Cable Sizing and Voltage Drop Calculator
Size low-voltage power cables the way a design office does. Enter the load, supply, route and installation conditions, and the calculator returns the design current, the ambient and grouping derating factors, a recommended conductor size, the voltage drop in volts and percent, and the power lost in the cable at full load — all based on the tabulated data of BS 7671 / IEC 60364, the basis of Kenyan wiring practice.
Looking to size a solar system instead? Try the Solar PV Calculator.
Cable schedule — this circuit CALC-02 · REV 2026-07
| Design current, Ib | – |
|---|---|
| Ambient correction, Ca | – |
| Grouping correction, Cg | – |
| Combined derating, Ca × Cg | – |
| Required tabulated rating, It = Ib ÷ (Ca × Cg) | – |
| Size required for current alone | – |
| Recommended cable size | – |
| Tabulated capacity of selected size, Iz | – |
| Effective capacity after derating, Iz × Ca × Cg | – |
| Voltage-drop factor used | – |
| Voltage drop over route | – |
| Voltage drop, percent of nominal | – |
| Power loss in cable at full load | – |
Guidance tool only. Values are typical tabulated figures for PVC-insulated multicore cable per BS 7671:2018 / IEC 60364-5-52 at 30 °C reference ambient. Final selection must be verified by a registered engineer against manufacturer data, earth-fault loop impedance, short-circuit withstand and the current edition of the applicable standard.
How this calculator works
The calculation follows the standard four-step LV design sequence:
- Design current, Ib. For single phase, Ib = P ÷ (V × pf). For three phase, Ib = P ÷ (√3 × V × pf). You can also enter the current directly if it is already known.
- Derating. The tabulated ratings assume 30 °C ambient and a single circuit. The ambient correction factor Ca (BS 7671 Table 4B1, PVC 70 °C, interpolated between table points) and the grouping factor Cg (Table 4C1, bunched circuits) reduce the usable capacity, so the cable must have a tabulated rating of at least It = Ib ÷ (Ca × Cg).
- Current-carrying capacity. The smallest conductor whose tabulated rating Iz meets It is selected from typical values for PVC-insulated multicore cable (BS 7671 Tables 4D2A copper, 4H2A aluminium) for reference installation methods A, B, C and E per IEC 60364-5-52.
- Voltage drop. Drop = mV/A/m × Ib × L ÷ 1000, using the tabulated millivolt factors (Tables 4D2B / 4H2B). If the thermally adequate size exceeds the selected limit — 3% for lighting or 5% for other circuits supplied from a public LV network — the calculator automatically upsizes until the drop is within limits, and tells you it did so.
Cable power loss is reported at full load (P = Ib² × mV × L ÷ 1000 for single phase; √3 × Ib² × mV × L ÷ 1000 for three phase) — useful for arguing the economics of one size up on long, heavily loaded runs.
Assumptions and limits
- PVC-insulated (70 °C) multicore cable. XLPE (90 °C) ratings are higher; contact us for a design using manufacturer data.
- Aluminium selections start at 16 mm², the smallest size in general LV use.
- Voltage-drop factors are the tabulated impedance values at conductor operating temperature; for very large cables at low power factor the reactive component can shift results slightly.
- The tool does not check earth-fault loop impedance, protective-device coordination or short-circuit withstand — all mandatory checks in a full design.
Need the full design, stamped and submission-ready? See our consultancy services or the design document library. To learn to do these calculations yourself, join a training program.