Cable voltage drop and size
Enter the current, route length and cable size to see the drop in volts and percent. The second mode picks the smallest standard size.
Result
- Voltage at the load
- 223.1 V
- Power lost in the cable
- 109.7 W
- Conductivity γ
- 56 m/(Ω·mm²)
ΔU = 2 · L · I · cos φ / (γ · S)
| Size | ΔU | ΔU % | Loss |
|---|---|---|---|
| 1.5 mm² | 11.4 V | 4.97% | 183 W |
| 2.5 mm² ← | 6.86 V | 2.98% | 110 W |
| 4 mm² | 4.29 V | 1.86% | 68.6 W |
| 6 mm² | 2.86 V | 1.24% | 45.7 W |
| 10 mm² | 1.71 V | 0.745% | 27.4 W |
| 16 mm² | 1.07 V | 0.466% | 17.1 W |
| 25 mm² | 0.686 V | 0.298% | 11 W |
| 35 mm² | 0.49 V | 0.213% | 7.84 W |
| 50 mm² | 0.343 V | 0.149% | 5.49 W |
| 70 mm² | 0.245 V | 0.106% | 3.92 W |
| 95 mm² | 0.18 V | 0.0785% | 2.89 W |
| 120 mm² | 0.143 V | 0.0621% | 2.29 W |
| 150 mm² | 0.114 V | 0.0497% | 1.83 W |
| 185 mm² | 0.0927 V | 0.0403% | 1.48 W |
| 240 mm² | 0.0714 V | 0.0311% | 1.14 W |
This method only uses conductor resistance, not reactance, so it underestimates the drop above roughly 50 mm². It doesn’t check current-carrying capacity or protection: confirm the cable choice against IEC 60364.
How voltage drop is calculated
Current through a cable causes a drop across its resistance. In a single-phase circuit the current goes out and back, hence the factor 2: ΔU = 2 · L · I · cos φ / (γ · S). In a balanced three-phase system the factor is √3 and the result refers to the line-to-line voltage.
γ is conductivity: 56 m/(Ω·mm²) for copper and 34 for aluminium at 20 °C. At operating temperature (up to 70 °C for PVC insulation) resistance rises by about 20%, and the drop with it.
What drop is allowed?
IEC 60364-5-52 recommends that the drop from the origin of the installation to the load stays within 3% for lighting and 5% for other uses when supplied from the public network. Many designers use 3% for final circuits.
Common questions
Which length do I enter?
The one-way route length from the board to the load. The factor in the formula already accounts for the return conductor.
Is this enough to size a cable?
No. The cable must also carry the current for its installation method, coordinate with the protective device and ensure automatic disconnection.