Voltage Drop Calculator
The voltage lost along a cable — from its length, the conductor size, the current and the material — as volts and as a share of the supply, with the power wasted as heat, whether it sits inside the usual limit, and the conductor size that would.
Every metre of copper has resistance, and current through it drops voltage: for a single-phase run the current flows out and back, so the drop is twice the length times the resistance per metre times the current.
How the voltage drop calculator works
Every metre of copper has resistance, and current through it drops voltage: for a single-phase run the current flows out and back, so the drop is twice the length times the resistance per metre times the current. Codes limit the drop to a few per cent of the supply so that motors start and lights do not dim. The cure is a larger conductor, and the page finds the smallest size that meets the limit.
Formula: V_drop = 2 × L × ρ × I / A (single phase); √3 × L × ρ × I / A (three phase); P_loss = V_drop × I
Worked examples
| Inputs | Voltage drop (V) | Note |
|---|---|---|
| 20 A over 40 m of 2.5 mm² | 11.008 | over 3% at 230 V |
| Upsized to 6 mm² | 4.587 | within the limit |
| Three-phase | 9.533 | a smaller drop |
FAQFrequently asked questions
What voltage drop is allowed?
Wiring rules typically limit the drop between the supply and the load to 3 to 5% of the nominal voltage, sometimes split between sub-mains and final circuits. Check the rule that applies to the installation.
Why does a long run need a bigger cable?
Because resistance grows with length and shrinks with cross-section. Doubling the length doubles the drop; the next cable size up brings it back down.
What resistivity does this use?
Copper at 0.0172 and aluminium at 0.0282 ohm·mm² per metre, at 20 °C. Warm conductors have a little more; cable tables give the exact figure per size.
Does this size the cable for safety?
No — current-carrying capacity, protection and installation method set the minimum size for safety, and are a job for the electrician and the wiring rules. Voltage drop is often the larger size for long runs, which is why it is checked separately.
What does the lost power cost?
The drop times the current is heat in the cable, paid for every hour the load runs. On long, heavily loaded runs a larger cable pays for itself in energy.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.