BS 7671 design tool
Voltage drop calculator
Check cable voltage drop against BS 7671 Appendix 12 limits. Enter cable size, design current and run length to see whether your circuit meets the 3% lighting or 5% power limit.
Circuit inputs
Results
Vd = (mV/A/m × Ib × L) ÷ 1000 = (18 × 20 × 25) ÷ 1000 = 9.00 V
BS 7671 Appendix 12 — 5% × 230 V nominal
mV/A/m value for 2.5 mm² (single-phase): 18 — BS 7671 Appendix 4
Note: resistive component only. Reactive component not included — results may be slightly optimistic for large cables on long runs.
Assumptions and limitations
What this tool calculates
- Voltage drop (V and %) for a cable of known size and run length
- Comparison against BS 7671 Appendix 12 default limits (3% lighting, 5% other)
Known limitations
- Resistive mV/A/m values only — reactive component not included
- Does not apply correction factors (temperature, grouping). Use the Cable Size tool if you need corrected Iz as well
- Cable data covers 1.5–95 mm² only
⚠️ Important notice
This is a preliminary design aid only. Verify all results against the current edition of BS 7671 and manufacturer data before carrying out or certifying any electrical work.
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