Electrical conductivity is a material's inherent ability to allow the flow of electric current, measured quantitatively as the reciprocal of electrical resistivity. When you are sizing a 400A service feeder or selecting busbar alloys for a custom power distribution unit, the units of electrical conductivity dictate your voltage drop, thermal limits, and physical footprint. The standard SI unit is Siemens per meter (S/m), but in practical electrical work and metallurgy, you will almost exclusively encounter Megasiemens per meter (MS/m) and the percentage of the International Annealed Copper Standard (% IACS). Understanding these units changes how you calculate voltage drop, determine ampacity derating, and select the physical cross-sectional area required to safely carry a load without exceeding insulation temperature ratings.
The Core Units of Electrical Conductivity Explained
To specify how well a material conducts electricity, engineers rely on three primary units depending on the industry context. The baseline SI unit is the Siemens per meter (S/m). Because S/m values for good conductors are massive (copper is roughly 58,000,000 S/m), the industry standardizes on Megasiemens per meter (MS/m) for readability.
However, the wire and cable manufacturing industry largely relies on % IACS (International Annealed Copper Standard). Established to standardize copper wire production, 100% IACS represents the conductivity of a perfectly annealed, pure copper standard at 20°C.
The Great Confusion: Conductivity vs. Conductance
The most common mistake hobbyists and junior engineers make is confusing conductivity with conductance. Think of conductivity as the speed limit and lane width of a specific type of asphalt (the intrinsic material property, measured in S/m), while conductance is the actual traffic throughput of one specific, physical highway you built (the component property, measured in Siemens). Conductivity tells you how good copper is; conductance tells you how much current a specific 10-foot length of 12 AWG copper wire will pass.
Worked Numeric Example: Sizing a 200A Feeder
Let's look at what conductivity units change in a real installation. Suppose you need to run a 200A, 240V single-phase feeder to a subpanel 100 feet away. The NEC recommends a maximum voltage drop of 3% for feeders (7.2V total loop drop).
We will compare Copper (100% IACS / 58 MS/m) against 1350 Aluminum (61% IACS / 35.4 MS/m). Using standard DC resistance values from NEC Chapter 9, Table 8 for uncoated conductors:
- 4/0 AWG Copper: 0.0766 Ω per 1,000 ft.
- 4/0 AWG Aluminum: 0.122 Ω per 1,000 ft.
The Copper Calculation (100% IACS):
One-way resistance for 100 ft = 0.0766 × 0.1 = 0.00766 Ω.
One-way voltage drop = 200A × 0.00766 Ω = 1.53V.
Total loop drop (line + neutral) = 1.53V × 2 = 3.06V.
Result: 3.06V is well under the 7.2V limit. 4/0 Copper passes.
The Aluminum Calculation (61% IACS):
One-way resistance for 100 ft = 0.122 × 0.1 = 0.0122 Ω.
One-way voltage drop = 200A × 0.0122 Ω = 2.44V.
Total loop drop = 2.44V × 2 = 4.88V.
Wait, 4.88V is also under 7.2V. But let's look at a 150-foot run.
At 150 feet, the Copper loop drop becomes 4.59V (passes). The Aluminum loop drop becomes 7.32V (fails the 3% rule). To fix the aluminum run, you must step up to 300 kcmil Aluminum (0.085 Ω/kft), which brings the 150-foot loop drop down to 5.10V.
The Takeaway: Because aluminum's conductivity is only 61% of copper's (35.4 MS/m vs 58 MS/m), you must increase the physical wire size by two full AWG steps to achieve the same voltage drop performance over longer distances.
Where You Meet This in Practice
You won't typically calculate MS/m from scratch on a jobsite, but you will interact with the units of electrical conductivity in three specific scenarios:
- Busbar and Metallurgy Verification: When sourcing copper busbars for high-current DC systems (like solar inverters or battery banks), suppliers use eddy current testing to verify the material. According to Iowa State's NDE eddy current resources, the meter reads conductivity directly in % IACS to ensure the copper isn't alloyed with impurities that would cause localized heating at bolted joints.
