Conduit Size Calculator

30/09/2026 (1.6.8): the printed report is now full size, with no text below 6 pt, "Page n of N" on every page and larger drawings; each preset's printed report is 3 pages, not 2. The PDF export and the emailed report no longer include the design notes; they stay in the tool and the Word, Excel, CSV and text exports, marked for the designer. In the tool, help tips stay inside the screen, the "Highest utilisation" tip can be reached with the keyboard, and the Options panel fits a screen 294 px wide. The calculations, sizes and verdicts are unchanged. 30/09/2026 (1.6.4 correction): this corrects the 29/09/2026 (1.6.4) entry. The design notes, with the On-Site Guide clip spacing, have never been on the printed report. In 1.6.4 they were in the tool and all the exports; from 1.6.8 the PDF export and the emailed report no longer include them. The rest of that entry stands. 30/09/2026 (1.6.7): in the tool, the printed report and the exports, "Highest utilisation" now shows the run nearest its own limit, or furthest over it. In 11,826 of 38,880 two-run test reports it shows the multicore run, not the singles run; in 4,188 of those, only the singles run passed. In 7,095 the size in the report title changed too. No run's size or verdict changed. Run cards name each figure's basis. The design notes now quote BS 7671 Regulation 521.10.202 NOTE 4, BS 8519:2020 15 f) and Guidance Note 1, Appendix G. The cable list group is now PD Cable Database. 30/09/2026 (1.6.6): with Auto, the smallest conduit is now 20 mm (the usual UK minimum in Project Design practice from site experience, not a requirement of a standard), or 25 mm where set in Options; 16 mm can be chosen by hand. The support and protective conductor notes now quote BS 8519:2020, BS 7671 Regulation 543.2 and Institution of Engineering and Technology Guidance Note 1. Runs sized at 16 mm with Auto now read 20 mm: the lighting radial preset reads 20 mm, PASS (21.7%), not 16 mm, PASS WITH WARNING (37.3%). Other preset sizes are unchanged. Please re-check these runs. 29/09/2026 (1.6.5): the conduit space factor limit is now 35% (Institution of Engineering and Technology Guidance Note 1, Appendix A, A5); 45% is for trunking. Some results change. A run with a sheathed cable or custom internal diameter filling 35% to 45% reads OVER 35%: for three FP200 Gold 2-core 1.5 mm² cables in 25 mm heavy gauge conduit (41.9%), 32 mm is given with Auto. In this tool, runs of singles passing the factor tables but filling over 35% read PASS WITH WARNING: for the lighting radial preset (37.3% in 16 mm), a note gives 20 mm. Please re-check these runs. 29/09/2026 (1.6.4): in the tool, the printed report and the exports, the design notes give the On-Site Guide clip spacing for fire alarm and emergency lighting cables. Where the manufacturer's figure differs, both are shown and the Guide's takes precedence in the tool: for FP200 Gold up to 9.0 mm overall diameter, the Guide gives 250 mm horizontal and Prysmian gives 300 mm. The drawing-in and steel conduit notes read shall, as BS 7671 does, and the protective device note gives the three conditions of Regulation 433.1.1. Most acronyms are spelt out at first use. The calculations, sizes and verdicts are unchanged. 27/09/2026 (1.6.3): runs of single-core non-sheathed cables sized by the On-Site Guide Appendix E factors are now checked against its 45% space factor for other cables, as PD good practice. Above 45%, the verdict is PASS WITH WARNING in amber, and a note gives the next size at 45% or less, or a split across two conduits. Fifteen 2.5 mm2 LSZH singles, 4.5 m with one bend, fill 25 mm steel conduit to 58.0%; the note gives 32 mm. In the cross-section drawing, no two cables now overlap. The existing calculations, the sizes given with Auto and every other verdict are unchanged. 26/09/2026 (1.6.2): the printed report had shown no sizing steps under its Sizing basis heading since 1.5.2 and now lists the steps and the notes for each run again; the steps were always shown in the tool and in the exports, and the calculations were never affected. In the check line shown when ELV control circuits share a conduit with LV circuits, the two methods from BS 7671:2018+A4:2026 Regulation 528.1 are now numbered 1 and 5, not (a) and (e), and worded as in the regulation. The fire alarm and emergency lighting check lines, the fire alarm banner on a run, the protective device rule, the design notes on steel conduit, fire-rated fixings and supports, and the help on the cable quantity and Serves fields now use the verb of the clause they cite: shall for BS 7671 Regulations 521.5.1, 433.1.1, 521.10.202 and 528.1, and should for BS 5839-1:2025 35.5, BS 5266-1:2025 8.2.6, BS 8519:2020 15 and 16 (cited before with no edition), and the On-Site Guide Table D3 note and Appendix D item (a). The banner and the field help are in the tool only; the other lines are also in the exports. In the emergency lighting check line, the advice that circuits fed from a central power supply system should have a conduit of their own is now a sentence of its own, since it is PD's reading of the separate wiring that 8.2.6 recommends. The help on the Serves field now reads "The segregation checks depend on it", not "This drives the segregation checks". The page had no main heading; the tool name in the dark bar at the top of the tool is now that heading, with the same look as before. Two changes to the look made in 1.6.1 were not named at the time: the text in the fields is 14 px on a desktop, as in the drop-down lists, and 16 px on a phone, so that the phone does not zoom in when a field is tapped; and the crosses used to remove a cable or a run are easier to tap on a touch screen. 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. The calculations, sizes, factors, limits and verdicts are unchanged. 