Voltage drop calculator: volts and per cent
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Supply | Single phase |
| Design current | 20 A |
| Cable length | 25 m |
| Cable size | 2.5 mm² |
| mV/A/m from the cable maker's data or BS 7671 | 18 mV/A/m |
| What the circuit supplies | Other uses |
| Voltage drop limit | 5 % |
| Supply voltage | 230 V |
Voltage drop: 9.00 V (3.91%)
- Within the guidance Voltage drop: 3.913 % UK-wide: BS 7671:2018+A4:2026 Appendix 12. Check with the source
| Result | Value |
|---|---|
| Voltage drop | 9 V |
| As a share of the supply | 3.91% |
| Longest run within the limit | 31.94 m |
| mV/A/m used | 18 mV/A/m |
Show the working
- mV/A/m: 18 (your figure)
- Voltage drop: 18 × 20 A × 25 m ÷ 1000 = 9 V
- As a share: 9 ÷ 230 V × 100 = 3.91%
- Longest run: 11.5 V × 1000 ÷ (18 × 20) = 31.9 m
How it works
- Voltage drop (V) = mV/A/m × design current Ib (A) × one-way length (m) ÷ 1000
- As a percentage = voltage drop ÷ supply voltage × 100
- Longest run = (limit % × supply voltage ÷ 100) × 1000 ÷ (mV/A/m × Ib)
Tabulated mV/A/m figures assume the conductor is at its maximum operating temperature, so this is on the safe side for most circuits.
Indicative only: an electrician checks the design against BS 7671 and the cable maker's data.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Supply voltage: Single phase | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Supply voltage: Three phase | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
| Voltage drop limit: Lighting | 3 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (lighting) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Voltage drop limit: Other uses | 5 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (other uses) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
Regulations and standards
- BS 7671:2018+A4:2026 Appendix 4 (voltage drop, mV/A/m)
- BS 7671:2018+A4:2026 Appendix 12 (voltage drop limits)
Explained: BS 7671 (the IET Wiring Regulations), Approved Document P (electrical safety).
BS 7671 is used across the UK. Who may do the work differs: Part P applies in England and Wales; Scotland and Northern Ireland have their own rules.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Voltage drop (mV/A/m) for flat twin and earth cable | BS 7671:2018+A4:2026 Appendix 4, Table 4D5 | UK-wide | 0/7 values entered Not yet checked against the source |