Enter the current, the one-way cable run length, the conductor material, and the supply voltage to get the minimum cable cross-section and the nearest standard size.
A common target is 3% for lighting circuits and 5% for power circuits, but always check the limit set by the standard that applies where the installation is built.
The required section exceeds the standard sizes listed below (50 mm2). Consult a qualified electrician for a larger conductor or a different cable arrangement.
Standard commercial cross-sections (mm2):
| Section (mm2) | Approx. AWG |
|---|
This calculation is indicative and covers voltage drop only. It does not replace a full sizing calculation by a qualified electrician, which must also account for thermal protection, installation method, ambient temperature, and grouping with other circuits under the wiring standard that applies locally.
Cable sizing under BS 7671 in the UK
UK domestic and commercial wiring is designed under BS 7671 (the 18th Edition Wiring Regulations), which sets voltage drop limits of 3% for lighting circuits and 5% for other circuits fed from a low-voltage supply. Twin and earth cable used behind sockets and lighting circuits comes in a standard set of sizes, typically 1.0, 1.5, 2.5, 4, 6, and 10 mm2, each paired with a matching circuit breaker or fuse rating and a maximum tabulated current-carrying capacity that also depends on how the cable is installed (clipped direct, in conduit, in insulation, and so on). This calculator gives the minimum section needed to keep voltage drop within your chosen limit; the current-carrying capacity check under the installation method is a separate step covered by BS 7671 tables.
Standard sections and typical protection
| Section (mm2) | Typical UK use | Common protective device |
|---|---|---|
| 1.0 | Lighting (older installs) | 6A MCB |
| 1.5 | Lighting circuits | 6A or 10A MCB |
| 2.5 | Ring or radial socket circuits | 20A or 32A MCB |
| 4 | Cooker or shower sub-circuits | 32A MCB |
| 6 | Showers, larger loads | 40A MCB |
| 10 | Cooker circuits, sub-mains | 45-50A MCB |
What this calculator cannot do
The result only accounts for resistive voltage drop along the conductor. It does not check the cable's current-carrying capacity against its installation method, does not apply grouping or ambient temperature correction factors, and does not size the protective device. Real cable resistivity also drifts with operating temperature, so a cable running hot under sustained load will show a slightly higher drop than this simplified figure. Always have the final circuit design checked against BS 7671 by a qualified electrician, in particular for fixed installations and anything feeding a consumer unit.
A worked example
A 230V single-phase circuit carrying 16A over a 20 metre one-way run, with copper conductors (resistivity about 0.0175 ohm.mm2/m) and a 3% drop limit, needs a minimum section of about (2 x 20 x 16 x 0.0175) / (230 x 0.03) = 11.2 / 6.9, close to 1.6 mm2, so the next standard size up, 2.5 mm2, is the one to use. For the current, voltage, and resistance relationship behind this formula, the Ohm's Law Calculator covers the basics in more detail.