How to use the Voltage Drop
- Enter the one-way run length; the calculation doubles it for the return path.
- Enter the load current in amps and the system voltage.
- Enter the conductor cross-section in mm² or select an AWG size.
- Compare the percentage drop against the limit in your wiring regulations.
How the calculation works
The current travels out and back, so the resistance in circuit is twice the one-way length divided by the cross-sectional area, scaled by the material's resistivity. Copper at 20 °C is about 1.72 × 10⁻⁸ Ω·m; aluminium is roughly 1.6 times more resistive, which is why aluminium runs need a larger conductor for the same drop.
Installation standards cap the drop rather than the absolute volts. Common practice allows around 3% for a final circuit and 5% overall from origin to load, because excessive drop causes motors to run hot, lighting to dim and electronics to reset. Resistivity also rises with temperature, so a cable running warm inside insulation drops more than a cold-calculation suggests.
V_drop = 2 × ρ × L × I / A (single-phase; ×√3 rather than 2 for three-phase line-to-line)Source: Copper resistivity 1.72 × 10⁻⁸ Ω·m at 20 °C; drop limits per IEC 60364 / NEC 210.19 recommendations. VERIFY the limit in your local wiring regulations.
Worked example
A 16 A load on a 230 V single-phase supply, 42 m from the board, wired in 2.5 mm² copper.
- Circuit length = 2 × 42 = 84 m.
- R = 1.72e−8 × 84 / 2.5e−6 = 0.578 Ω.
- V_drop = 16 × 0.578 = 9.25 V.
- As a percentage: 9.25 / 230 = 4.02%.
About 9.25 V dropped, or 4% — over the usual 3% final-circuit guidance, so step up to 4 mm².
Frequently asked questions
What voltage drop is acceptable?+
Typically 3% on a final circuit and no more than 5% from the supply origin to the load, though the exact figures are set by your national wiring regulations and by equipment tolerance.
Why is the run length doubled?+
Current flows to the load and back, so both conductors contribute resistance. Using the one-way length halves the calculated drop and produces an undersized cable.
Does this apply to DC systems like solar or 12 V vehicles?+
Yes, and it matters more at low voltage: a 1 V drop is 0.4% of 230 V but 8% of 12 V. Low-voltage runs usually need conductors far larger than the current alone would suggest.
Should I use the cold or hot resistivity?+
Calculate at operating temperature for a realistic result. Copper resistance rises roughly 0.39% per °C, so a conductor at 70 °C drops about 20% more voltage than the same one at 20 °C.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.