BS 7671 Voltage Drop Calculator
Professional BS 7671 voltage drop calculator for UK electrical installations. Compute voltage drop in volts and percent, check compliance with 3%/5% limits, and find the minimum cable size based on BS 7671 Appendix 4 tabulated mV/A/m values.
Circuit data
Use BS 7671 Appendix 4 coefficients or provide a custom mV/A/m to confirm compliance with the selected limit.
Use line-to-neutral for single-phase and line-to-line for three-phase circuits.
Appendix 4 factors already include the return path for the selected circuit type.
Select the standard cable construction referenced in Appendix 4.
Provide the required input data to see the compliance details and conductor notes.
How to Use This Calculator
Gather your circuit data: the system voltage, design current, one-way length, and the BS 7671 Appendix 4 tabulated mV/A/m coefficient for your cable. Choose the compliance limit (3% for lighting circuits or 5% for other uses) and pick the cable family plus conductor size the standard prescribes.
Click Calculate to see the voltage drop in volts and percent, compare it to the selected limit, and find the maximum allowable length for the chosen conductor. You can also download the CSV and observe how the drop grows with length via the influence table.
Methodology
The calculator applies the tabulated mV/A/m factors from BS 7671 Appendix 4 (single- and three-phase copper circuits). The core calculation is ΔV = k × Ib × L / 1000; the percent drop is ΔV divided by the nominal voltage.
Maximum allowable length for a limit p% is derived from Lₘₐₓ = ((p/100) × Vn × 1000) / (k × Ib). When the computed percent drop exceeds the limit, you either need a larger conductor CSA, a shorter run, or a cable with a lower coefficient.
Glossary
- k (mV/A/m): Voltage drop coefficient from BS 7671 Appendix 4 for the selected family, CSA, and system.
- ΔV (V): Voltage drop over the length L.
- ΔV%: ΔV expressed as a percentage of Vn.
- Limit (p%): Contractors typically apply 3% for lighting and 5% for other circuits.
- Lₘₐₓ: Maximum one-way length that stays within the chosen limit.
- Minimum CSA: Smallest conductor in the table that satisfies the limit for the inputs provided.
How It Works: A Step-by-Step Example
Inputs: single-phase, Vn = 230 V, Ib = 20 A, L = 30 m, limit = 3% (lighting), family = PVC 70°C copper, size = 4 mm² with k ≈ 7.4 mV/A/m.
Calculation: ΔV = 7.4 × 20 × 30 / 1000 = 4.44 V; ΔV% = 4.44 / 230 × 100 = 1.93%.
Result: 1.93% ≤ 3% ⇒ compliant. The calculator also estimates how long the cable can be while remaining within the limit and what conductor CSA would be required if you needed to stay under 3% with the same current and length.
Frequently Asked Questions (FAQ)
Do the tabulated mV/A/m values already include the return path?
Yes. BS 7671 Appendix 4 factors account for the return path of the specific circuit type.
Should I use 230 V or 400 V?
Single-phase circuits use 230 V (line-to-neutral); three-phase circuits use 400 V (line-to-line) unless your specification states otherwise.
What if my installation uses XLPE 90°C cables?
Select the XLPE 90°C option. These cables typically have lower voltage drop coefficients than PVC 70°C. Always verify the exact coefficient from BS 7671 Appendix 4.
How is the minimum CSA determined?
The tool scans the tabulated sizes for the chosen family and system and reports the smallest CSA that keeps the percent drop within the chosen limit for the provided Ib and length.
Can I override the mV/A/m value?
Yes. Enable "Use custom mV/A/m" to enter a proprietary or site-specific coefficient from BS 7671 or manufacturer data.
Is this calculator sufficient for full compliance?
No tool replaces professional judgment. Verify inputs, cable data, ambient conditions, and compliance with the latest edition of BS 7671 and any project-specific requirements.
Where can I read more?
Visit the IET publication page for BS 7671:2018+A2:2022 and NAPIT's guidance on voltage drop limits for further context.
Quality note: Coefficients are representative of BS 7671 Appendix 4 PVC 70°C and XLPE 90°C copper data. Always cross-check with your licensed tables.
Strumento sviluppato da Ugo Candido. Contenuti verificati da Elek Engineering Editorial Board.
Ultima revisione per l'accuratezza: .