26/09/2026 (1.6.1): BS 7671 Regulation 521.6 is now cited for circuits sharing a conduit, not 521.7, which applies to circuits sharing a multicore cable. The On-Site Guide is cited without page numbers, with Appendix D item (a) for non-sheathed cables and item (h) for thermosetting and PVC sheathed cables. The compliance record now gives the 2015 edition of BS 7629-1. The BS 8519:2020 note reads should, not need. The conduit drawings and printed report now use the PD Power Distribution tool colours, with the four cable service colours kept. The calculations, sizes, factors, limits and verdicts are unchanged. 26/09/2026 (1.6.0): The tool now looks like the PD Power Distribution tool, with the same colours, font, buttons, cards and tables. The Start again button is now in a dark bar at the top of the tool, with the tool name and version. The Cross-section and Run detail switch is now shown as tabs. The calculations, the wording of every check line, the printed report, the exports and the saved reports are unchanged. 16/09/2026 (1.5.8): 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 checks are called checks. The calculations, the printed report and the exports are unchanged. The 2,190 scenarios, the input audit and the term-of-art check were re-run on 1.5.8 with no findings; verified live after upload. 15/09/2026 (1.5.7): 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 2,190 scenarios were re-run on 1.5.7 with no findings; verified live after upload. 15/09/2026 (1.5.4 to 1.5.6): every citation of the fire alarm and emergency lighting standards was read back against the current editions, and the conduit is now described by its internal diameter throughout. BS 5839-1:2025 replaced the 2017 edition: the rule against sharing a conduit with other services is cited to 35.5, conduit to BS EN 61386 to 35.7 and the 1 mm2 conductor minimum to 25.7. BS 5266-1:2025 replaced the 2016 edition: emergency lighting fed from a central power supply system gets a conduit of its own (8.2.6), fire-resistant cables in screwed steel conduit are named as one of the listed cable systems (8.2.2), and supply cables to self-contained luminaires may be installed as a normal lighting circuit (8.1). A report saved before 1.5.6 opens with its custom internal diameter as entered. Three 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 earlier builds; the 2,190 scenarios, the input audit and the term-of-art check were re-run on 1.5.6 with no findings; no size, factor or limit changed; verified live after upload. 10/09/2026 (1.5.3): a check across every PD tool for a question asked of every user that only a minority case needs, and for a figure used in an answer that the reader never saw. A sheathed multicore run was found to refuse a size until a length was entered, although the length serves only the On-Site Guide Appendix E draw-in envelope that the singles method reads; a sheathed run with a blank length is now sized and prints a warning that the envelope has not been checked, and a singles run still asks for the length. The grouping factor from BS 7671 Table 4C1 was found to rest on a circuit count the reader was never shown; the rating line now states the count and its basis, estimated from the cables on the run or entered in a new Circuits field on the run, and the subscriber rating table and the editable exports print the same basis. The 2,190 scenarios, the input audit and a new automated check for both paths were run on 1.5.3 with no findings, and the new check fails on the 1.5.2 build; no factor, size or limit changed; verified live after upload. 04/09/2026 (1.5.2): the sizing basis is now headed How this size was reached and laid out as numbered steps for each run, with the run'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 2,190 scenarios and the input audit were re-run on 1.5.2 with no findings; no factor, size or limit changed; verified live after upload. 04/09/2026 (1.5.1): a check of whether every input the standard names by a term of art is defined in the tool, printed back in the result and questioned where a wrong entry can be seen. Two were not. The custom internal diameter field took the nominal size, which BS 4568 uses for the outside diameter, so a 20 entered for a 20 mm conduit sized on 314 mm2 where the heavy gauge internal diameter gives 222 mm2. The custom cable diameter field took the conductor size in place of the overall diameter. Both fields now show help naming the figure the standard means and the entry that is wrong, and an internal diameter equal to a standard nominal size or a diameter equal to a conductor size prints a check line with the figure the tool holds while still giving the answer. The 2,190 scenarios and the input audit were re-run on 1.5.1 with no findings; no factor, size or limit changed; verified live after upload. 03/09/2026 (1.5.0): a check across every PD tool that every printed value is reached through the input the standard derives it from. The support spacing for a 16 mm conduit was found to print the 20 to 25 mm row; On-Site Guide Table D3 is now held in full (four rows, rigid metal, rigid insulating and pliable) with a material choice per run. Current ratings are now given only for the FP200 cable the datasheet covers, other cables are pointed to their BS 7671 Appendix 4 table; an ambient temperature input applies Table 4B1. Blank lengths and blank custom internal diameters ask for the value instead of sizing at a default. The 2,190 scenarios and a new audit of the input paths were run with no findings; verified live after upload. 02/09/2026: the same 2,190 scenarios were re-run after the 1.4.3 wording revision with no findings; no size or factor changed. July 2026: the tool was checked against the ESC C157-3 worked examples and 21 scenarios were audited on the live tool. August 2026: 2,190 scenarios were driven through the tool's own interface and checked against independent calculations written from the On-Site Guide factors and the BS 4568 internal diameters, with no findings.