Trunking Size Calculator

IET On-Site Guide Appendix E (Tables E5/E6, the 45% space factor rule with the major-axis definition); ESC Technical Manual T113-1 and T113-3 (cable factors extended to 240 mm2, spare capacity rule, internal areas); BS 7671 (521.5.1, 521.7, 521.10.1, 521.10.202, 527.2.2, 528.1, Appendix 4 Reference Method B and Table 4C1); BS 5839-1:2025 35.5, BS 5266-1:2025 8.2.6, BS 8519 cl.15/16 and Annex B; Legrand Salamandre 2019 standard range; MK PVCu 45% areas; BS 7629-1 and Prysmian FP200 Gold dimensions; OSG Table D4 supports; Quelfire tested firestop details.

26/09/2026 (1.4.2): the printed report, which is part of the Engineering Toolkit, had shown no sizing steps, and none of the check lines marked NOTE, since 1.3.2, released on 4 September 2026. In 1.3.2 the report section with the sizing steps was renamed "How this size was reached", and the printed report was not changed to match, so the SIZING BASIS heading was printed with nothing under it. The calculations, the wording shown in the tool, the Excel, Word, CSV and Text exports and the report emailed to free users were never affected by this fault. Each section of the printed report is now found by a fixed internal name rather than by its heading, and the sizing steps and those check lines are printed again. A long page of the printed report is no longer shrunk to fit one sheet: what does not fit on one sheet continues on the next, so a printed report can run to more pages than before. The text in the printed tables is now 9.5 px; in 1.4.1 it was 9 px, which printed at 6.75 pt, or smaller on a long page. Every result line, check line and sizing step now prints at 7 pt or more. The following now read should, the verb of the clause they cite: the check line, the design note and the help on intermediate cable supports in a vertical run over 5 m (ESC S309-1); the check line, the help and the design note on the offset of two cable diameters in a riser with straight runs over 32 m (BS 8519:2020 15 i); the two help lines, the warning and the check line on the fire alarm compartment (BS 5839-1:2025 35.5), and the line on the fire alarm cable colour (BS 5839-1:2025 25.9); the help and the check line on the partition for emergency lighting fed from a central power supply system (BS 5266-1:2025 8.2.6); and the design note on trunking bends (On-Site Guide Appendix E). The help on the Serves field now reads "The segregation checks depend on it", not "This drives the segregation checks". Where a line on BS 7671 Regulations 521.5.1, 521.10.1, 521.10.202, 526.8 or 527.2.2 said must or must never, it now reads shall or shall not, as in the regulation. Two statements now use must, as the On-Site Guide does: on lighting suspension trunking, where the tabulated support spacings do not apply, the manufacturer's instructions must be followed (Table D4 note 2); and thermosetting cables that share trunking with thermoplastic (PVC) cables must be rated as thermoplastic (Appendix E). In the How this works list, flat twin-and-earth is now "taken as a circle of diameter equal to its MAJOR axis, as the Guide states", not "counted on its MAJOR axis as the Guide requires". The rating line that names a cable's own BS 7671 Appendix 4 table now cites Regulations 523.1 and 523.2, and states that the current-carrying capacity shall be selected from that table for Reference Method B, with the Table 4C1 factor, or determined in accordance with Regulation 523.3, which the line had not mentioned. The factor of 0.5 on the free-air current rating inside a fire-rated protective enclosure is now cited to BS 8519:2020 clause 6.2 NOTE 2, which states that, for cables in an enclosure of passive fire protection material, this factor may be used unless the manufacturer of the protective system advises a different one. The lines that stated the factor had given it without that condition, and six lines had cited Annex B as its source; Annex B is the worked example (390 A in free air becomes 195 A). The value 0.5 is unchanged. Each changed verb and citation was checked against the text of the clause. Where the printed report, the exports or the report emailed to free users include a reworded line, it is reworded there too. The tool name in the dark bar, Trunking Size Calculator, is now the main heading of the page; it looks the same and its words are unchanged. The green for a pass, the amber for a point to check or a warning, and the grey for a neutral mark are now darker, so that any white text on them meets the 4.5:1 contrast that the Web Content Accessibility Guidelines 2.2 require at level AA for text of 12 px; this change applies only to the colours, not to any calculation, result, wording or layout. Amber text on a light background is also a darker amber, with a contrast of at least 4.5:1. The Cross-section and Run detail drawings, in the tool and in the printed report, and the cross-section drawing in the exports are now drawn in the darker green and amber. The panel for saving and exporting, below the results, keeps its present colours for now. Apart from these changes, the calculations and every size, factor, limit and result are unchanged, and so are the text, dimensions and values in the drawings. The 1,538 scenarios in each tier, the derived-input check, the term-of-art check, the length and storey height checks added in 1.3.3 and 1.3.4, and the two citation checks for BS 5839-1:2025 and BS 5266-1:2025 were re-run on 1.4.2 with no findings. The automated check that reads every line that can be shown in the tool now also includes each change in this release. In a new automated check, a set of cases is printed and compared with the tool: every result line, check line and sizing step shown in the tool is in the printed report at 7 pt or more, and the check fails on a copy with the 1.3.2 fault put back. The automated check of how the tool looks now also includes the colours and the main heading. All three pass on 1.4.2 and fail on 1.4.1; verified live after upload. 