Conductor resistance is a key input for cable voltage drop, power loss, short-circuit, and thermal calculations. It depends on conductor material, cross-sectional area, conductor construction, frequency, and operating temperature.
Use this guide to select a resistance value at 20 °C, adjust it to the conductor operating temperature, and account for skin and proximity effects in AC cables.
Resistance is used in voltage drop and power loss calculations; for resistance and reactance together, see Cable Impedance.
D.C. resistance
For a uniform conductor, the basic resistivity relationship estimates its d.c. resistance per unit length at 20 °C:
| R20 | D.C. resistance per unit length at 20 °C, Ω/m |
| ρ20 | Electrical resistivity at 20 °C, Ω·m |
| S | Conductor cross-sectional area, m2 |
This equation uses S in m2: 1 mm2 = 10−6 m2. For a length L in metres, total conductor resistance is Rlength = R20 × L when R20 is in Ω/m.
EC 60228 Resistance
For finished cable conductors, IEC 60228 specifies maximum d.c. resistance at 20 °C. These limits account for conductor class and construction, and will not equal a simple resistivity calculation using nominal CSA. Use the applicable cable data and design standard. myCableEngineering uses IEC 60228 values where they apply and calculates resistance outside the tabulated range. See how myCableEngineering cable sizing software uses cable data for current capacity, voltage drop, and fault checks.
IEC 60228 conductor classes
- Class 1: solid conductors.
- Class 2: stranded conductors.
- Class 5: flexible conductors.
- Class 6: flexible conductors with greater flexibility than class 5.
IEC 60228 maximum d.c. resistance at 20 °C
The selected values below were checked against BS EN IEC 60228:2024: Tables 3 and 4 for classes 1 and 2, and Tables 5 and 6 for flexible copper. This is a selected resistance reference, not the complete construction specification.
Units: all resistance values in the next two tables are Ω/km, numerically identical to mΩ/m. Divide by 1,000 to obtain Ω/m. CSA means nominal cross-sectional area. A dash means no value is tabulated for that combination in the cited table; it does not mean zero resistance.
Plain copper and class 2 aluminium
| CSA, mm2 | Plain copper class 1 | Plain copper class 2 | Aluminium class 2 |
|---|---|---|---|
| 0.5 | 36.0 | 36.0 | — |
| 0.75 | 24.5 | 24.5 | — |
| 1 | 18.1 | 18.1 | — |
| 1.5 | 12.1 | 12.1 | — |
| 2.5 | 7.41 | 7.41 | — |
| 4 | 4.61 | 4.61 | — |
| 6 | 3.08 | 3.08 | — |
| 10 | 1.83 | 1.83 | 3.08 |
| 16 | 1.15 | 1.15 | 1.91 |
| 25 | 0.727 | 0.727 | 1.20 |
| 35 | 0.524 | 0.524 | 0.868 |
| 50 | 0.387 | 0.387 | 0.641 |
| 70 | 0.268 | 0.268 | 0.443 |
| 95 | 0.193 | 0.193 | 0.320 |
| 120 | 0.153 | 0.153 | 0.253 |
| 150 | 0.124 | 0.124 | 0.206 |
| 185 | 0.101 | 0.0991 | 0.164 |
| 240 | 0.0775 | 0.0754 | 0.125 |
| 300 | 0.0620 | 0.0601 | 0.100 |
| 400 | 0.0465 | 0.0470 | 0.0778 |
| 500 | — | 0.0366 | 0.0605 |
| 630 | — | 0.0283 | 0.0469 |
| 800 | — | 0.0221 | 0.0367 |
| 1000 | — | 0.0176 | 0.0291 |
| 1200 | — | 0.0151 | 0.0247 |
The aluminium column here is for class 2; class 1 aluminium and aluminium alloys have additional qualifications in the standard. For stranded aluminium alloy, agree the resistance with the manufacturer.
The class 2 copper values at 800, 1,000 and 1,200 mm2 are included here; the standard also provides larger class 2 sizes beyond this selected extract.
Flexible copper: classes 5 and 6
| CSA, mm2 | Plain copper | Metal-coated copper |
|---|---|---|
| 0.5 | 39.0 | 40.1 |
| 0.75 | 26.0 | 26.7 |
| 1 | 19.5 | 20.0 |
| 1.5 | 13.3 | 13.7 |
| 2.5 | 7.98 | 8.21 |
| 4 | 4.95 | 5.09 |
| 6 | 3.30 | 3.39 |
| 10 | 1.91 | 1.95 |
| 16 | 1.21 | 1.24 |
| 25 | 0.78 | 0.795 |
| 35 | 0.554 | 0.565 |
| 50 | 0.386 | 0.393 |
| 70 | 0.272 | 0.277 |
| 95 | 0.206 | 0.210 |
| 120 | 0.161 | 0.164 |
| 150 | 0.129 | 0.132 |
| 185 | 0.106 | 0.108 |
| 240 | 0.0801 | 0.0817 |
| 300 | 0.0641 | 0.0654 |
| 400* | 0.0486 | 0.0495 |
| 500* | 0.0384 | 0.0391 |
| 630* | 0.0287 | 0.0292 |
The flexible-copper values apply to both classes 5 and 6 up to 300 mm2. Entries marked * (400, 500 and 630 mm2) apply to class 5 only. “Metal-coated” includes tinned copper.
