Conduit Size Calculator
Size a conduit for the cables it must hold: singles by the On-Site Guide factors, FP200-type multicore by the space-factor method. Print the record that proves it.
How this works
- Add a run. A run is one conduit between draw-in points: choose the conduit material (steel, PVC or pliable), set its length and how many bends or sets it takes (a double set counts as one bend), then drop cables in, or pick a typical situation and edit it.
- The method follows the cable type. Single-core non-sheathed cables (BS 6004 PVC / BS 7211 LSZH) use the exact On-Site Guide Appendix E unit system: the conduit factor falls as the run gets longer and gains bends. Sheathed multicore cables (FP200-type to BS 7629-1, any LSZH) use the 45% space-factor method with a geometric fit check; on a long or bendy pull the PD guide's working target is printed as a note.
- Read the verdicts and print the record: conduit size per run, the factor or space-factor calculation, the segregation checks (fire alarm gets its own conduit), the enclosed current-rating consequence, and the drawings.
Free to run for everyone. Engineering Toolkit subscribers get the compliance record with verbatim clause citations, the enclosed rating check, editable Excel / Word / CSV / Text exports and My Projects.
Options: conduit system
How this size was reached
Enclosed current rating: the transition consequence
Compliance record: the clauses, quoted verbatim
Design notes
- 2,190scenarios checked against independent calculations on 1.5.8, no findings
- 4 rows x 3 columnsOn-Site Guide Table D3 support spacings held in full, by conduit material
- E1 to E4On-Site Guide factor tables held
- 2standards read back on 1.5.7, BS 5839-1:2025 and BS 5266-1:2025: every citation checked against the 2025 editions
- Last reviewed
- September 2026
- Data basis
- OSG Appendix E factors (E1 to E4), BS 4568 conduit IDs (BSRIA TN 10/92), BS 7629-1 and FP200 Gold dimensions, FP200 enclosed ratings
- Validated against
Show the full record
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.
- Steel, PVC or pliable conduit for singles (OSG unit system); space-factor method on 16 to 32 mm standard steel IDs, manufacturer ID entered for plastic conduit. Multicore up to 4 mm2 (BS 7629-1 types, FP200 Gold, custom OD). Sizes the containment only: the circuit design of record (Ib, In, Iz, voltage drop, Zs) remains the designer's separate calculation; the in-conduit rating shown is the FP200 datasheet figure with the grouping and ambient factors applied, other cables are pointed to their BS 7671 table.
What's changed
- 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 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 conduit is described by its internal diameter throughout: in the tool, in the help, in the check lines, in the exports and in the notes written inside the program code. Reports saved before this version open with their custom internal diameter as entered. No size, factor or limit changed
- The emergency lighting lines now cite BS 5266-1:2025 and say which wiring they apply to: a separate conduit for circuits fed from a central power supply system (8.2.6), with fire-resistant cables in screwed steel conduit as one of the listed cable systems (8.2.2); supply cables to self-contained emergency luminaires may be installed as a normal lighting circuit (8.1). The BS 8519 fixing line reads should, not must. No factor, size or limit changed.
- The fire alarm segregation lines now cite BS 5839-1:2025, which replaced the 2017 edition: 35.5 for the rule against sharing a conduit with other services, 35.7 for conduit to BS EN 61386 and 25.7 for the 1 mm2 conductor minimum. The earlier citation used the 2017 clause number, which in the 2025 edition covers radio-linked systems. No factor, size or limit changed
- A sheathed or multicore run is sized on the 45% space factor whether or not the length is entered, since that limit does not read the length; a blank length then prints a note that the Appendix E envelope has not been checked. A single-core run still asks for the length, which selects its Appendix E factors. Each run has a Circuits (for Cg) field: blank is the estimate the tool always made (each multicore cable one circuit, three singles one circuit), now stated on the rating line, and a figure entered overrides it. No factor or limit changed
- 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 top of its card and every figure in bold. The steps themselves, and the report section that shows them, are the same calculations as before. No factor, size or limit changed
- The custom ID field and the custom cable overall diameter field now say what they take and what they do not: the internal diameter, not the nominal size, which BS 4568 uses for the outside diameter, and the overall diameter over the sheath, not the conductor size. An entered ID equal to a standard nominal size, or an overall diameter equal to a conductor size, prints a check line with the figure the tool holds and still gives the answer. No factor, size or limit changed
- Support spacing now reads all four rows and all three columns of On-Site Guide Table D3 for the conduit material chosen on each run (steel, PVC or pliable), so a 16 mm steel conduit prints 0.75 m horizontal and 1 m vertical; the enclosed rating figure applies to FP200 products only and every other multicore cable is pointed to its own BS 7671 Appendix 4 table; a blank length, internal diameter, cable diameter or quantity is asked for instead of defaulted; the BS 8519 fixing centres print for fire alarm and emergency lighting services only; a run passes or fails on the 45% limit alone, with the 30% working target printed as the PD guide's note; the largest device figure is replaced by the Ib <= In <= Iz rule; the ambient temperature is asked, default 30 C, and the Table 4B1 factor is applied and stated
- The wording was revised in plain UK English: the worst run is labelled highest utilisation, a limit is called a limit, plurals written out, empty fields show a plain hyphen and the on-screen text has no dashes. No size or factor changed
- The does-not-fit warning and banner lead with the instruction; proper plurals throughout
- The not-suitable messages now read plainly: the conduit tool is not suitable for this arrangement, please use the cable tray or cable duct tool, with the supporting numbers in a second sentence
- A run that no conduit can take now says so with its numbers instead of going blank: cables too big for conduit are named as tray or duct work with a link to those calculators, a run over the 45% limit at 32 mm shows the figure at 32 mm (space-factor sizing stops there: BS 4568 publishes no internal diameter above 32 mm), and the outside-the-tables length and bend combinations get the add-a-draw-in-box verdict for sheathed cables and singles alike
- The core count numerals on the cable discs read clearly again: the white label background added in 1.3.0 no longer applies to white text on a coloured disc
- The recommended conduit sizes stay in view in a bar that follows you down the page while you edit the runs. Drawing labels have a white background so they read clearly over the cables, and a review flag on a run reads as plain text rather than a dash
- Free use is now measured in sessions: three days of anonymous use, then your email continues free use with no password and no account. The enclosed rating headline (the Iz and the largest device per run) is now free in the tool; the full rating table and the verbatim compliance record stay with the Engineering Toolkit, and the report goes by email for free users with print as part of the Toolkit
- Subscriber-facing copy states what the Engineering Toolkit adds rather than what the free preview lacks
- Data, AV and fibre cables from the shared library can now be selected on conduit runs, sized by the 45% space factor on the datasheet overall diameter
- Run detail: the bend note now reads "bend inner radius >= 62.5 mm (2.5 x OD)" and sits clear of the saddle ticks
- Internal diameter written ID throughout; Run detail redrawn with saddles placed as the On-Site Guide places them, within 300 mm of every box and bend and then at the Table D3 maximum spacing
- First release: conduit sizing for singles by the On-Site Guide Appendix E factors (run length and bends included) and for FP200-type and LSZH multicore by the 45% space factor, with presets, fire alarm segregation, true-scale drawings and reports