26/09/2026 (1.4.1): the rule that allows two or more circuits in the same trunking system is now cited to BS 7671 Regulation 521.6 rather than 521.7, which is the rule for two or more circuits in one multicore cable. The correction is in the Design notes, the subscriber compliance record, the list of the compliance record's clauses shown to free users and the References line in the printed report, the exports and the report emailed to free users; the quoted regulation text was already correct. The note on steel trunking as the protective conductor now cites Regulation 543.2.5 rather than 543.2.2, which is about the enclosure or frame of switchgear, controlgear or a busbar trunking system. The sources listed after the rating table now cite Installation Methods 6 to 9 of Table 4A2 rather than 6 and 7 only; 8 and 9 are multicore cable in trunking on a wall. The three corrected BS 7671 citations were checked against the text of BS 7671:2018+A4:2026. Two fire alarm lines were corrected against BS 5839-1:2025 35.5: the help on each cable's "In" field (the compartment the cable runs in) reads should, not must, and the fire alarm compartment check line ends "reserved solely for the fire alarm cables". Other lines in the tool and the reports were corrected for grammar, punctuation, repeated words, missing degree signs and one idiom, and the blurred example line shown to free users now names the figure as Iz in the trunking, which is a current-carrying capacity, not a current. The Cross-section and Run detail drawings, in the tool and in the printed report, are now drawn in the font and colours of the PD Power Distribution tool, and so is the cross-section drawing in the exports. The cable service colours are a colour code and are unchanged. In the printed report, the accents, the ruled lines and the table headers, now light grey with dark text, are in the new colours. Apart from the drawings and the corrected citations and wording, the text, layout, font, font sizes and page breaks of the printed report are unchanged. The printed report for each of the eight typical situations was saved as a PDF from 1.4.0 and from 1.4.1 and compared: each 1.4.1 copy has the same number of pages as its 1.4.0 copy, and the text differs only by the corrected citations and wording. The text in the tool's fields is now 14 px; in 1.4.0 some fields were 12 or 13 px. When the page is under 768 px wide, as on a phone, the text in the tool's fields is 16 px, so a phone does not zoom the page when a field is tapped. The panel for saving and exporting, below the results, keeps its present colours for now. The calculations and every size, factor, limit and result are unchanged, and so are the text, dimensions and values in the drawings. Apart from the cross-section drawing in the exports, the exports and the report emailed to free users change only in the corrected citations and wording. The 1,538 scenarios in each tier, the derived-input check, the term-of-art check, the length and storey height checks added in 1.3.3 and 1.3.4, and the two citation checks for BS 5839-1:2025 and BS 5266-1:2025 were re-run on 1.4.1 with no findings. Every line that can be shown in the tool is now read by a new automated check, and the automated check of how the tool looks now also includes the drawings and the printed report. Both pass on 1.4.1 and fail on the 1.4.0 build; verified live after upload. 26/09/2026 (1.4.0): the tool now looks like the PD Power Distribution tool, with the same font and colours, and the same style of buttons, fields, tables, result tiles and check lines. The tool name and version are shown in a new dark bar at the top of the tool, and the Start again button was moved into that bar from beside the description; it works as before. The calculations, every result, the drawings, the wording shown in the tool (apart from the tool name and version in the dark bar), the printed report, the report emailed to free users and the exports are unchanged. The 1,538 scenarios in each tier, the derived-input check, the term-of-art check, the length and storey height checks added in 1.3.3 and 1.3.4, and the two citation checks for BS 5839-1:2025 and BS 5266-1:2025 were re-run on 1.4.0 with no findings. A new automated check of how the tool looks, run at desktop width and at a 390 px phone width, passes on 1.4.0 and fails on the 1.3.8 build; verified live after upload. 16/09/2026 (1.3.8): the wording in the tool was revised in plain UK English. The compliance record line and the How this works list now say in the tool, and the segregation and compartment checks are called checks. The calculations, the printed report and the exports are unchanged. The 1,538 scenarios in each tier, the derived-input check and the term-of-art check were re-run on 1.3.8 with no findings; verified live after upload. 15/09/2026 (1.3.7): the notes written inside the program code were rewritten in plain UK English. The calculations, the wording shown in the tool, the printed report and the exports are unchanged. The 1,538 scenarios in each tier were re-run on 1.3.7 with no findings; verified live after upload. 15/09/2026 (1.3.5 and 1.3.6): every citation of the fire alarm and emergency lighting standards was read back against the current editions. BS 5839-1:2025 replaced the 2017 edition: the compartment rule is now cited to 35.5, non-metallic trunking to BS EN 50085-1 (35.7) and the support note to 35.3, which says non-metallic trunking as the sole means of support is likely to be precluded. BS 5266-1:2025 replaced the 2016 edition: its clause 8.2 applies to emergency lighting fed from a central power supply system, so the partition, the marking of reserved trunking and the non-combustible support are now stated for that case, and supply cables to self-contained luminaires are named as installed like a normal lighting circuit (8.1); the subscriber compliance record quotes clause 8.2.6 of the 2025 edition word for word. Two new automated checks read every citation, and the wording of every program file, in the tool, the report and the printed sheet, and fail on the 1.3.4 and 1.3.5 builds; the 1,538 scenarios, the derived-input check and the term-of-art check were re-run on 1.3.6 in both tiers with no findings; no factor, size or limit changed; verified live after upload. 