BS 7671:2018+A4:2026 Tables 54.1, 54.7 and 54.8 for conductor sizing. The earthing conductor is sized from the line conductor under Table 54.7; on a PME supply the Table 54.8 value applies as well, and where it is buried the Table 54.1 minimum for the burial method applies as well (Reg 542.3.1): the greater value is returned. Table 54.8 is keyed on the PEN conductor of the supply, the DNO's PEN at the cut-out, service head or substation, in copper equivalent; where an installation is fed from a landlord or BNO sub-main, that sub-main has separate neutral and earth conductors and no PEN, and the DNO's PEN at the building intake is entered instead, since PME conditions apply at each installation where the intake has a PME earth terminal (ENA G87 Issue 2, 5.2 and 5.4). Main protective bonding on TN-S and TT is half the earthing conductor under Reg 544.1.1, not less than 6 mm² copper and need not exceed 25 mm² copper; on PME it follows Table 54.8 from the copper-equivalent PEN. Supplementary bonding is sized from the CPC under Regs 544.2.1 to 544.2.3, with the 4 mm² minimum without mechanical protection applied as IET Guidance Note 1 Table 3.9 does. The decision paths cite Regs 415.2.2, 542.2 to 542.4.2, 543.1.1 to 543.5.1, 544.1.1 to 544.2.5, 545.1.1 and 545.1.2, 514.4.2, 514.13.1, 708.415.1, 709.531.2, 710.415.2.101, 722.411.4.1 and 722.531.3.101, and Table 51 for conductor identification. Guidance: BS 7671 Section 545, IET Guidance Notes 1 and 8, BS 6701, BS EN 50310 and BS EN 62305-3. 17 guided questions lead to 22 outcomes, with 9 warnings raised where the answer depends on something the tool cannot see.

15/09/2026 (1.12.1): the wording in the tool, in the result notes, in the readme and in the notes written inside the program code was revised in plain UK English. The identification notes now say a conductor has no protective function (Table 51, FE) or performs one (Reg. 543.5.1), and the functional earth bar note says no general protective bonding shall be connected to the bar. A new automated check reads every program file and the 12,539 tool pages on the Toolkit tier and 12,290 on the free tier that the full set of automated tests produces, and fails on the 1.12.0 build; the tests were re-run with 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, no findings. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.12.0): the confirmation screen after the supply and location questions was found to be a Continue click for every user that repeated the two answers, while its purpose, to stop the reader early with a warning, applies only where the combination raises one. It is now shown only where the supply and location raise a warning (PME at an EV charging point, a caravan park or marina, a petrol filling station or a medical location, any TT supply, medical on TN-S); on a plain building on PME or TN-S the role question follows the location directly and Back returns to the location question. The automated tests gained a check on every test run that the confirmation screen is present exactly where a warning applies and absent elsewhere, and that Back after a skipped screen returns to the location with its answer cleared; 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier, 1,260 on the free tier, no findings, and the same tests fail on the 1.11.0 build. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.11.0): the question about how the installation is supplied, added in 1.6.1 and reworded twice, was found to serve the multi-occupancy case only while being asked of every PME user, and to confuse the case it did not serve. It is removed. The PEN question is asked once per role, defined as the PEN conductor of the DNO supply at the cut-out, service head or substation in copper equivalent, and names the exception in one sentence: a landlord or BNO sub-main has a separate neutral and earth and no PEN, so the DNO's PEN at the building intake is entered instead (ENA G87 Issue 2, 5.2 and 5.4). The result prints the PEN used and prints a check line whenever the PEN entered equals the line conductor. The automated tests gained checks that no such question exists on the PME path, that the path runs straight from the installation method to the line conductor and the PEN, that the PEN question opens with its definition and names the exception with its basis, that the PEN is echoed on the result, and that the check line prints when the PEN equals the line conductor and never otherwise; 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier, 1,260 on the free tier, no findings, and the same tests fail on the 1.10.0 build. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.10.0): on a PME supply the question before the PEN entry was found to offer only the small-building form of a DNO supply (the cut-out), so an office block supplied from a DNO transformer on site fitted neither option. It was reworded with a description under each option. The worked example for the case entered now sits under a button on the result and the fixed reference example is no longer shown in the tool; the Back and Start again buttons sit at the top right of the tool. The automated tests gained checks that the two navigation buttons sit in the header and Print summary alone in the footer, and that the worked example sits under a button on every kind of result, closed until opened, with no reference example in the tool; 315 enumerated outcomes and 1,307 numeric test runs on the Toolkit tier, 1,259 on the free tier, no findings. No value, clause or verdict changed; verified live after upload. 