A cable is not sized by BS 7671 — it is sized by ONE of its checks, whichever asks for the most copper, and knowing which one tells you what to change. Work out the smallest conductor each check accepts on its own and the binding constraint falls out, along with how far it pushed the cable past everything else. Plus the two quantities that belong to the path rather than the circuit, which is why a row-by-row calculation misses them.
Every cable management system has to be held up, and the spacing table only answers half of it: the On-Site Guide Appendix D spacings meet Chapter 52 and, in the IET's own words, not Regulation 521.10.202. What the clip, conduit and trunking tables say, how the load path changes between clipping direct, a cantilever bracket and a suspended trapeze, and what the fire condition does to each link in it.
An RCD only trips on the shapes of residual current it was built to see, and a Type AC sees exactly one of them. What Reg 531.3.3 really says, the topology-to-type table buried in Annex A53, what EV charging actually requires, and why fitting a Type B downstream can blind the older device upstream.
Regulation 443.4.1 has two live triggers, not the three most guidance still quotes — and the standard's own figure of about 1000 V per metre of connecting lead means a fully compliant SPD can hand your equipment more voltage than it is rated to withstand. What Section 534 actually requires, and the two different lengths that decide whether any of it works.
A trunking sized correctly to the 45% rule looks half empty — and 41% of the section is 91% of the permitted fill. Where the limit comes from, why conduit is not sized on a fill percentage at all but on GN1 factors that fall with run length and bends, when the width method replaces both on tray and basket, and how the fill you choose feeds straight back into the cable size through Cg.
Regulation 311.1 requires maximum demand to be determined and permits diversity — then publishes no table, no allowances and no method. Where the numbers really come from, how the rules change with the type of building, and why diversity sizes the supply but never the board.
Interactive guide to the BS 7671 rating factors — Ca, Cg, Ci, Cf and the buried set. Why they multiply, why you divide the device rating rather than the load, and the get-outs almost nobody explains. Watch a cable lose half its capacity, factor by factor.
Interactive guide to the BS 7671 earthing codes — TN-S, TN-C-S (PME), TN-C, TT and IT. Explore each system in a live diagram, identify yours on site, and see max Zs, disconnection times and the open-PEN risk.
Interactive guide to BS 7671 Table 4Ab — the 3% and 5% limits, the 6%/8% private-supply row, the extra allowance beyond 100 m, and why a sub-main and its final circuit share one budget. Explore any circuit in a live voltage profile.
How bright should it be? Recommended lux levels for offices, warehouses and more, plus the glare, uniformity and maintenance factors a lux number leaves out.
XLPE gives you a 90 °C cable. Reg 512.1.5 usually gives you a 70 °C circuit — because the rule is about the terminal, not the cable. What you give back, what you keep, and which table Table 4A3 sends you to.
BS 7671 publishes exactly one numeric selectivity ratio, and it is not for circuit-breakers. Where the 1.6:1 rule really comes from, why it does not transfer, and what to do for the pairings the standard does not answer.
An earthing study is only as good as its soil data. How the Wenner and Schlumberger four-probe methods work, and what a good soil resistivity survey should deliver.
Emergency lighting is a life-safety system with its own standard. How BS 5266 durations and coverage work, and the mistakes that fail at building control.
Fire-survival cables need supports that survive the fire too. Why BS 8519 forces closer spacing and bigger rods, and where standard bracket tables fail.
Everyone knows Cmin 0.95. Almost nobody has seen Cmax 1.1 — and they are 16 % apart at the same point. Plus the new Appendix 14: root 3 or 2, why the earth fault counts too, and the 16 kA exemption.
BS 7671 requires every conductor to withstand the electromechanical forces of a fault, in four separate places, and gives no method in any of them. The force goes as the square of the PEAK current — so the RMS figure your fault study quotes understates it about five times.
Honest, indicative 2026 price ranges for earthing studies, cable calculation packs and DIALux lighting design — and what drives a project up or down the range.
Inject an earth fault into a joint bay earth grid on a 275 kV route and it leaves two ways — metallically along the ECC, or into the soil. Only the soil part raises the earth potential. How that split is set, why a real route beats the hand calculation, and what actually moves the answer.
What earth potential rise is — and the part most guides leave out. BS 7671 Section 442 says where that rise LANDS in the low voltage installation: on equipment insulation as a stress voltage, or, in exactly one arrangement, on people as a fault voltage.