Resistivity of Metals Chart: Values, Derating, and Selection Guide

The metal with the lowest electrical resistivity at room temperature is silver ($1.59 \times 10^{-8} \Omega\cdot m$), but copper ($1.68 \times 10^{-8} \Omega\cdot m$ for pure, $1.72 \times 10^{-8} \Omega\cdot m$ for standard ETP) is the definitive practical choice for 95% of electrical wiring due to its superior cost-to-conductivity ratio. When sizing wire or designing traces, relying solely on a 20°C baseline chart will result in undersized conductors for real-world operating temperatures. You must apply temperature derating to the base resistivity value to calculate true voltage drop and ampacity limits.

The Resistivity of Metals Chart (20°C Baseline)

The following table provides the baseline DC resistivity for common electrical metals. Source Standard: Values are aligned with ASTM B193 (Standard Test Method for Resistivity of Electrical Metallic Materials) and referenced against the International Annealed Copper Standard (IACS).

Table 1: DC Resistivity and Conductivity of Common Metals at 20°C (68°F)

Metal / Alloy	Resistivity ($\rho$) at 20°C ($\Omega\cdot m \times 10^{-8}$)	Conductivity (% IACS)	Temp Coefficient ($\alpha$) per °C	Primary Application
Silver (Pure)	1.59	105%	0.0038	RF contacts, aerospace, high-end audio
Copper (Pure / OFHC)	1.68	102%	0.00393	PCB traces, magnet wire, precision shunts
Copper (ETP / Standard)	1.72	100%	0.00393	THHN/NM-B building wire, busbars
Gold (Pure)	2.44	70%	0.0034	Corrosion-proof edge connectors, IC bonding
Aluminum (Pure)	2.65	61%	0.00429	High-voltage transmission lines
Aluminum (AA-8000 Series)	2.85	57%	0.00400	NEC-compliant branch/feeder building wire
Tungsten	5.60	31%	0.0045	Incandescent filaments, high-temp probes
Nichrome 80 (80% Ni, 20% Cr)	110.0	1.5%	0.0004	Toaster elements, 3D printer hotends, dummy loads

How to Read This Table

The Resistivity ($\rho$) column is your raw physics value, measured in ohm-meters. For practical wire math, you will usually convert this to ohms per circular mil-foot or ohms per 1000 feet based on AWG cross-sections. The % IACS column is the industry shorthand for conductivity; standard annealed copper is defined as exactly 100% IACS. If a material is 61% IACS (like pure aluminum), it means it has 61% of the conductance of standard copper, requiring a larger cross-sectional area to carry the same current. The Temp Coefficient ($\alpha$) is the multiplier used to adjust the baseline resistivity when the conductor heats up under load.

Applying Temperature Derating to Base Values

A common mistake on the workbench and in CAD software is using the 20°C resistivity value to calculate voltage drop for a wire that will actually operate at 60°C or 75°C. As a metal heats up, atomic lattice vibrations increase, scattering electrons and raising resistance.

To find the true operating resistivity ($\rho_T$), use this formula:

$\rho_T = \rho_{20} \times [1 + \alpha(T - 20)]$

Worked Example: 12 AWG Copper at 75°C

Suppose you are running a 12 AWG ETP copper branch circuit in a hot attic, and the terminations are rated for 75°C.

• Baseline: 12 AWG copper at 20°C is 1.588 $\Omega$ / 1000 ft.
• Temp Coefficient ($\alpha$): 0.00393 (from the chart above).
• Delta T: $75°C - 20°C = 55°C$.
• Calculation: $1.588 \times [1 + (0.00393 \times 55)] = 1.588 \times 1.216 = $ 1.93 $\Omega$ / 1000 ft.

Decision Tree: Which Metal for Your Installation?

Use this decision path to terminate your material selection. Do not default to 'it depends'—match your specific physical constraints to the concrete pick below.

