Cable — Calculations Bs7671

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.”

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

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. At 9 a

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. “But Dave’s house hasn’t burned down yet

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

“Dave’s 2.5mm would be a fuse on a wire,” Tom muttered. “Death in a plastic sheath.”