Temperature adjustment
Conductor d.c. resistance increases with temperature. To adjust the 20 °C value to the conductor operating temperature t:
| Rt | D.C. resistance per unit length at temperature t |
| R20 | D.C. resistance per unit length at 20 °C, in the same units |
| t | Conductor temperature, °C |
| α20 | Temperature coefficient of resistance at 20 °C, K−1 |
Use α20 = 0.00393 K−1 for annealed copper (plain or metal-coated), and 0.00403 K−1 for aluminium. These correspond to the material-specific correction relationships in Annex B of BS EN IEC 60228:2024. Obtain aluminium-alloy data from the manufacturer.
Cable operating temperature
For a simple steady-state estimate, assume fixed ambient and installation conditions and a temperature rise proportional to current squared. Neglecting other heat sources and losses, the conductor is approximately at ambient temperature at zero current and at its permitted operating temperature when carrying its sustained rating:
| Ib | Cable design current, A |
| Iz | Sustained current rating for the actual installation and ambient conditions, including applicable correction factors, A |
| Ta | Ambient temperature, °C |
| Tc | Maximum permitted conductor operating temperature for the cable and application, °C |
Use this as an approximation for 0 ≤ Ib ≤ Iz, not as a transient or cyclic thermal model. Other heat sources, temperature-dependent losses and changing installation conditions can alter the result. For detailed temperature and rating calculations, use the applicable thermal method and cable data.
For related calculations, see Cable Thermal Analysis and Estimating Cable Life.
Worked example of calculating resistance
Consider a 10 mm2 class 2 plain copper conductor, 100 m long, operating at 90 °C. Its tabulated maximum d.c. resistance at 20 °C is 1.83 Ω/km. Using the copper coefficient:
- Temperature multiplier: 1 + 0.00393 × (90 − 20) = 1.2751.
- D.C. resistance at 90 °C: 1.83 × 1.2751 = 2.3334 Ω/km.
- Convert to Ω/m: 2.3334 ÷ 1,000 = 0.0023334 Ω/m.
- Resistance of one 100 m conductor: 0.0023334 × 100 ≈ 0.233 Ω.
For an identical outgoing and return conductor, each 100 m long, the d.c. loop resistance is approximately 0.467 Ω. For an AC calculation, use the temperature-adjusted value in Ω/m to calculate skin and proximity effects before multiplying by length.
A.C. resistance
At the same conductor temperature, a.c. resistance is generally higher than d.c. resistance because of skin and proximity effects. For small conductors at power frequency, the difference can be negligible. The IEC 60287 conductor-resistance relationship is:
| Rac | A.C. resistance per unit length at conductor temperature t, Ω/m |
| Rt | D.C. resistance per unit length at the same temperature t, Ω/m |
| ys | Skin effect factor, dimensionless |
| yp | Proximity effect factor, dimensionless |
Calculate Rt at the operating temperature first, then calculate Xs, Xp and their correction factors at the required frequency. Recalculate these factors if the temperature or frequency changes.
Skin effect
As frequency increases, current tends to concentrate nearer the conductor surface. The effect becomes more significant for larger conductors. For Xs ≤ 2.8:
Here f is frequency in Hz, Rt must be in Ω/m, and ks is a dimensionless construction coefficient. Xs is dimensionless. For Xs > 2.8, use the other skin-effect expressions in IEC 60287-1-1 rather than extrapolating this expression.
Proximity effect
Nearby conductors change the magnetic field and current distribution within a conductor. The following proximity expressions apply to circular conductors, with Xp ≤ 2.8, using the appropriate cable arrangement:
For two-core cables or two single-core cables carrying the outgoing and return currents:
For three-core cables or three single-core cables in an equally spaced trefoil arrangement carrying balanced three-phase currents:
For both cases:
| f | Frequency, Hz |
| dc | External diameter of the circular conductor |
| s | Centre-to-centre spacing of adjacent conductors; use the same length unit as dc |
| kp | Dimensionless proximity-effect construction coefficient |
| Xp | Dimensionless proximity parameter; calculate with Rt in Ω/m |
Flat formations, unequal spacing, sector-shaped conductors, Milliken conductors and pipe-type cables require the relevant construction-specific IEC 60287 provisions. Do not apply the circular-conductor equations or a geometric mean spacing substitution indiscriminately.
For spacing concepts, see Geometric Mean Distance.
Skin and proximity coefficients
| Material | Conductor form | ks | kp |
|---|---|---|---|
| Copper | Circular solid or stranded (non-segmental) | 1 | 1 |
| Aluminium | Circular solid or stranded (non-segmental) | 1 | 1 |
The coefficients above are for the stated non-segmental circular constructions. For segmental/Milliken, hollow or other special conductors, select coefficients from the applicable IEC 60287-1-1 provisions or manufacturer data. A single generic segmental value is not suitable for every cable construction.
Screens, sheaths and armour can introduce additional AC losses, depending on the material and bonding arrangement. These losses are separate from the conductor skin and proximity corrections discussed here. For effective resistance in short-circuit calculations, use the applicable IEC 60909 method for the cable and screen arrangement.
For the material property behind conductor resistance, see Electrical Resistivity.
For background on the basic resistance formula and temperature coefficient, see Electrical Resistance.
References
- BS EN IEC 60228:2024, Conductors of insulated cables: Tables 3–6 and Annexes A and B (BSI subscription access). Identical to IEC 60228:2023.
- IEC 60287-1-1:2023: conductor resistance, skin/proximity effects and cable-loss calculations. Use the full applicable provisions for the construction and arrangement being designed.