Calculator route
Voltage Drop (1φ / 3φ)
Estimate cable voltage drop using UK-default assumptions and metric inputs. Use this for quick design checks and communication, not compliance sign-off.
Estimate only — not a BS 7671 compliance check.
UK load presets
UK CSA presets
Design current at the load in amperes.
One-way cable length in metres.
UK mm² ladder sizes.
Advanced (material, system voltage, temperature)
Result
This estimate is at or below 3%, often used as a lighting-circuit design reference in UK guidance.
How it's calculated
Vd = k × I × L × ρ(T) / A, with k = 2 (1φ) or √3 (3φ)
Resistivity is temperature-adjusted as ρ(T) = ρ₂₀ × (1 + α × (T − 20)).
- Estimate only — this is guidance, not a compliance verdict.
- Resistivity is temperature-adjusted from 20 °C using a linear approximation.
- This result does not replace BS 7671 design and verification checks.
Formula version: 1.0.0
UK FAQ
What BS 7671 voltage-drop limits should I compare against?
Common UK design references are 3% for lighting circuits and 5% for other final circuits from origin to load. Treat those as design checks, not an automatic pass/fail from this tool.
Why does temperature change the drop result?
Conductor resistivity increases with temperature. A hotter cable has higher resistance, so the same current and length produce a larger voltage drop.
When should I model aluminium instead of copper?
Use aluminium when the cable specification actually calls for it. Aluminium has higher resistivity than copper, so drop tends to rise for the same size and run length.
Continue with resistance & power loss or review methodology and assumptions.