Cable Sizing · 21 July 2026 · For contractors and project engineers
A cable that's big enough to carry the current can still be too small — because by the time the supply reaches the far end of a long run, the voltage has sagged below what the equipment needs. BS 7671 puts a limit on that sag. Here's how the 3% and 5% rules work, how to calculate voltage drop, and what to do when a circuit fails it.
For an installation supplied directly from the public LV network, the permitted voltage drop from the origin to any point of use is:
| Circuit | Limit | 230 V | 400 V |
|---|---|---|---|
| Lighting | 3% | 6.9 V | 12 V |
| Other uses (power) | 5% | 11.5 V | 20 V |
Fed from a private source (own transformer or generator) instead of the public network, the limits rise to 6% and 8%.
Every cable size has a tabulated voltage-drop figure in millivolts per ampere per metre (mV/A/m) in BS 7671 Appendix 4. The drop is:
Vd = (mV/A/m × Ib × L) / 1000
with Ib the design current and L the length in metres. For sizes of 25 mm² and above the tables split the value into resistive (r) and reactive (x) parts because inductance starts to matter, and you combine them with the load's power factor; below that, the single (z) figure is fine.
The limit applies to the whole path, not each circuit in isolation. A final circuit fed through a long submain shares the submain's drop, so a final circuit that passes on its own can fail once the submain is added. On a real network this bookkeeping is where hand calculations go wrong — the drop at a socket depends on every cable upstream of it, and a change to the submain silently changes every circuit it feeds.
Voltage drop is one of the five checks in a compliant cable calculation, alongside current-carrying capacity (with derating factors), earth-fault loop impedance and the adiabatic check. Because it's cumulative and network-wide, it's exactly the sort of thing calculation software gets right and spreadsheets get wrong when a design changes.
For an installation supplied from the public low-voltage network, BS 7671 Regulation 525 limits voltage drop to 3% for lighting and 5% for other uses, measured from the origin of the installation to the load. At 230 V single-phase that is 6.9 V and 11.5 V; at 400 V three-phase, 12 V and 20 V.
Voltage drop = (mV/A/m × Ib × L) / 1000, where mV/A/m is the cable's tabulated value from BS 7671 Appendix 4, Ib is the design current in amps, and L is the length in metres. Divide the result by the nominal voltage to get the percentage.
From the origin of the installation to the point of use — so it is cumulative. The drop across a submain and the final circuit it feeds must be added together and the total kept within the limit. That is why a final circuit that passes on its own can still fail once the submain drop is included.
Increase the conductor size, shorten the cable route, move the distribution board nearer the load, or split the load across more circuits. On a network it is often cheaper to uprate one submain than several final circuits.
Yes. Where the installation is fed from a private source such as its own transformer or a generator rather than the public network, BS 7671 allows higher limits — 6% for lighting and 8% for other uses — because the supply voltage is under the installation owner's control.