Cable Calculations Bs7671 2021 Direct

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.”

That evening, Tom sat in his van again. He’d run the cables, drilled the holes, and tested everything – continuity, insulation resistance, RCD trip times. All passed.

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. cable calculations bs7671

The job was a simple garage conversion. That’s what Tom told his wife, anyway. But for an electrician, “simple” is a trap.

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. At 9 a

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.

Ashworth looked at the dense figures: Ib ≤ In ≤ Iz , the volt drop, the adiabatic. He signed the order. “But Dave’s house hasn’t burned down yet

He circled the final design: 16mm² twin and earth. 50A Type C RCBO. Earthing via TN-C-S, but only after verifying the DNO’s maximum Ze.