Voltage Drop Calculator
Every metre of wire is a small resistor, so current through a long or thin conductor arrives dimmer than it left — the missing volts become heat in the cable. Lamps glow amber, motors draw more to compensate, and low-voltage DC gear trips early. Electricians size conductors to keep the loss inside a few percent. This calculator returns the wire's resistance, the drop, the percentage lost, and what reaches the load.
Enter values
Results
- Voltage drop (V)
- 1.4
- Drop as % of supply
- 0.61
- Voltage at the load (V)
- 228.6
- Wire resistance (Ω)
- 0.14
How is the Voltage Drop calculated?
Enter the load current in amps, the ONE-WAY length of the wire run in metres — the return leg is counted automatically — the conductor's cross-section in square millimetres, and the material: copper or aluminium, which sets the resistivity. Add the supply voltage. The headline result is the drop in volts, followed by the same drop as a percentage of supply, the voltage that finally reaches the load, and the wire's total resistance for cross-checking.
Formula
R = ρ × 2L ÷ A · Drop = I × R where: ρ = resistivity (copper 0.0175, aluminium 0.0282 Ω·mm²/m) L = one-way wire length in metres — doubled for the return leg A = conductor cross-section in mm² I = load current in amps The ×2 is the way back: current returns to the source through the same size of wire, so both legs resist.
Example
Current: 10 A · One-way length: 10 m · 2.5 mm² copper · 230 V supply R = 0.0175 × 20 ÷ 2.5 = 0.14 Ω · Drop = 10 × 0.14 = 1.4 V That is 0.61% of the supply, leaving 228.6 V at the load — comfortably inside the ~3% guideline. The same run in aluminium gives 0.2256 Ω and a 2.26 V drop, which is why aluminium feeders are specified at a larger cross-section for the same job.
Things to remember
Aim to keep the drop under about 3% — a widely used rule of thumb, not a universal code — and remember the two levers: doubling the length doubles the drop, doubling the cross-section halves it. Undersized wiring also runs hot, which is the real hazard. Match the conductor to the current, not just the voltage.
Frequently Asked Questions
What is an acceptable voltage drop?
A common guideline keeps branch-circuit drop under about 3% and the total under 5% — roughly 7 V on a 230 V supply. This calculator prints your drop as a percentage of the system voltage, so the comparison against that guideline is immediate.
Why is the wire length doubled in the formula?
Because current makes a round trip: out along the live or positive conductor and back along the neutral, ground or return leg. Both directions have resistance, so the one-way length you enter is doubled before resistivity and cross-section are applied.
Copper or aluminium: which drops less voltage?
Copper, at about 0.0175 Ω·mm²/m against aluminium's 0.0282 — roughly 60% more resistance for the same size and length. Aluminium is lighter and cheaper, so it earns its place at larger cross-sections, mostly in overhead distribution and big feeders.
Does voltage drop waste electricity?
Yes — the dropped voltage becomes heat in the wire, and those losses scale with the square of the current (P = I² × R). It also starves the load: a starved motor draws more current, which increases the drop further until protection or overheating intervenes.
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