Cable size and voltage drop calculator
Enter the load current, run length and cable size to get the voltage drop in volts and percent, the receiving-end voltage and the I²R loss. It also finds the smallest copper or aluminium size that keeps the drop within your limit, for LV feeders and solar DC strings.
Basis
Conductor resistance: IEC 60228 / IS 8130 class 2 maximum DC resistance at 20 °C, corrected to operating temperature.
Reference values for planning and comparison. Confirm against the current standard, the purchaser’s specification and the manufacturer’s datasheet before ordering.
Conductor resistance used (Ω/km)
| Size | Cu Ω/km 20 °C | Cu 70 °C | Cu 90 °C | Al Ω/km 20 °C | Al 90 °C |
|---|---|---|---|---|---|
| 1.5 mm² | 12.1 | 14.4776 | 15.4287 | — | — |
| 2.5 mm² | 7.41 | 8.8661 | 9.4485 | — | — |
| 4 mm² | 4.61 | 5.5159 | 5.8782 | — | — |
| 6 mm² | 3.08 | 3.6852 | 3.9273 | — | — |
| 10 mm² | 1.83 | 2.1896 | 2.3334 | 3.08 | 3.9489 |
| 16 mm² | 1.15 | 1.376 | 1.4664 | 1.91 | 2.4488 |
| 25 mm² | 0.727 | 0.8699 | 0.927 | 1.2 | 1.5385 |
| 35 mm² | 0.524 | 0.627 | 0.6682 | 0.868 | 1.1129 |
| 50 mm² | 0.387 | 0.463 | 0.4935 | 0.641 | 0.8218 |
| 70 mm² | 0.268 | 0.3207 | 0.3417 | 0.443 | 0.568 |
| 95 mm² | 0.193 | 0.2309 | 0.2461 | 0.32 | 0.4103 |
| 120 mm² | 0.153 | 0.1831 | 0.1951 | 0.253 | 0.3244 |
| 150 mm² | 0.124 | 0.1484 | 0.1581 | 0.206 | 0.2641 |
| 185 mm² | 0.0991 | 0.1186 | 0.1264 | 0.164 | 0.2103 |
| 240 mm² | 0.0754 | 0.0902 | 0.0961 | 0.125 | 0.1603 |
| 300 mm² | 0.0601 | 0.0719 | 0.0766 | 0.1 | 0.1282 |
| 400 mm² | 0.047 | 0.0562 | 0.0599 | 0.0778 | 0.0997 |
| 500 mm² | 0.0366 | 0.0438 | 0.0467 | 0.0605 | 0.0776 |
| 630 mm² | 0.0283 | 0.0339 | 0.0361 | 0.0469 | 0.0601 |
Worked example: 100 A over 150 m at 415 V on 50 mm² copper
- R at 90 °C = 0.387 × (1 + 0.00393 × 70) = 0.4935 Ω/km; X = 0.08 Ω/km; cos φ = 0.85, sin φ = 0.53.
- Vd = √3 × 100 × 0.150 km × (0.4935 × 0.85 + 0.08 × 0.53) = 11.99 V.
- 2.89 % of 415 V: within a 3 % limit, so 50 mm² passes on voltage drop. Check current rating separately.
What voltage drop is allowed in a cable?
Many specifications allow 3 % to 5 % from the source to the load for LV circuits, and solar designs often target 1 % to 2 % on DC strings. Use the limit in your project specification or the applicable code.
How is three-phase voltage drop calculated?
Vd = √3 × I × L × (R cos φ + X sin φ), with R the conductor resistance at operating temperature and X the reactance per metre. Single-phase uses 2 instead of √3; DC uses 2 × I × L × R.
Does this calculator check current rating?
No. It sizes by voltage drop only. The cable must also carry the current under its installation conditions, so check the manufacturer’s current rating tables or send us the schedule.