Underground duct sizing, arrangement and coordination for power cables, grounded on the Project Design Underground Duct Selection guide (BS 7671, ENA TS 12-24, IET GN6, DNO and IDNO practice). Work in runs: add a run, choose its authority, cables per duct and arrangement (flat, trefoil, quad or grid), then drop cables in. The tool tiles the run across as many ducts as the cables need and shows the fill. It sizes the ducts, the cover depth, the spacing, the colour, and the BS 7671 depth and grouping factors, and draws a true coordination section. Free to run; Engineering Toolkit subscribers can save to a project, export editable Excel, Word, CSV and text, and get the full NJUG coordination guidance and the detailed duct section.
- 1,993scenarios enumerated in each tier on 3.7.4 against an independent replica, no difference
- 122 of 122hand-built cases re-driven through the tool's own interface, no difference
- 6 DNOs + IDNOsadoptable duct tables held
- 4,157tool pages, reports and printed sheets read by the new wording check on 3.7.4, no finding
- Last reviewed
- September 2026
- Data basis
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IET Guidance Note 6, 10th Edition, section 2.10 | IET Guide to Cables and Cable Management, section 8 | ENA TS 12-24 (duct class and duct size range), BS EN 61386-24 (N450 and N750 mechanical class), ENA ER G87 (building entry) and ESQCR 2002 (marking, screening, records)| DNO and IDNO adoptable duct tables: UK Power Networks, SP Energy Networks, Northern Powergrid, NGED, SSEN, ENWL and the IDNOs | ERA quad-bundle current ratings
- Validated against
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15/09/2026 (3.7.4): the wording in the tool, in the report, in the help and in the notes written inside the program code was revised in plain UK English, and the duct is now described by its internal diameter throughout. The tool now states that an adoptable route always includes at least one spare duct, that extra depth reduces the rating, that single-core cables on AC need a defined arrangement, and that the cable sizing is done separately. A new automated check reads every program file and 4,157 tool pages, reports and printed sheets across the 1,993 scenarios in both tiers, and fails on the 3.7.3 build; the 1,993 scenarios were re-run in both tiers with no difference; no size, count or factor changed; verified live after upload. 10/09/2026 (3.7.3): the rating record now states that soil thermal resistivity is not applied, the BS 7671 Appendix 4 buried ratings being tabulated at 2.5 K.m/W, and names Table 4B3 as the source of the factors for other resistivities; a service-size run of 35 mm2 or below sharing a trench with a larger cable prints a mixed-trench note explaining that the larger cable's spacing floor applies to the whole trench. Automated checks were added for both lines and fail on the 3.7.2 build; the 1,993 scenarios were re-run in both tiers with no difference; no size, count or factor changed; verified live after upload. 10/09/2026 (3.7.2): a check across every PD tool for a question asked of every user that only a minority case needs. The page-level default duct size control was found to apply only on a private route, where the size select on each duct and the private-route minimum already set the size, and its wording described a starting size for new ducts that the tool never used. It is removed; a private run still prints the minimum it applies and why. The 1,993 scenarios were re-run on 3.7.2 in both tiers with no difference, and an automated check now confirms that the control is absent and the private-route minimum still prints; no size, count or factor changed; verified live after upload. 04/09/2026 (3.7.1): a check of whether every input the standard names by a term of art is defined in the tool. The depth of lay, the clear spacing, the spare capacity and the duct catalogue, which holds the internal diameter and the outside diameter of every duct, all name their term. The manual size list did not: it now prints the internal diameter on every option, as the catalogue and the result already did. The 1,993 scenarios were re-run on 3.7.1 with no difference; no size, count or factor changed; verified live after upload. 04/09/2026: the full input range was enumerated, 1,993 scenarios in each tier, generated from the same independent replica: every multicore SWA size on every authority at three spare percentages, singly and in bundles, at two and three cables per duct; every single-core AWA size in flat, trefoil and quad formation; multicore and CPC pairs; concentric and Waveform cables; data and fire-performance cables in bundles up to 48; 11 kV and 33 kV on every DNO and IDNO; every standard duct size entered manually; mixed bundles; and spare capacity from 0 to 100%. Each case was driven through the tool's own interface and the printed duct size, working and spare duct counts, total, fill percentage, over-capacity state and circuits per duct were compared with the replica. No difference was found. 03/09/2026 (3.7.0): a check across every PD tool that every printed value is reached through the input the standard derives it from. The grouping factor was found to count cores rather than circuits, so a trefoil of three single-core cables read as three circuits; each cable line now states the cores forming one circuit and the record prints the derivation, with Table 4C3 named as an approximation where it is applied to a single-core set. Blank spare, spacing and depth ask for the value instead of sizing at a default; a small-power private run is no longer floored at 100 mm; the pulling stress is cited to BS EN 50565-1 and the unsourced aluminium figure is removed; the bending radius and clearance notes state their sources; the voltage is a visible choice; a mixed trench checks each authority's cover minimum. The 122 replica scenarios were re-run and all passed, with 20 small-power private cases returning the smaller duct now allowed; verified live after upload. 02/09/2026 (3.6.2): every cited clause was read back against BS 7671:2018+A4:2026; the single-core bonding note now cites Reg 523.201 (the earlier Reg 524.201 was a wrong clause number); the 122 scenarios were re-run and all passed; no size, count or factor changed. The sizing was validated on 122 scenarios (25/07/2026) against an independent replica of the sizing engine that reads the served configuration and the shared cable data rather than a copy of them: every SWA size as a single cable on a private route and on UK Power Networks, all six DNOs and an IDNO, mixed SWA and CPC pairs, bundles of 2 to 20, single-core sets in flat, trefoil and quad, concentric and Waveform cables, manual sizes including undersize, 2, 3, 4 and 6 cables per duct, spare capacity at 0, 10, 25 and 50%, service band and 11 kV and 33 kV minimums, and data cables. Every case is also checked against rules taken from the PD underground duct selection guide: working ducts never exceed the number of cables, one cable gives one working duct on every authority at every spare percentage, the internal diameter is at least 1.5 times the overall diameter and at least the authority minimum, the next standard size down must fail (the snug rule), spare ducts are the working count times the spare percentage rounded up with a DNO minimum of one, and a private route never totals more than the UK Power Networks equivalent. Thirteen further cases are hand calculated. All 122 were then driven through the tool's own interface with no difference in duct size, duct count, spare or fill. Grouping is verified against BS 7671 Table 4C3. The 122 replica scenarios were re-run on 12/08/2026 after the interface update and all passed. They were re-run again on 02/09/2026 after the 3.6.1 wording revision and all 122 passed; no size, count or factor changed.
