealed) | 1.724 | 100.0% | 0.00393 |
| Copper (OFE C10100, Annealed) | 1.712 | 100.7% | 0.00393 |
| Gold (Pure, Annealed) | 2.44 | 70.7% | 0.00340 |
| Aluminum (1350-H19, Hard Drawn) | 2.82 | 61.2% | 0.00403 |
| Aluminum (1350-O, Annealed) | 2.65 | 65.1% | 0.00429 |
| Tungsten (Pure, Drawn) | 5.60 | 30.8% | 0.00450 |
| Iron (Pure, Annealed) | 9.71 | 17.8% | 0.00651 |
| Nichrome 80 (80% Ni, 20% Cr) | 108.0 | 1.6% | 0.00040 |
Notice the distinction between hard-drawn and annealed aluminum. The mechanical process of drawing wire into a specific AWG size introduces crystalline lattice defects that increase resistivity. If you are calculating voltage drop for standard THHN aluminum building wire, use the hard-drawn 1350-H19 values, not the annealed values often cited in generic physics textbooks.
Applying Temperature Derating to Base Values
The values in the chart above are strictly for a 20°C ambient environment. In a real-world installation, conductors heat up due to $I^2R$ (Joule) heating and ambient environmental factors. Which column applies to your installation? For DC or low-frequency AC wiring, the Resistivity at 20°C column gives your baseline, while the Temp Coefficient ($\alpha$) column is the multiplier you must apply if the wire operates above 20°C.
To find the operational resistivity ($\rho_T$) at a specific temperature ($T$), use the linear approximation formula derived from the Callendar-Van Dusen equation for metals:
$\rho_T = \rho_{20} \times [1 + \alpha \times (T - 20)]$
Worked Numeric Example:
You are sizing a 12V DC solar array combiner box using 4 AWG copper wire. The wire is routed through an attic where the ambient temperature is 45°C, and the wire itself heats to 65°C under peak load. What is the actual resistivity of the copper at this operating temperature?
- Base Resistivity ($\rho_{20}$): 1.724 $\mu\Omega\cdot$cm (from the chart)
- Temp Coefficient ($\alpha$): 0.00393
- Target Temp ($T$): 65°C
- Calculation: $\rho_{65} = 1.724 \times [1 + 0.00393 \times (65 - 20)]$
- Step 1: $65 - 20 = 45$
- Step 2: $0.00393 \times 45 = 0.17685$
- Step 3: $1 + 0.17685 = 1.17685$
- Final: $1.724 \times 1.17685 = \mathbf{2.028 \mu\Omega\cdot cm}$
At 65°C, the copper's resistivity has increased by nearly 18%. If you used the 20°C baseline to calculate your voltage drop, your actual voltage drop under load would be 18% higher than calculated, potentially pushing a sensitive 12V inverter into a low-voltage brownout shutdown. How derating modifies the base value: The $\alpha$ coefficient acts as a linear scalar. For every 1°C increase above 20°C, copper's resistivity increases by roughly 0.393%. Always derate to the maximum expected operating temperature, which is typically the temperature rating of your termination lugs (e.g., the 75°C column in NEC 310.16).
What This Chart Cannot Tell You (Installation Limits)
While this resistivity chart of metals provides the raw physics of the materials, it is not a substitute for electrical code tables or AC impedance calculations. Here is what the raw data cannot tell you:
1. Ampacity and Thermal Dissipation Limits
Resistivity tells you how much heat a metal will generate per meter, but it does not tell you how much heat the insulation can survive. A silver wire and a copper wire of the exact same AWG and insulation type (e.g., THHN) will have the exact same NEC ampacity rating, even though silver has 5% lower resistivity and runs slightly cooler. Ampacity is governed by the thermal degradation threshold of the polymer insulation (60°C, 75°C, or 90°C), not just the metal's conductivity. Always defer to NFPA 70 (NEC) Article 310 for maximum allowable continuous currents.
2. AC Skin Effect and Proximity Effect
The resistivity values above are strictly for Direct Current (DC) or very low-frequency AC. At 60Hz mains frequency, alternating current tends to migrate toward the outer surface of the conductor—a phenomenon known as the skin effect. For wire gauges smaller than 1/0 AWG, the AC resistance is virtually identical to the DC resistance. However, for large feeders (e.g., 500 kcmil or larger), the effective AC resistance can be 10% to 20% higher than the DC resistivity suggests. For precise AC voltage drop on massive feeders, you must consult the IEEE standard AC resistance tables, which factor in the skin and proximity effects of the specific cable geometry.
3. Alloy Impurities and Contact Resistance
The chart assumes pure, standardized metallurgy. In practice, cheap imported busbars or unbranded wire may contain trace impurities (like phosphorus or iron in copper) that drastically reduce the % IACS rating. Furthermore, this chart measures the bulk material; it does not account for contact resistance at termination points. A perfectly conductive silver wire terminated with a loose, un-torqued aluminum lug will create a high-resistance joint that generates localized heat. Always use a calibrated torque screwdriver on lugs and apply antioxidant compound (like Noalox) when terminating aluminum to prevent galvanic corrosion, which effectively alters the surface resistivity over time.
