Cable Calculations Bs7671 'link' May 2026

Ashworth looked at the dense figures: Ib ≤ In ≤ Iz , the volt drop, the adiabatic. He signed the order.

At 9 a.m., he knocked on Ashworth’s door. “What’s the damage?” the client asked. “More than Dave’s quote,” Tom said, showing the scribbled page of calculations. “But Dave’s house hasn’t burned down yet. That’s just luck, not engineering.”

Adiabatic equation. The one that stops you dying. [ S = \frac{\sqrt{I^2 \times t}}{k} ] He measured the earth fault loop impedance (Zs) at the board: 0.35Ω. A 48A load meant a 230A fault current. The 32A Type B MCB would trip in 0.1 seconds. Copper k factor = 115. [ S = \frac{\sqrt{230^2 \times 0.1}}{115} = \frac{72.7}{115} = 0.63\text{mm}^2 ] His 16mm² earth was massively overkill. But if he’d used a cheap 1.5mm? Zap. No second chances. cable calculations bs7671

The client, Mr. Ashworth, wanted a 7.4 kW car charger, a mini workshop, and LED spotlights. “Just wire it in,” he’d said. “My mate Dave says 2.5mm cable is fine.”

Tom snorted. Dave wasn’t here. Dave didn’t have to sign the Electrical Installation Certificate. Dave wouldn’t get sued if the cable melted and burned the house down. Ashworth looked at the dense figures: Ib ≤

He flipped open the regs to Section 433 – Protection against overcurrent . Then to Appendix 4, the cable rating tables. He grabbed his notepad, the one with coffee stains and a torn spine.

He turned to Table 4Ab. For 16mm² cable, volt drop was 2.8 mV/A/m. Run length: 35 meters. [ Vd = (2.8 \times 48 \times 35) / 1000 = 4.7 \text{ volts} ] Max allowed? 5% of 230V = 11.5V. He was safe. But if he’d used 10mm²? 4.4 mV/A/m would give 7.4V – still legal, but pushing it under full load. “What’s the damage

Tom sat in his van at 6 a.m., rain hammering the roof. On the passenger seat lay the BS 7671:2018+A2:2022 – the big red book. The Wiring Regulations. His Bible and his courtroom judge.