Relative conductivity is the ratio of a specific material's ability to carry electrical current compared to a standardized baseline of pure, annealed copper (defined as 100% IACS). When you are sizing feeders, building battery banks, or choosing busbars, this single percentage dictates whether your wire will run cool or melt its insulation.
What Relative Conductivity Actually Means (And What It Isn't)
To compare apples to apples across different metals, the electrical industry uses the International Annealed Copper Standard (IACS). Under this standard, a specific grade of pure, annealed copper at 20°C is assigned a baseline value of exactly 100%. Every other conductive material is graded as a percentage of that baseline. 100% IACS equals a resistivity of 1.7241 µΩ·cm at 20°C.
What this changes in a real circuit is your required cross-sectional area. If you swap a 100% IACS conductor for a 61% IACS conductor, you must increase the wire's physical cross-section by roughly 60% to achieve the exact same resistance and voltage drop.
- Absolute Conductivity: Relative conductivity is a dimensionless percentage (% IACS). Absolute conductivity is measured in Siemens per meter (S/m) and changes with temperature.
- Thermal Conductivity: A metal's ability to conduct heat does not perfectly mirror its electrical conductivity, though they often correlate in pure metals.
- The '100% Limit' Myth: Many assume copper is the absolute physical ceiling for conductivity. It isn't. Pure silver sits at roughly 105% IACS, but its cost relegates it to specialized RF contacts and high-end audio relays, not branch wiring.
The Math: A Worked Numeric Example
Let's look at what relative conductivity actually does to power loss (heat) in a real branch circuit. We will compare 100 feet of 10 AWG Copper (100% IACS) against 100 feet of 10 AWG Aluminum (61% IACS) carrying a continuous 30A load.
According to standard wire tables, 10 AWG copper has a DC resistance of approximately 1.018 ohms per 1,000 feet at 20°C. For a 100-foot run, the resistance is 0.1018 ohms.
Because the aluminum wire has only 61% of the conductivity of copper, its resistance is inversely proportional: 1 / 0.61 = 1.639 times higher. Therefore, the 10 AWG aluminum wire has a resistance of 0.1668 ohms for the same 100-foot run.
Now we calculate the power lost as heat using the formula P = I²R:
- Copper Heat Loss: 30² × 0.1018 = 91.6 Watts
- Aluminum Heat Loss: 30² × 0.1668 = 150.1 Watts
That is a 58.5-watt difference. If you undersized your aluminum wire by forgetting to account for its lower relative conductivity, you are trapping an extra 58 watts of heat inside your walls or conduit. Over time, that excess heat degrades insulation and accelerates termination loosening.
Where You Meet This in Practice
You will encounter relative conductivity tradeoffs in three primary scenarios on the jobsite and at the workbench:
1. Subpanel Feeders (SER Cable)
When pulling a 100A feeder to a detached garage, copper SER cable is heavy, stiff, and expensive. Because electrical-grade aluminum alloy (AA-8176 series) has a relative conductivity of 61%, you simply step up two AWG sizes (e.g., from 3 AWG Copper to 1 AWG Aluminum) to match the ampacity. The aluminum is lighter, cheaper, and perfectly safe when terminated with an anti-oxidant compound like Noalox and torqued to the breaker's exact inch-pound specification.
2. Solar PV Combiner Runs
In large solar arrays, the home-run from the combiner box to the inverter can easily exceed 150 feet. Voltage drop is the enemy here. Using copper for a 200-foot, 60A DC run requires massive, expensive wire. By calculating the voltage drop using aluminum's 61% IACS rating, you can step up to a thicker aluminum conductor (like 1/0 AWG XHHW-2) for a fraction of the copper cost, keeping your voltage drop under the recommended 1.5% for DC strings.
3. Battery Bank Interconnects
For short, high-current jumps between 4/0 lithium cells, relative conductivity dictates that you stick to copper. The runs are under 2 feet, so the cost savings of aluminum vanish, and copper's superior flexibility and 100% IACS rating ensure minimal voltage sag during heavy inverter surge loads.
