Cable Voltage Drop Calculator
By Santaji GadepatilinUpdated October 27, 2026
Every cable has resistance, so some voltage is lost between the supply and the load. Too much drop makes motors overheat and lights dim, which is why standards limit it to about 3–5%. Learn the method in this voltage drop calculation guide and how it fits with choosing cable size for load current. This free cable voltage drop calculator solves for whichever value you leave blank and shows the formula and steps.
Voltage drop (% of supply) (%) vs Cable length (one way) (m) — other inputs held at your current values.
How to use the cable voltage drop calculator
Choose the circuit type and conductor material, then enter the supply voltage, current, one-way cable length and cross-section. The voltage drop appears in volts and as a percentage, with a colour guide. To size a cable, press × on the cross-section and type your allowed drop %.
For more background, read Cable Sizing for Motor Feeders.
Cable voltage drop formula
Single-phase: Vd = 2 × L × I × ρ ÷ A
Three-phase: Vd = √3 × L × I × ρ ÷ A
- ρ — resistivity: copper 1.72 × 10⁻⁸ Ω·m, aluminium 2.82 × 10⁻⁸ Ω·m
- L — one-way length
- A — conductor cross-section
Worked examples
Single-phase: 16 A at 230 V over 30 m of 2.5 mm² copper: Vd = 2 × 30 × 16 × 1.72 × 10⁻⁸ ÷ 2.5 × 10⁻⁶ = 6.6 V (2.9%)
Three-phase: 32 A at 415 V over 50 m of 6 mm² copper: Vd = 1.732 × 50 × 32 × 1.72 × 10⁻⁸ ÷ 6 × 10⁻⁶ = 7.9 V (1.9%)
Standard cable sizes (mm²)
1.5 · 2.5 · 4 · 6 · 10 · 16 · 25 · 35 · 50 · 70 · 95 · 120 · 150 · 185 · 240
Copper resistance per conductor size
| Size (mm²) | Resistance at 20 °C (Ω/km) | Typical max current, clipped direct (A) |
|---|---|---|
| 1.5 | 12.1 | 19–20 |
| 2.5 | 7.41 | 26–27 |
| 4 | 4.61 | 35–36 |
| 6 | 3.08 | 45–46 |
| 10 | 1.83 | 61–63 |
| 16 | 1.15 | 81–85 |
| 25 | 0.727 | 105–112 |
Resistance values follow IEC 60228 for plain copper. Current ratings depend heavily on installation method, grouping and ambient temperature — check your local wiring rules.
Voltage drop limits
Common design limits (as a percentage of supply voltage):
| Circuit | Typical limit |
|---|---|
| Lighting | 3% |
| Other final circuits | 5% |
| Motor starting | 10–15% (briefly) |
Limits vary by country. IEC 60364 and BS 7671 use 3% for lighting and 5% for other uses from the origin of the installation.
Temperature effect
Copper resistance rises about 0.39% per °C. At a conductor temperature of 70 °C, resistance is about 20% higher than at 20 °C, so voltage drop is higher in fully loaded cables.
Choosing a cable size
- Size for current-carrying capacity first.
- Check voltage drop over the run length.
- If the drop is too high, step up one or two sizes.
- Check fault-loop impedance and protective device coordination.
Remember
Voltage drop is only one check. The cable must also carry the current safely (current rating with installation derating) and allow protective devices to disconnect a fault in time.
Frequently asked questions
What is an acceptable voltage drop?
Many wiring codes recommend about 3% for lighting and 5% for other circuits from the origin of the installation. Check your local regulations.
How do I find the minimum cable size?
Press × on Conductor cross-section, then type the maximum voltage drop % you allow. Pick the next standard size up.
Does this account for cable temperature and reactance?
It uses resistivity at about 20 °C and ignores reactance, which is fine for small and medium cables. For large cables or high temperatures, use the manufacturer's mV/A/m tables.
What is an acceptable voltage drop?
Typically 3% for lighting and 5% for other circuits, but follow your local electrical code.
Does a longer cable increase voltage drop?
Yes. Voltage drop is proportional to length, so doubling the run doubles the drop.