Voltage Drop Calculator
Calculate the electrical voltage drop in a cable by length, cross-section and current.
Tip: Rule of thumb: for every 30 metres of cable length, go up one cross-section size — e.g. if 2.5 mm² is enough for 15 m, use 4 mm² for 30+ m.
Did this tool help?
About this digital tool
Every electrical cable has resistance — the longer and thinner the cable, the more voltage is lost along the way. A voltage drop above 3–5% causes problems: appliances don't work well, lights dim, and energy is wasted as heat. This calculator helps you choose the right cable cross-section for your installation, working out the drop from the length, current and material (copper or aluminium).
How to use
- Enter the system voltage (230V, 400V)
- Enter the current (A) or the power (W)
- Enter the cable length (m)
- Choose the material (copper/aluminium) and the cross-section (mm²)
- See the voltage drop (V and %)
Frequently asked questions
How much voltage drop is acceptable?
The NEC/IEC standard recommends a maximum of 3% for individual branches and 5% in total (from the panel to the appliance). In Albania, the Electrotechnical Code follows IEC standards.
How is voltage drop calculated?
Vd = 2 × I × R × L / 1000, where I = current (A), R = cable resistance (Ω/km depending on the cross-section), L = one-way length (m). The factor 2 accounts for the outward and return runs.
Copper or aluminium?
Copper has 40% lower resistance (0.0175 vs 0.0283 Ω·mm²/m), but costs more. Aluminium is used for long lines (transmission) where weight and cost are the priority.
Does voltage drop depend on the length of the cable?
Yes, the longer the cable, the bigger the drop, because resistance rises with length. The tool uses the formula 2 × current × resistivity × length ÷ cross-section. For long lengths choose a larger cross-section.