Conductor Selection Decision Tree
Use this decision matrix to terminate your material selection process with a concrete pick. Do not mix metals at a single termination point without bimetallic lugs.
| Condition / Constraint | If True... | Concrete Pick (Material & Type) |
|---|---|---|
| Run is under 20 feet AND current exceeds 100A | Cost/weight savings of Al are negated by termination hardware costs and physical space limits in the panel. | 1/0 AWG THHN Copper (or equivalent size for your specific ampacity). |
| Run exceeds 50 feet AND current is 100A - 200A | Voltage drop requires upsizing anyway; Aluminum's lower cost per pound heavily outweighs the upsizing penalty. | 4/0 AWG XHHW-2 Aluminum (Compact stranded, rated 90°C). |
| Environment has high vibration, movement, or thermal cycling | Aluminum's higher coefficient of thermal expansion causes terminations to loosen faster under mechanical stress. | 2 AWG Class K Stranded Copper with crimped ferrules. |
| Building a DIY busbar for a 48V LiFePO4 system | Space inside the battery box is highly constrained; you need maximum current density per square millimeter. | 1/4' x 2' C11000 Copper Busbar (100% IACS). |
Common Material Benchmarks (% IACS)
When reading datasheets for busbars, wire, or PCB traces, you will see these standard ASTM B193 benchmark values referenced. Note that alloying a metal to increase its tensile strength almost always destroys its relative conductivity.
| Material | Relative Conductivity (% IACS) | Common Electrical Application |
|---|---|---|
| Silver (Pure) | 105% | RF contacts, high-end relay wipers, aerospace slip rings. |
| Copper (Annealed) | 100% (Baseline) | Branch wiring, motor windings, PCB traces, battery links. |
| Copper (Hard Drawn) | 97% | Overhead transmission lines (sacrifices 3% conductivity for tensile strength). |
| Gold (Pure) | 70% | Low-voltage signal contacts (chosen for oxidation resistance, not conductivity). |
| Aluminum (EC Grade 1350) | 61% | High-voltage overhead transmission, older residential branch wiring. |
| Aluminum (AA-8176 Alloy) | 61% | Modern residential/commercial feeders (XHHW-2, SER cable). |
| Brass (Cartridge) | 28% | Terminal blocks, plug prongs, fuse clips. |
| Stainless Steel (304) | 2.5% | Structural hardware, NOT for current carrying paths. |
Frequently Asked Questions
Does temperature change a material's relative conductivity percentage?
No. The % IACS rating is a ratio measured at a standard 20°C. However, the absolute resistance of both metals increases as they heat up. Copper's resistance increases by roughly 0.39% per degree Celsius, while aluminum increases by about 0.40% per degree Celsius. Because they track so closely, the 61% ratio remains practically stable across normal operating temperatures (up to 90°C in THHN/XHHW insulation).
Why do we use gold on audio connectors if it only has 70% IACS?
Gold is not used for its conductivity; it is used for its chemical inertness. Copper and silver oxidize and tarnish when exposed to air, creating a high-resistance surface layer that chokes low-voltage audio and data signals. Gold does not oxidize, ensuring a clean contact surface, even though the bulk metal beneath it is less conductive than copper.
What is the default rule if I am stuck between copper and aluminum for a home project?
Stop guessing and apply this default rule: Use copper for all indoor branch circuits (15A/20A receptacles and lighting), battery interconnects, and any run under 30 feet. Use AA-8176 series aluminum exclusively for long, high-ampacity feeder runs (100A+ subpanels, HVAC disconnects, and solar home-runs) where the cost and weight savings of aluminum justify the upsized wire gauge. For detailed ampacity tables and installation practices, always cross-reference the Southwire Aluminum vs Copper technical guidelines and your local NEC articles.
