Quick answer: A 30 m run of 2.5 mm² copper carrying 16 A on 230 V single-phase drops 2 × 16 × 30 × 0.020629 ÷ 2.5 = 7.92 V at 70 °C, which is 3.44% of the supply.
How it's calculated
A cable is a resistor: its resistance is resistivity × length ÷ cross-section. In a single-phase circuit current flows out and back, so the drop is Vd = 2 × I × L × ρ ÷ A, with L the one-way length. In a balanced three-phase circuit the line-to-line drop is Vd = √3 × I × L × ρ ÷ A.
Resistivity is 0.017241 Ω·mm²/m for copper and 0.028264 Ω·mm²/m for aluminium at 20 °C, and it rises with temperature by about 0.4% per °C. A PVC cable at full load runs at about 70 °C, which gives copper 0.0206 Ω·mm²/m — about 20% more drop than the cold value. Some calculators use the 20 °C value; enter 20 °C to compare.
IEC 60364-5-52 suggests keeping the drop from the supply point to the load within 3% for lighting and 5% for other uses. Local codes (for example the Saudi Building Code SBC 401 or DEWA regulations) may set their own limits. The formula ignores cable reactance, so for cables above about 35 mm² the real drop is a little higher.
Worked examples
- 2.5 mm² copper, 30 m, 16 A, 230 V, 70 °C: 2 × 16 × 30 × 0.020629 ÷ 2.5 = 7.92 V (3.44%)
- Same run in 4 mm²: 2 × 16 × 30 × 0.020629 ÷ 4 = 4.95 V (2.15%)
- 16 mm² copper, three-phase, 50 m, 60 A, 400 V, 70 °C: √3 × 60 × 50 × 0.020629 ÷ 16 = 6.70 V (1.67%)
Conductor resistivity used
| Conductor | ρ at 20 °C (Ω·mm²/m) | α (per °C) | ρ at 70 °C |
|---|---|---|---|
| Copper | 0.017241 | 0.00393 | 0.020629 |
| Aluminium | 0.028264 | 0.00403 | 0.033959 |
Frequently asked questions
What is an acceptable voltage drop?
IEC 60364-5-52 suggests at most 3% for lighting and 5% for other loads, measured from the supply point to the equipment. Check the code that applies where you build.
Is the length one way or the round trip?
One way: the distance from the board to the load. The formula multiplies by 2 for single-phase (out and back) and by √3 for three-phase.
Why does temperature matter?
Metal resistance rises as it warms. A cable at its 70 °C operating temperature has about 20% more resistance than at 20 °C, so it drops more voltage.
Does this size the cable for current?
No. Voltage drop is one check; the cable must also carry the current safely for its installation method, which comes from the current-rating tables of your wiring code.
What is the drop on 30 m of 2.5 mm² copper at 16 A?
About 7.9 V, or 3.4% on a 230 V single-phase circuit with the cable at 70 °C. That's fine for sockets, but more than the 3% usually suggested for lighting.
How do I reduce voltage drop?
Go up a cable size, shorten the run, or for big loads use a higher voltage or three-phase. Doubling the cross-section roughly halves the drop.