BS 7671 calculators for UK electricians

No. These are preliminary design aids to support early-stage circuit design and checking. All results must be verified against the current edition of BS 7671, manufacturer data and your project-specific assumptions by a qualified electrician before any certification is issued.

BS 7671:2018+A2:2022 (the 18th Edition with Amendment 2). Always check you are working to the current edition.

Cable sizing to BS 7671 involves five steps: (1) Calculate the design current Ib from the load. (2) Select a protective device rating In ≥ Ib. (3) Apply correction factors for ambient temperature (Ca), grouping (Cg), thermal insulation (Ci) and installation method (Cm) to find the required tabulated current It = In ÷ (Ca × Cg × Ci × Cm). (4) Select the smallest cable whose tabulated current-carrying capacity ≥ It. (5) Verify voltage drop and adiabatic checks pass. Our cable size calculator automates all five steps.

BS 7671 Appendix 12 sets the default maximum voltage drop at 5% of the nominal supply voltage for power circuits — that is 11.5 V on a 230 V single-phase supply or 20 V on a 400 V three-phase supply. For lighting circuits the limit is 3% (6.9 V on 230 V). These are default limits; tighter limits may apply for sensitive equipment or where the designer specifies otherwise.

The adiabatic equation (BS 7671 Reg 543.1.3) is S = (I²t)^0.5 ÷ k, where S is the minimum conductor cross-sectional area in mm², I is the fault current in amps, t is the disconnection time in seconds, and k is a material constant from BS 7671 Tables 54.2–54.4 (115 for copper/PVC, 143 for copper/XLPE). It is used to verify that the CPC (earth wire) can withstand the energy of a fault without damage.

Zs is the earth fault loop impedance — the total impedance of the path that fault current follows from the source, through the protective conductor, and back to source. A lower Zs means more fault current flows, which causes the protective device to operate faster. BS 7671 Tables 41.2–41.4 give maximum permissible Zs values for each device type and rating to ensure disconnection within the required time (0.4 s for socket circuits, 5 s for fixed equipment in TN systems).

0.4 s applies to socket outlet circuits and other circuits up to 32 A in TN systems — the most common domestic and light commercial scenario. 5 s values for fixed equipment and submains are not yet included. Check BS 7671 Tables 41.2–41.4 directly for 5 s values.

EOC London — a London-based electrical contractor specialising in domestic, commercial and EV charging installations across London. We built these tools to help electricians and engineers with preliminary BS 7671 design checks.