Cable Voltage Drop
Voltage drop along a low-voltage cable run and the smallest section that keeps it within the limit, using the resistive method common in installation practice.
Voltage drop1.67 %
- Voltage drop ΔU
- 6.7 V
- Line current I
- 25.5 A
- Power loss in cable
- 348 W
- Section needed for the limit
- 3.35 mm²
- Suggested standard section
- 4 mm²
- Drop 1.67 % — limit 3.0 %
- Check current-carrying capacity for the installation method separately (manufacturer tables)
METHOD
How it is calculated
Single-phase: ΔU = 2 · L · I · cos φ / (κ · S) Three-phase: ΔU = √3 · L · I · cos φ / (κ · S) I = P / (U · cos φ) · I = P / (√3 · U · cos φ)
ASSUMPTIONS & NOTES
- κ: copper 56, aluminium 35 m/(Ω·mm²) at 20 °C. Resistance rises ≈ 20 % at operating temperature.
- Line reactance is ignored; include it for sections above 50 mm² and long runs.
- Common limits: lighting 1.5 %, power 3 %; current-carrying capacity must also be checked.
Results are for preliminary design and checking; base the final design on the governing standard and safety factors.
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