Case 1
- Current (A)
- 10A
- One-way length (m)
- 20m
- Resistivity (Ω·m)
- 1.68e-8Ω·m
- Cross-section (m²)
- 0.000002m²
3.360V
Open with these valuesVoltage Drop Calculator
3.360V
Result: 3.360 VTen amperes over 20 m of 2 mm² copper lose 3.36 V. Enter the one-way distance: the formula doubles it, because the current flows out to the load and back through the second conductor. The power burnt in the cable is the drop times the current, here 33.6 W.
Worked examples
Case 2
- Current (A)
- 15A
- One-way length (m)
- 30m
- Resistivity (Ω·m)
- 1.68e-8Ω·m
- Cross-section (m²)
- 0.000004m²
3.780V
Open with these valuesCase 3
- Current (A)
- 20A
- One-way length (m)
- 50m
- Resistivity (Ω·m)
- 2.82e-8Ω·m
- Cross-section (m²)
- 0.000006m²
9.400V
Open with these valuesHow it's calculated
Vdrop = 2 × L × I × ρ / A
- StepEnter the current the cable carries, in amperes.
- StepEnter the ONE-WAY length; the formula doubles it for the return conductor.
- StepEnter the resistivity and the cross-section in m² — 1 mm² is 0.000001 m².
- ResultRead the drop in volts; multiply it by the current for the watts lost.
Reference table
| Current, one-way length, ρ, area | What that run is | Voltage drop (V) |
|---|---|---|
| 5, 10, 0.0000000168, 0.0000015 | 5 A over 10 m of 1.5 mm² copper | 1.12 |
| 30, 25, 0.0000000168, 0.00001 | 30 A over 25 m of 10 mm² copper | 2.52 |
| 10, 20, 0.0000000168, 0.000002 | 10 A over 20 m of 2 mm² copper | 3.36 |
| 15, 30, 0.0000000168, 0.000004 | 15 A over 30 m of 4 mm² copper | 3.78 |
| 20, 50, 0.0000000282, 0.000006 | 20 A over 50 m of 6 mm² aluminium | 9.40 |
Questions
How do I calculate voltage drop?
Multiply two times the one-way length by the current and the resistivity, then divide by the cross-sectional area: Vdrop = 2 × L × I × ρ / A. The factor of two covers the round trip out and back. Ten amperes over 20 m of 2 mm² copper give 3.36 V.
Why does the formula multiply the length by two?
Because the current has to travel out to the load and back to the source. A circuit needs two conductors, so the wire the current actually passes through is twice the one-way distance. Enter the one-way length and the calculator handles the round trip.
What is voltage drop?
Voltage drop is the voltage lost as current flows through the resistance of the conductor. The longer or thinner the cable, the less voltage is left for the device at the far end. It is measured in volts, and it is the practical reason cable size matters on long runs.
How much power does the cable waste?
The power lost in the cable is the voltage drop multiplied by the current, so the 3.36 V example at 10 A burns 33.6 W as heat. Halving the drop halves the wasted power at the same current. That heat is the reason undersized cable runs warm.
How do I reduce voltage drop?
Use a thicker conductor, shorten the run, or switch from aluminium to copper. The drop is inversely proportional to the cross-section, so doubling the area halves it. Raising the supply voltage also helps, because the same power then needs less current.
What units should I use?
Amperes for the current, metres for the one-way length, ohm-metres for the resistivity and square metres for the cross-section, which gives the drop in volts. Convert mm² to m² by dividing by one million, so a 2.5 mm² conductor is 0.0000025 m².
Sources and last check
Information, not professional advice.