Cable Calculations Bs7671 ^new^ -

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.

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. cable calculations bs7671

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. He circled the final design: 16mm² twin and earth

The big red book sat on the dashboard. Not a manual. A lifeline. BS 7671 isn’t red tape. It’s the difference between a switch that works and a funeral you pay for. Tom snorted

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.”

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.”