- Wire Datasheets: When buying bulk magnet wire or high-voltage transmission cable, the spec sheet will list conductivity in % IACS. Aluminum conductor steel-reinforced (ACSR) cables specify the aluminum strands' IACS rating to calculate line losses.
- PCB Trace Plating: In high-frequency RF or high-current PCB design, surface finishes matter. ENIG (Electroless Nickel Immersion Gold) has a nickel layer with a conductivity of roughly 14 MS/m (24% IACS), which is vastly inferior to bare copper (58 MS/m). This matters for high-current traces where the skin effect pushes current to the surface.
Conductor Selection Decision Tree
Use this decision path to select your conductor material and insulation type based on your installation constraints and the conductivity properties of the metals.
| Installation Constraint | Material Choice (Conductivity) | Terminating Concrete Pick |
|---|---|---|
| Run is > 100 ft, budget is tight, and conduit has extra fill capacity. | Aluminum (61% IACS / 35.4 MS/m). Cheaper per pound, but requires larger AWG to offset lower conductivity. | 4/0 AWG XHHW-2 Aluminum (Compact stranded, rated 90°C in wet/dry locations). |
| Panel space is extremely tight, run is < 50 ft, and termination lugs are small. | Copper (100% IACS / 58 MS/m). Higher conductivity allows smaller physical wire size, saving bend radius and panel space. | 2/0 AWG THHN Copper (Smaller diameter, easier to terminate on compact breakers). |
| High-vibration environment (e.g., marine or mobile solar) with frequent thermal cycling. | Copper (100% IACS). Aluminum creeps under pressure and thermal cycling, leading to loose terminations and arcing. | 2 AWG Welding Cable (Fine Stranded Copper) with adhesive-lined heat shrink lugs. |
Unit Conversion Reference Chart
When reading international datasheets or older metallurgy texts, you will encounter various units. The NIST guide to non-SI units and ASTM B193 standards govern how these are tested and converted. Use this chart to translate between them.
| Material (at 20°C) | Conductivity (MS/m) | Conductivity (% IACS) | Resistivity (nΩ·m) |
|---|---|---|---|
| Silver (Pure) | 63.0 | 108.6% | 15.87 |
| Copper (Annealed Std) | 58.0 | 100.0% | 17.24 |
| Gold (Pure) | 45.2 | 78.0% | 22.14 |
| Aluminum (1350 Alloy) | 35.4 | 61.0% | 28.25 |
| Tungsten | 18.9 | 32.6% | 52.80 |
| Nickel (Pure) | 14.3 | 24.7% | 69.90 |
Frequently Asked Questions
Why do we still use % IACS instead of just sticking to SI units?
% IACS remains the dominant unit in wire manufacturing because it provides an immediate, intuitive benchmark. When a wire engineer sees '61% IACS', they instantly know it behaves like standard electrical aluminum and requires a 1.6x cross-sectional area increase over copper. It is a practical shorthand that avoids floating-point math on the factory floor.
Does temperature change the units of electrical conductivity?
The units (S/m or % IACS) remain the same, but the value changes drastically. Copper's conductivity drops by roughly 0.4% for every 1°C rise in temperature. This is why ampacity tables in the NEC are based on specific temperature columns (60°C, 75°C, 90°C); as the wire heats up from I²R losses, its conductivity falls, resistance rises, and it generates even more heat.
What is the default recommendation for residential branch circuits?
For standard 15A and 20A residential branch circuits under 100 feet, always default to Copper (100% IACS). The cost savings of aluminum at these small gauges is negligible (often less than $15 per roll), but copper's superior conductivity and mechanical stability at small screw terminations eliminate the risk of thermal creep and outlet fires. Save aluminum for 100A+ feeders and service entrance cables where the weight and cost savings scale meaningfully.
