Engineering Reference
Wire Resistivity, Resistance and Temperature Reference
Lookup reference for copper and aluminum resistivity, conductivity, density, temperature coefficient, resistance per length, mass per length, and temperature correction boundaries.
- Materials
- Copper / aluminum
- Reference
- Approx. 20°C
- Updated
- September 28, 2026
Wire Property Terminology
| Property | Symbol | Unit | Meaning |
|---|---|---|---|
| Electrical resistivity | ρ | Ω·m | Material opposition to current at a stated temperature |
| Electrical conductivity | σ | S/m | Reciprocal of resistivity in consistent SI units |
| Resistance | R | Ω | Depends on material, conductor length, and conductive area |
| Temperature coefficient | α | 1/°C or 1/K | First-order resistance change about a stated reference temperature |
| Mass density | ρm | kg/m³ | Material mass per volume; distinct from electrical resistivity |
| IACS conductivity | % IACS | % | Conductivity relative to the stated annealed-copper reference convention |
| Resistance per length | R/L | Ω/m | Resistivity divided by conductive cross-sectional area |
| Mass per length | m/L | kg/m | Density multiplied by conductive cross-sectional area |
Shared Material Values
These values come from the same shared material records used by ECParts wire calculators. They are engineering references at approximately 20°C, not guaranteed values for every alloy, temper, plating, or product.
| Material | ρ Ω·m | σ MS/m | % IACS | Density kg/m³ | α /°C |
|---|---|---|---|---|---|
| Copper | 1.7240e-8 | 58.005 | 100 | 8960 | 0.00393 |
| Aluminum | 2.8200e-8 | 35.461 | 61 | 2700 | 0.00403 |
Canonical Relationships
| Quantity | Relationship | Boundary |
|---|---|---|
| Resistance | R = ρL/A | Uniform conductor at the resistivity reference condition |
| Conductivity | σ = 1/ρ | Use consistent SI units |
| Measured resistivity | ρ = RA/L | Excludes contact and lead resistance unless included in R |
| Linear temperature correction | R(T) = Rref[1 + α(T − Tref)] | First-order approximation over the applicable range |
| Volume | V = AL | Conductive material only |
| Mass | m = ρmAL | Does not include insulation, plating, connectors, or shields |
Selected AWG Resistance and Mass
Values are generated from the canonical AWG area and shared material properties. Resistance is milliohms per metre; mass is conductor grams per metre.
| Gauge | Material | Area mm² | Resistance mΩ/m | Mass g/m |
|---|---|---|---|---|
| AWG 10 | Copper | 5.2612 | 3.2768 | 47.140 |
| AWG 10 | Aluminum | 5.2612 | 5.3600 | 14.205 |
| AWG 12 | Copper | 3.3088 | 5.2104 | 29.647 |
| AWG 12 | Aluminum | 3.3088 | 8.5228 | 8.934 |
| AWG 18 | Copper | 0.8230 | 20.9466 | 7.374 |
| AWG 18 | Aluminum | 0.8230 | 34.2629 | 2.222 |
| AWG 24 | Copper | 0.2047 | 84.2083 | 1.834 |
| AWG 24 | Aluminum | 0.2047 | 137.7422 | 0.553 |
First-Order Temperature Example
| Material | Reference | Resistance at 75°C Ω | Resistance at 100°C Ω |
|---|---|---|---|
| Copper | 1.0000 Ω at 20°C | 1.216150 | 1.314400 |
| Aluminum | 1.0000 Ω at 20°C | 1.221650 | 1.322400 |
Common Reference Mistakes
- Using resistivity without its reference temperature.
- Confusing electrical resistivity with mass density.
- Mixing Ω·m, Ω·mm²/m, µΩ·cm, and nΩ·m without conversion.
- Using outside cable diameter as conductive metal area.
- Forgetting to double one-way length for a two-conductor loop.
- Treating percent IACS as ampacity or efficiency.
- Applying a linear TCR model over an unsupported range.
- Ignoring alloy, strand, plating, and manufacturing differences.
- Including insulation mass in a conductor-only calculation.
- Ignoring contact and connector resistance in measured geometry.
- Rounding AWG area before engineering calculations.
- Using these material values as code-compliant current ratings.
Support reference
FAQ
What is electrical resistivity?
Resistivity is a material property in ohm-metres that relates conductor resistance to length and conductive area at a stated condition.
What is electrical conductivity?
Conductivity in siemens per metre is the reciprocal of resistivity when consistent SI units are used.
Why must resistivity include a temperature?
Metal resistivity changes with temperature. A value without its reference condition can produce an inconsistent resistance estimate.
How is wire resistance calculated?
For a uniform conductor, R = ρL/A. A two-conductor loop normally uses twice the one-way conductor length.
How does copper resistance change with temperature?
A common first-order model is R(T) = Rref[1 + α(T − Tref)]. The shared copper dataset uses α = 0.00393/°C near its reference condition.
Is mass density the same as electrical resistivity?
No. Density describes mass per volume in kg/m³; resistivity describes electrical opposition in Ω·m.
What does percent IACS mean?
It expresses conductivity relative to an annealed-copper reference convention. It is not conductor purity, efficiency, or ampacity.
Why does aluminum wire need more area than copper?
The shared aluminum reference has higher resistivity, so more conductive area is required for the same resistance and length.
Does resistance per metre include insulation?
No. Electrical resistance uses conductive metal area. Insulation affects outside diameter, thermal behavior, and installation but does not carry normal conductor current.
Does wire mass include insulation and connectors?
No. The tabulated mass examples represent conductor metal only.
Can the linear temperature model be used at any temperature?
No. It is a first-order approximation. Wide ranges, alloys, or precision work require material-specific data.
Does this Reference provide allowable current ratings?
No. Ampacity depends on insulation, installation, ambient temperature, bundling, cooling, regulations, and safety requirements; those topics belong to the separate installation reference.
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