Cable Size and Voltage Drop Calculator
Calculate the voltage drop along a cable and find the smallest standard size that keeps it within your limit. Conductor resistance is corrected to operating temperature.
- ρ at 70 °C = 0.017241 × (1 + 0.00393 × (70 − 20)) = 0.020629 Ω·mm²/m
- Vd = √3 × I × L × (R cos φ + X sin φ), X = 0.08 Ω/km assumed
- Vd = 9.856 V = 2.37 % of 415 V
Voltage drop for each standard size
| Size | Resistance | Voltage drop | Percent |
|---|---|---|---|
| 1.5 mm² | 13.75 Ω/km | 101.6 V | 24.48 % |
| 2.5 mm² | 8.252 Ω/km | 61.11 V | 14.72 % |
| 4 mm² | 5.157 Ω/km | 38.33 V | 9.24 % |
| 6 mm² | 3.438 Ω/km | 25.67 V | 6.19 % |
| 10 mm² | 2.063 Ω/km | 15.55 V | 3.75 % |
| 16 mm² | 1.289 Ω/km | 9.856 V | 2.37 % |
| 25 mm² | 0.8252 Ω/km | 6.439 V | 1.55 % |
| 35 mm² | 0.5894 Ω/km | 4.704 V | 1.13 % |
| 50 mm² | 0.4126 Ω/km | 3.402 V | 0.82 % |
| 70 mm² | 0.2947 Ω/km | 2.534 V | 0.61 % |
| 95 mm² | 0.2171 Ω/km | 1.963 V | 0.47 % |
| 120 mm² | 0.1719 Ω/km | 1.63 V | 0.39 % |
| 150 mm² | 0.1375 Ω/km | 1.377 V | 0.33 % |
| 185 mm² | 0.1115 Ω/km | 1.186 V | 0.29 % |
| 240 mm² | 0.08595 Ω/km | 0.9977 V | 0.24 % |
| 300 mm² | 0.06876 Ω/km | 0.8711 V | 0.21 % |
Voltage Drop Formulas
Single phase: Vd = 2 × I × L × (R cos φ + X sin φ)
Three phase: Vd = √3 × I × L × (R cos φ + X sin φ)
R is the conductor resistance per metre at operating temperature, from ρ = ρ20 × (1 + α(T − 20)) ÷ A, with ρ20 = 0.017241 Ω·mm²/m for copper and 0.028264 for aluminium. Reactance X is taken as 0.08 Ω/km, typical of multicore LV cable.
Worked example
A 415 V three phase load draws 50 A over 100 m of 16 mm² copper at 70 °C, pf 0.85. R = 1.289 Ω/km and the drop is about 9.9 V, or 2.4 %, within a 3 % limit.
Frequently Asked Questions
What is the maximum allowed voltage drop?
Many standards and specifications allow 3 % for lighting and 5 % for other loads from the supply point. Check your local wiring rules and project specification.
Why is cable resistance corrected for temperature?
Copper resistance rises about 0.4 % per °C. A cable at 70 °C has about 20 % more resistance than at 20 °C, so using 20 °C values underestimates voltage drop.
Does this calculator check cable ampacity?
No. It checks voltage drop only. Current carrying capacity depends on insulation, installation method, grouping and ambient temperature and must be checked against the cable standard.
Learn more on the blog
Cable sizing and ampacity explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article