If your primary constraint is...	And the environment is...	Then your concrete pick is...
High-Frequency RF / Skin Effect	Aerospace, ham radio, or high-end audio interconnects where surface conductivity dictates signal loss.	Silver-plated copper wire (e.g., MIL-W-16878 / PTFE insulated). Pure silver is too soft; plating gives the RF surface benefit with copper's tensile strength.
Standard Branch Circuits / Mains	Residential or commercial walls, conduit, NM-B, or THHN in standard ambient temperatures.	ETP Copper (THHN/THWN-2). It is the undisputed baseline. Size according to NEC 310.16 75°C column.
Heavy Feeders / Weight & Cost	Service entrance conductors, long underground feeder runs (200A+), where copper cost or weight is prohibitive.	AA-8000 Series Aluminum (XHHW-2). Never use pure aluminum for building wire. Size exactly two AWG sizes larger than the equivalent copper (e.g., 2/0 Al for 100A instead of 3 AWG Cu).
Intentional Heat Generation	Toasters, 3D printer hotends, kiln elements, or high-wattage dummy loads.	Nichrome 80 (80% Ni, 20% Cr). Its high resistivity (110 $\times 10^{-8} \Omega\cdot m$) and stable $\alpha$ prevent thermal runaway and oxidation at red-hot temperatures.
Corrosion Resistance / Micro-Signals	Low-voltage edge connectors, PCB contacts, or marine environments where copper oxide would cause data errors.	Gold-flashed contacts (minimum 30 $\mu$in thickness). Gold does not oxidize, ensuring reliable low-current mating cycles.

What This Chart Cannot Tell You (The Blind Spots)

A DC resistivity chart is a physics baseline, not a complete engineering specification. Here is what the numbers above hide:

1. The AC Skin Effect

This chart lists DC resistivity. At 60 Hz (mains power), the skin effect in standard AWG wire is negligible. However, at high frequencies (e.g., 100 kHz+ in switch-mode power supplies or RF transmitters), current migrates to the outer skin of the conductor. The effective cross-sectional area shrinks, drastically increasing AC resistance. For high-frequency builds, you must use Litz wire or hollow copper tubing, regardless of the base DC resistivity.

2. Alloy Impurities and Mechanical Creep

The chart lists 'Pure Aluminum' at 2.65 $\times 10^{-8} \Omega\cdot m$. But pure aluminum (1350 series) suffers from severe mechanical creep and galvanic oxidation at screw terminals, which historically caused house fires. Modern electrical codes require AA-8000 series aluminum alloys. These alloys add trace iron and copper to stop creep, but they slightly increase resistivity to ~2.85 $\times 10^{-8} \Omega\cdot m$. Always use the alloy value for voltage drop calculations, not the pure metal value.

3. Contact Resistance

Resistivity measures the bulk material. It does not account for the micro-ohms of resistance introduced at a crimp, a solder joint, or a mechanical lug. A poorly torqued aluminum lug will introduce more resistance than 50 feet of the wire itself. Always follow manufacturer torque specs (e.g., 25 in-lbs for a standard 12 AWG copper terminal).

Quick-Jump Reference: The Most Queried Metals

Bookmark this section for fast lookups when ordering materials or debugging a circuit.

• Copper (ETP): The 100% IACS baseline. $\rho = 1.72 \times 10^{-8} \Omega\cdot m$. Use for 99% of DIY electronics, home wiring, and motor windings. If you are unsure, use copper.
• Aluminum (AA-8000): $\rho = 2.85 \times 10^{-8} \Omega\cdot m$. Use strictly for heavy feeders (>100A) to save money and weight. Requires anti-oxidant paste (Noalox) and specific AL/CU rated lugs.
• Silver: $\rho = 1.59 \times 10^{-8} \Omega\cdot m$. Only 6% more conductive than copper but 50x more expensive. Justifiable only for RF skin-effect plating or critical aerospace contacts.
• Nichrome 80: $\rho = 110 \times 10^{-8} \Omega\cdot m$. The standard for heating. If you need a 10-ohm heating element, you need a very long, thin piece of copper, but just a short, manageable coil of Nichrome.

For further reading on standard testing methods and material properties, consult the Georgia State University HyperPhysics wire properties database or review the NEC Chapter 9, Table 8 for the practical translation of these resistivity values into AWG ohms-per-thousand-feet.