Copper vs Aluminium Cable: Resistance, Size and Voltage Drop

Aluminium conducts less well than copper: at 20 °C its resistivity is 1.639 times that of annealed copper, so an aluminium conductor needs about 64 % more cross-section for the same resistance. It is also much lighter, which is why it dominates overhead lines and is common in large feeders, while copper remains the norm for building wiring and small sizes.

Every number below is computed from the resistivities and temperature coefficients used by the site's voltage drop calculator, so the comparison can be checked line by line.

Last reviewed 10 November 2026

DC resistance of one conductor from ρ20 and α20; 70 °C is the maximum operating temperature of PVC insulation. Aluminium values are shown for every size for comparison, although small aluminium conductors are rarely used in building wiring.
Size (mm²)	Cu at 20 °C (Ω/km)	Cu at 70 °C (Ω/km)	Al at 20 °C (Ω/km)	Al at 70 °C (Ω/km)	Al size with R ≤ Cu (mm²)	Cu mass (kg/km)	Mass of that Al size (kg/km)
1.5	11.49	13.75	18.84	22.64	2.5	13.34	6.757
2.5	6.896	8.252	11.31	13.58	6	22.23	16.22
4	4.31	5.157	7.066	8.49	10	35.56	27.03
6	2.874	3.438	4.711	5.66	10	53.34	27.03
10	1.724	2.063	2.826	3.396	25	88.9	67.58
16	1.078	1.289	1.767	2.122	35	142.2	94.6
25	0.6896	0.8252	1.131	1.358	50	222.3	135.2
35	0.4926	0.5894	0.8075	0.9703	70	311.2	189.2
50	0.3448	0.4126	0.5653	0.6792	95	444.5	256.8
70	0.2463	0.2947	0.4038	0.4851	120	622.3	324.4
95	0.1815	0.2171	0.2975	0.3575	185	844.6	500.1
120	0.1437	0.1719	0.2355	0.283	240	1,067	648.7
150	0.1149	0.1375	0.1884	0.2264	300	1,334	810.9
185	0.09319	0.1115	0.1528	0.1836	400	1,645	1,081
240	0.07184	0.08595	0.1178	0.1415	400	2,134	1,081

DC resistance of one conductor from ρ20 and α20; 70 °C is the maximum operating temperature of PVC insulation. Aluminium values are shown for every size for comparison, although small aluminium conductors are rarely used in building wiring.

Size (mm²)

Cu at 20 °C (Ω/km)

Cu at 70 °C (Ω/km)

Al at 20 °C (Ω/km)

Al at 70 °C (Ω/km)

Al size with R ≤ Cu (mm²)

Cu mass (kg/km)

Mass of that Al size (kg/km)

1.5

11.49

13.75

18.84

22.64

2.5

13.34

6.757

2.5

6.896

8.252

11.31

13.58

22.23

16.22

4.31

5.157

7.066

8.49

35.56

27.03

2.874

3.438

4.711

5.66

53.34

27.03

1.724

2.063

2.826

3.396

88.9

67.58

1.078

1.289

1.767

2.122

142.2

94.6

0.6896

0.8252

1.131

1.358

222.3

135.2

0.4926

0.5894

0.8075

0.9703

311.2

189.2

0.3448

0.4126

0.5653

0.6792

444.5

256.8

0.2463

0.2947

0.4038

0.4851

120

622.3

324.4

0.1815

0.2171

0.2975

0.3575

185

844.6

500.1

120

0.1437

0.1719

0.2355

0.283

240

1,067

648.7

150

0.1149

0.1375

0.1884

0.2264

300

1,334

810.9

185

0.09319

0.1115

0.1528

0.1836

400

1,645

1,081

240

0.07184

0.08595

0.1178

0.1415

400

2,134

1,081

How the values are calculated

R_T (Ω/km) = 1000 × ρ20 × (1 + α20 × (T − 20)) / A Cu: ρ20 = 0.017241 Ω·mm²/m, α20 = 0.00393 /K Al: ρ20 = 0.028264 Ω·mm²/m, α20 = 0.00403 /K ρ_Al / ρ_Cu = 1.639 at 20 °C, 1.646 at 70 °C Mass (kg/km) = density (g/cm³) × A (mm²); Cu 8.89, Al 2.703

The equal-resistance column takes the smallest standard IEC size of aluminium whose resistance does not exceed that of the copper conductor. Because standard sizes come in steps, that is the first standard size above 1.64 × the copper size, which is often two sizes up: 16 mm² copper needs 35 mm² aluminium on resistance alone. An aluminium conductor of exactly equal resistance weighs about 50 % of the copper one.

Voltage drop on the same run

Three-phase 400 V, 150 A at power factor 0.85, 120 m route, conductor at 70 °C, reactance 0.08 mΩ/m for all three cables:

Conductor	R at 70 °C (mΩ/m)	Voltage drop (V)	Voltage drop (%)
Copper 70 mm²	0.2947	9.12	2.28
Aluminium 70 mm²	0.4851	14.2	3.54
Aluminium 120 mm²	0.283	8.81	2.2

ΔV = √3 × I × L × (R cosφ + X sinφ) Cu 70: ΔV = 1.73205 × 150 × 120 × (0.0002947 × 0.85 + 0.00008 × 0.5268) = 9.12 V

Swapping copper for aluminium of the same size raises the drop from 2.28 % to 3.54 %; moving up to 120 mm² aluminium brings it back to 2.2 %. Current-carrying capacity must still be checked separately, because aluminium tables differ from the copper ones.

IEC 60228 and real conductor resistance

IEC 60228 defines conductor classes (solid, stranded, flexible) and the maximum DC resistance at 20 °C for each size of copper and aluminium conductor. Those maxima are close to, but not identical with, the resistivity values used here, because stranding and manufacturing tolerances are built in. For a final design use the IEC 60228 table or the cable maker's data, which the voltage drop calculator accepts as a resistance input.

Practical differences

Copper

Smaller cross-section for the same resistance and current, so smaller glands, conduits and terminals.
Forgiving terminations: standard lugs and terminals, low risk of loosening over time.
Usual choice for final circuits, flexible cords and control wiring.
Heavier, and the material cost per metre is generally higher; the gap depends on market prices at the time of purchase.

Aluminium

About 50 % of the mass of copper for equal resistance, which eases pulling and supports on long, large feeders.
Larger diameter for the same duty: check glands, lug sizes, tray fill and the switchgear terminal space.
Terminations need care: the oxide film, creep under pressure and galvanic action with copper call for lugs and connectors rated for aluminium (bimetallic where they meet copper), the maker's contact compound and tightening torque.
Material cost per metre for equal resistance is often lower, but this depends on market prices; compare installed cost, including the larger accessories.