05/09/2026 (1.9.0): the Engineering Toolkit gains a PEN cable helper under the PEN questions, built on a database of DNO service and mains cables with a source on every row: concentric service cables to BS 7870-3.11 from 4 to 35 mm² (copper equivalent of the concentric copper conductor by equal conductance against IET Guidance Note 3 Table B1, from the maximum resistance in the manufacturer datasheets), waveform mains and intake cables to BS 7870-3.40 and 3.50 from 95 to 300 mm² (the copper wires of the combined neutral and earth conductor, 22 or 42 wires of 1.85 mm, UK Power Networks EDS 02-0033), and the split concentric and 4-core cables that have no PEN, where the reader is sent to the DNO for the figure; an aluminium figure is converted by conductance (× 0.61, GN3 Table B1). The result, the worked example, the report and the printed sheet name the cable the figure came from. The automated tests gained their own transcription of every source and check against it the database, the helper on both tiers, every cable's fill and result band, the aluminium conversion at every Table 54.8 band edge, and a figure typed over the helper's value; 315 enumerated outcomes and 1,304 numeric test runs on the Toolkit tier, 1,256 on the free tier, no findings, and the same tests fail on the 1.8.1 build. No table value, clause or verdict changed; verified live after upload, with the helper tried while logged in. 05/09/2026 (1.8.1): on a PME supply the question before the PEN entry was found to read as ENA G87 text and to change nothing the reader could see. It now asks where the installation is supplied from, direct from the DNO cut-out or from a landlord or BNO sub-main, in plain words; each branch asks its own PEN question, the sub-main branch opening with the reminder that the sub-main neutral is not the PEN and that the PEN size at the building intake comes from the BNO or the DNO; the result on that branch shows a designer note naming the PEN used, in the tool, in the report and on the printed sheet, and the check line on a PEN equal to the sub-main line conductor reads as an instruction. The automated tests gained checks that each branch reaches its own PEN question, that the reminder opens the sub-main question and is absent from the cut-out question, that the feed question has no citation, and that the sub-main note reaches the result; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.8.0 build. No value, clause or verdict changed; verified live after upload. 05/09/2026 (1.8.0): the option buttons have shorter labels on every question so they read at a glance and never clip on a phone, with the detail kept in the question help and in the result, and every result with a size now opens with a one-line answer, the conductor role and its size, which stays in view while the rest of the result is read, with the full sizing line beneath it; the report and the printed sheet show the same answer line. The automated tests gained a check that the answer line is present wherever the sizing line opens with a figure, shows the same figure and the right role, and reaches the report and the printed sheet; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.7.0 build. No value, clause or verdict changed; verified live after upload. 04/09/2026 (1.7.0): every result now shows a worked example built from the case entered rather than a fixed illustration: the supply arrangement and location, each answer given, the table or regulation applied to the figures entered with the value at each step, the size that results and its identification, in the tool, in the emailed and exported report and on the printed sheet. The fixed examples stay below as reference examples. The automated tests gained a check that the worked example is present on every result, restates every figure entered, states the size the result prints and cites the table or regulation it came from; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.6.1 build. No value, clause or verdict changed; verified live after upload. 04/09/2026 (1.6.1): a landlord board fed through a building network was found to be answered from the sub-main neutral entered as the PEN. The PEN question now asks for the distributor's PEN at the intake in the standard's own term, names the sub-main and building network trap (ENA G87 5.2 and 5.4), and the result prints the PEN it used and checks a PEN equal to the line conductor on a sub-main. The automated tests gained a check that every input the standard names by a term of art is defined in the tool, echoed in the result and challenged where a substitution can be seen; 1,255 numeric test runs and 315 enumerated outcomes in each tier, no findings, and the same tests fail on the 1.6.0 build. No table value changed. 03/09/2026 (1.6.0): supplementary bonding now asks the CPC size and returns the Reg 544.2 size as a number, checked cell by cell against IET Guidance Note 1 Table 3.9; 1,245 numeric test runs in each tier, no findings. 03/09/2026: a buried earthing conductor on a TT supply was found to return the Table 54.1 minimum as its size. It was fixed in 1.5.3: every buried path now asks for the line conductor (and the PEN on PME) and returns the greater of the Table 54.7, Table 54.8 and Table 54.1 values. The automated tests gained a check that every derived size is reached through its input; 1,147 numeric test runs in each tier, no findings. 02/09/2026: every cited clause, table and note was read back again against BS 7671:2018+A4:2026, IET Guidance Note 8 (6th Edition) and the On-Site Guide; every computed size was also checked against On-Site Guide Tables 4.4(i) and 4.4(ii) as a second source, 28 further checks in each tier. Four wording corrections were released in 1.5.2 with no value or clause number changed; the full set of automated tests was re-run in both tiers, 634 scenarios each, with no findings. 12/08/2026: the full set of automated tests was re-run in both tiers after the limit of three anonymous sessions and the subscriber-only print were introduced, 634 scenarios each, with no findings; the free tier checks now confirm that the print button stays hidden and the email note is shown. August 2026 audit: 634 scenarios in each tier, free and Toolkit, were checked against hand transcriptions of Tables 54.1, 54.7 and 54.8, and every cited clause was read back against the standard itself.