11/09/2026 (1.3.4): a check across every PD tool for an input shown to every user that only a minority case reads. The storey height field was shown on every vertical section, although only a vertical section that penetrates a fire-rated element reads it, for the slab crossings in the support note. It now shows only once that box is ticked on a vertical section, and ticking the box reveals it at once. An automated check for it was added and fails on the 1.3.3 build; the 1,538 scenarios, the derived-input check and the term-of-art check were re-run on 1.3.4 in both tiers with no findings; no factor, size or limit changed; verified live after upload. 10/09/2026 (1.3.3): a check across every PD tool for a figure printed as entered that the user never gave. Every new section started with a 6 m length that was printed in the check row, the schedule and the report as if entered, and a vertical section left at it was told to provide intermediate supports for a 6 m run nobody had specified. A new section now starts with the length blank: the capacity result does not depend on it, and the support count, the pin racks and the fire-stopping positions ask for it until it is entered. An automated check now tests a fresh section in both tiers (blank field, the request line, no vertical note without a length, the note once a length is entered, blank again after Start again and on an added section) and fails on the 1.3.2 build; the 1,538 scenarios, the derived-input check and the term-of-art check were re-run on 1.3.3 with no findings; no factor, size or limit changed; verified live after upload. 04/09/2026 (1.3.2): the sizing basis is now headed How this size was reached and laid out as numbered steps for each section, with the section's size and verdict at the head of its card and every figure in bold, so the calculation behind the answer reads as a worked example for the case entered. The steps are the same calculations as before and the report shows them under the same heading. The 1,538 scenarios were re-run on 1.3.2 in both tiers with no findings; no factor, size or limit changed; verified live after upload. 04/09/2026 (1.3.1): a check of whether every input the standard names by a term of art is defined in the tool, printed back and questioned where a wrong entry can be seen. The custom internal width and depth fields took the catalogue size, which names trunking by its outside dimensions, so 75 x 50 entered gave 3750 mm2 where the tool's own Table E6 figure for that size is 3456 mm2; the custom cable diameter field took the conductor size in place of the overall diameter. Both now show help naming the figure meant and the entry that is wrong. A custom steel or PVCu size equal to one on the standard list, or a diameter equal to a conductor size, prints a check line with the figure the tool holds while still giving the answer; a fire-rated system, always entered by hand, is not questioned this way. The 1,538 scenarios were re-run on 1.3.1 with no findings; no factor, size or limit changed; verified live after upload. 04/09/2026: the full input range was enumerated, 1,538 scenarios in each tier: every single-core size from 1.5 to 240 mm2 in PVC and LSZH, solid and stranded, FP200 and BS 7629 multicores, ESC three-core and flat cables, mixed sets, at four quantities, three spare capacities, one and two compartments, steel and PVCu, every standard steel size checked manually, the fire-rated path with and without the manufacturer size, and blank or nonsense spare entries. The proposed size, the verdict word, the support centres against On-Site Guide Table D4, the grouping factor against BS 7671 Table 4C1 and the Table E5 and E6 figures were each recomputed independently from the source tables and compared with what the tool prints. No difference was found. 03/09/2026 (1.3.0): a check across every PD tool that every printed value is reached through the input the standard derives it from. A fire-rated system was found to be sized and passed on the steel Table E6, with its current rating printed without the BS 8519 Annex B correction: it now asks for the manufacturer's internal size and tested support centres and prints the rating rule instead of a figure. Current ratings are given only for the FP200 cable the datasheet covers, other cables are pointed to their BS 7671 Appendix 4 table. Blank spare and blank length ask for the value; circuits beyond Table 4C1 say so; compartment widths can be entered; Band I cables state whether they are insulated for 300/500 V before the Reg 528.1 note applies; the 528.1 methods are numbered 1 to 6 as the A4 text has them. The audit checks were run in both tiers; verified live after upload; no size or factor changed on steel or PVCu. 02/09/2026 (1.2.3): every cited BS 7671 clause was read back against BS 7671:2018+A4:2026, BS 5266-1:2025 and BS 5839-1:2025; the result summary names the section with the highest utilisation and the trunking cover rule is cited to Reg 530.4.3; no size or factor changed. 02/09/2026 (1.2.2): the revised wording was checked in the tool against the house style, using the FF core transition preset; no size or factor changed. 33 checks were run: both ESC worked examples (unit system and space factor), the RM Technical chart example, a real project cable schedule, and the published internal-area figures (75x50 = 3455 mm2, 75x75 = 5270 mm2, reproduced by factor / 0.45). All 37 steel factors were verified against On-Site Guide Table E6. The grouping check was verified against the FP200 Gold datasheet and Table 4C1.