BS 7671 design guides, with the working shown.
Worked calculations and amendment changes for UK electrical engineers, referenced to BS 7671:2018+A4:2026.
Coming next
- BS 7671
Required versus actual disconnection times
BS 7671:2018+A4:2026 · Table 41.1
The voltage drop tables are pessimistic on purpose
Appendix 4 of BS 7671 gives two corrections that sharpen the tabulated mV/A/m, for conductor operating temperature and load power factor. What they are, the conditions attached to them, and a worked example where they save a cable size.
Amendment 4: The Changes That Alter a BS 7671 Design Calculation
What BS 7671:2018+A4:2026 changed for design calculations: D1 and D2 buried cable ratings, no calculated risk level for SPDs, and the A3 withdrawal.
Selectivity: Why a Fault on One Circuit Takes Out the Whole Board
What Section 536 requires: the coordination rules, the four permitted verification routes, the selectivity limit current, and where the rule is simple.
Diversity for Dwellings: Why Eight Houses Do Not Need Eight Houses' Worth of Supply
Two-layer diversity for dwellings: within-dwelling allowances, the Figure 3.4 ADMD curves, the judgement that more than doubles the supply, and new loads.
The Seven Factors That Quietly Derate Your Cable to BS 7671
The seven correction factors that decide what a cable can carry to BS 7671: ambient, grouping, insulation, soil, depth, the 0.9 factor and harmonics.
Voltage Drop to BS 7671: The Method, the Limits, and the Correction Most Designers Skip
The mV/A/m method with worked examples, the 3 and 5 percent limits, three-phase and large conductor complications, and the Ct correction.
Earth Fault Loop Impedance: The Calculation That Decides Whether Protection Actually Protects
Zs, disconnection times, where maximum Zs values come from, a worked example with hot conductor resistances, and the maximum circuit length.
Reference Method D No Longer Exists: What Amendment 4 Changed for Buried Cables
BS 7671:2018+A4:2026 splits Reference Method D into D1 and D2. What changed for buried cable sizing, the corrections designers miss, and the deadline.
Put it into practice.
The methods in these articles run in CurrentProof's calculators, referenced to BS 7671:2018+A4:2026.