Cable Voltage Drop and Size Selection Calculator
Find the smallest copper or aluminium cable that keeps the voltage drop within your limit. The workbook calculates the drop for every standard size from 1.5 to 300 mm² at the conductor operating temperature.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 66 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Voltage drop for each size
| Size (mm²) | Copper R20 (Ω/km) | Aluminium R20 (Ω/km) | R at temperature (Ω/km) | Voltage drop (V) | Voltage drop (%) | Within limit |
|---|---|---|---|---|---|---|
| 1.5 | 12.1 | 14.4777 | 115.49 | 27.83 | CHECK | |
| 2.5 | 7.41 | 8.8661 | 70.88 | 17.08 | CHECK | |
| 4 | 4.61 | 5.5159 | 44.25 | 10.66 | CHECK | |
| 6 | 3.08 | 3.6852 | 29.69 | 7.15 | CHECK | |
| 10 | 1.83 | 2.1896 | 17.80 | 4.29 | CHECK | |
| 16 | 1.15 | 1.91 | 1.3760 | 11.33 | 2.73 | OK |
| 25 | 0.727 | 1.2 | 0.8699 | 7.31 | 1.76 | OK |
| 35 | 0.524 | 0.868 | 0.6270 | 5.38 | 1.30 | OK |
| 50 | 0.387 | 0.641 | 0.4630 | 4.08 | 0.98 | OK |
| 70 | 0.268 | 0.443 | 0.3207 | 2.94 | 0.71 | OK |
| 95 | 0.193 | 0.32 | 0.2309 | 2.23 | 0.54 | OK |
| 120 | 0.153 | 0.253 | 0.1831 | 1.85 | 0.45 | OK |
… and 4 more rows in the download.
About this Calculator
Long motor and instrument power feeders are usually sized by voltage drop rather than current. Seeing the drop for every standard size side by side makes it easy to choose and justify the cable, and to see what one size up or down would mean.
Basis: Conductor resistances from IEC 60228 (class 2, at 20 °C), corrected to operating temperature. Current carrying capacity is not checked: confirm it with IEC 60364-5-52 or your cable data.
How to use
- Select the system and enter voltage, current, power factor and route length.
- Choose copper or aluminium and the conductor operating temperature (70 °C PVC, 90 °C XLPE).
- Set the allowed voltage drop (often 3 % for lighting and 5 % for power, per project).
- Read the smallest size within the limit, then confirm its current rating for your installation method.
Formulas used
- R at temperature = R20 × (1 + α (T − 20)), α = 0.00393 (Cu) or 0.00403 (Al).
- ΔV = k × I × L × (R cos φ + X sin φ) ÷ 1000.
- Smallest size = first row with ΔV % within the limit (INDEX and MATCH).
Related tools
Frequently Asked Questions
What voltage drop is allowed in a cable?
Many standards and projects allow 3 % for lighting and 5 % for other loads from the origin of the installation, but check your local rules.
Why use resistance at operating temperature?
Copper resistance rises about 0.39 % per °C, so a cable at 70 °C has about 20 % more resistance than at 20 °C.
Does this sheet check current capacity?
No. Confirm the current rating for your installation method and grouping from IEC 60364-5-52 or the cable manufacturer.
Learn more on the blog
Cable sizing for motor feeders: worked example
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article