Cable Sizing Calculator — BS 7671 18th Edition

This calculator uses current carrying capacity data from BS 7671 18th Edition (IET Wiring Regulations). Cable sizing depends on four key factors: the design current of the circuit, the installation method, the cable type, and the conductor material.

How cable sizing works

Step 1: Determine the design current (Ib) — the maximum current the circuit will carry under normal conditions.

Step 2:Select a protective device rating (In) that is greater than or equal to Ib but does not exceed the cable’s capacity (Iz), so that Ib ≤ In ≤ Iz. For example, if Ib = 28A you would typically select a 32A MCB.

Step 3: Using the appropriate BS 7671 table for your installation method and cable type, find the smallest cable whose current carrying capacity (Iz) meets the requirement.

Note: this tool sizes the cable so that its tabulated capacity (Iz) is at least your design current (Ib). It does not pick the protective device for you — once you have chosen a standard device rating (In), confirm that Iz ≥ In for your cable and method before relying on the result.

BS 7671 reference tables

• Table 4D5: Flat twin and earth (6242Y), copper, 70°C thermoplastic — Reference Methods C (clipped), B (conduit/trunking) and 100 (in insulation)
• Table 4D4A: 2-core PVC steel-wire-armoured (SWA), copper — Method C (clipped direct) and Method D (direct in ground)
• Table 4F3A: Flexible cords (3183Y / 3093Y), copper, single-phase
• Tables 4D5 / 4D4B: Voltage-drop (mV/A/m) columns used for the volt-drop check

The calculator compares your design current against the tabulated current carrying capacities for each standard UK cable size and recommends the smallest cable that meets the requirement. Tabulated ratings assume a 30°C ambient with no grouping or thermal-insulation derating — apply BS 7671 Appendix 4 correction factors (Ca, Cg, Ci, Cc) for your conditions. Always verify with a qualified electrician and check voltage drop for longer cable runs. For the full Table 4D5 figures at a glance, see our BS 7671 cable size reference table.