Provides an indicative shortlist based on simplified current-carrying capacity and voltage-drop checks.
Final design must be verified to BS 7671, manufacturer data, and project specification.
Edit reference tables (advanced)
For design guidance only. Always verify with a qualified engineer to BS 7671 and project specification.
About this cable sizing calculator
This free cable sizing calculator produces an indicative shortlist of cable cross-sectional areas for a circuit, screening each size against two simplified checks: current-carrying capacity and voltage drop. It is aimed at UK building-services engineers, electrical contractors and estimators who need a quick tender-stage sanity check on conductor size before committing to a full design. You enter the system (single- or three-phase, supply voltage, copper or aluminium), the load as a design current or as kW with a power factor, the one-way cable length, your maximum permitted volt drop, and the installation method, grouping and ambient temperature. Results are indicative only and must be verified to BS 7671, manufacturer data and the project specification.
How it works
For each tabulated size the tool applies your installation, grouping and ambient correction factors to a base current rating, then accepts the size only where adjusted capacity is at least the design current (Ib) and the calculated volt drop stays within your limit. Volt drop is mV/A/m × Ib × length ÷ 1000, expressed as a percentage of supply voltage. Copper values are simplified from BS 7671 Table 4D2A (PVC, 70 °C); aluminium is derived from copper by ratio, and the ambient factor Ca follows BS 7671 Table 4B1. It does not check protective device selection, fault current, Zs, thermal constraints or harmonics.
Frequently asked questions
Which standard does it use?
It uses simplified copper values from BS 7671 Table 4D2A (PVC, 70 °C, clipped), with aluminium derived from copper and the ambient correction Ca taken from BS 7671 Table 4B1.
Can I enter the load in kW instead of amps?
Yes. Choose the kW input and the calculator converts it to a design current using your supply voltage, phase and power factor before sizing.
Is the result a finished design?
No. It is an indicative shortlist for tender-stage screening only. It omits protective device coordination, fault current, earth-loop impedance, thermal limits and harmonics, so always verify with a qualified engineer to BS 7671.