Free tools
Inspection and testing calculators
The four calculation engines from the app, working in your browser. Formulas and reference tables come straight from BS 7671:2018+A4:2026.
Maximum Zs
Max permitted Zs = 0.8 × tabulated Zs
Look up the tabulated maximum earth fault loop impedance for a protective device from BS 7671 Table 41.3, apply the 0.8 rule-of-thumb correction for conductor operating temperature, and check a measured reading against it.
BS 7671 Table 41.3 (0.4 s, 230 V); correction per Reg 411.4.5 / Appendix 3
Open calculatorRing Final Circuit
Expected reading at furthest point = (r1 + r2) / 4
Check the line and neutral end-to-end readings of a ring final circuit are balanced, and work out the expected r1+r2 reading at the furthest point of the ring using the figure-of-eight method.
BS 7671 / IET Guidance Note 3 — ring final circuit continuity test
Open calculatorAdiabatic Equation
S = √(I²t) / k
Calculate the minimum cross-sectional area a protective conductor needs to survive a fault, from the fault current, the disconnection time and the material/insulation k factor.
BS 7671 Regulation 543.1.3; k from Table 54.3
Open calculatorVoltage Drop
Drop (V) = (mV/A/m × Ib × L) / 1000
Work out the voltage drop along a circuit from its tabulated mV/A/m figure, design current and length, and express it as a percentage of the nominal supply voltage.
BS 7671 Appendix 4 (mV/A/m tables)
Open calculatorThese calculators are revision and cross-checking aids. They do not replace the published tables in BS 7671, the requirements of the specific installation, or the judgement of a competent person.
Same calculators, on site
In the app they keep your favourites to hand and produce a result sheet you can share straight from the job.