- Sizes underground ducts for LV power cables, and the DNO or IDNO HV duct interface. You build the run: the authority, the cables per duct and the arrangement (flat, trefoil, quad or grid, for single-core or multicore). The result gives the duct internal diameter, the number of ducts, the cover to the crown at the chosen depth of lay, the duct-to-duct spacing, the duct colour and the spare ducts, with the run tiled across as many ducts as the cables need. It includes a coordinated trench and duct-section drawing, a BS 7671 depth-of-lay (Cd) and grouping (Cg) design record that reflects the real cables per duct and the arrangement, and each cable's minimum bending radius and maximum pulling tension.
- It sizes the duct, not the cable: the Cd and Cg factors are a record for the cable sizing you do separately. The figures are indicative guidance. The duct size, cover depth, spacing and colour follow the DNO or adopting authority specification and the current issue of each standard; the bending radius and pulling tension are the installed and conductor-strength values, and the cable manufacturer data and the DNO pulling limits take precedence. Confirm these before construction. The designer remains responsible for the final design.
What's changed
- Each line that cites a source was read against it. The ENA ER G87 lines now give G87's recommendation: for most installation arrangements the duct should terminate inside the building below the cut-out, in a draw pit or a 900 mm minimum radius slow bend. The ESQCR line now gives the requirements of Regulations 13, 14 and 15 and, from the 2002 DTI guidance, the order of preference for marking and protection. The UK Power Networks standard is now cited as ECS 02-0019. The single-core arrangement lines cite BS 7671 Reg 521.5 in place of 523.6, and the BS 7671 Appendix 10 line says the current sharing may be more like 70/30 than 50/50. The printed network list and the Cover to crown box have no dashes, and the printed spare line is in plain words. 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 has a contrast of 5.37:1 on the green, 4.68:1 on the amber and 5.93:1 on the grey, above the 4.5:1 that the Web Content Accessibility Guidelines 2.2 require at level AA for text of 12 px; before, it was 4.38:1, 3.46:1 and 2.56:1. This change applies only to the colours, not to any calculation, result, wording or layout, in the tool or when printed. Apart from these changes, the calculations, the wording shown in the tool, the printed report and the exports are unchanged
- The tool now looks like the PD Power Distribution tool, with the same font and colours. The Start again button is now in a dark header bar with the tool name and version. The Trench positioning and Duct section (detail) views of the coordination drawing are now chosen from tabs. Each result figure is now above its label, and each check line's opening words, such as DUCTS SIZED or NOTE, are in a coloured tag. On a phone there is no longer a blank band below the introduction, and the Cores forming one circuit field has its own row so the page does not scroll sideways. The coordination drawing, the printed report and the detailed duct section in the exports are in the new colours, with the NJUG service colours unchanged. The panel for saving and exporting, below the results, keeps its present colours for now. Apart from the new colours and the header bar, the calculations, every result, the wording shown in the tool, the text, dimensions and figures in the drawing, the printed report and the exports are unchanged
- The wording in the tool, in the report and in the notes written inside the program code was revised in plain UK English: an adoptable route always includes at least one spare duct, extra depth reduces the rating, single-core cables on AC need a defined arrangement, and the cable sizing is done separately. The duct is described by its internal diameter throughout. No size, count or factor changed
- The rating design record now states that soil thermal resistivity is not applied in it: BS 7671 Appendix 4 tabulates the buried ratings at 2.5 K.m/W and Table 4B3 gives the factors for other resistivities, to be applied in the cable sizing. Where a service-size run shares a trench with a larger cable, the notes say that the spacing floor is the larger cable's figure for the whole trench and that the 100 mm service spacing applies only between service ducts in a trench of their own. No size, count or factor changed
- The page-level Default duct size control is removed. It applied only on a Private route, where the size select on each duct and the private-route minimum already set the size, and its wording described a starting size for new ducts that the tool never used. No size, count or factor changed
- The duct size list names the internal diameter on every option, as the catalogue and the result already do. No size, count or factor changed
- The grouping factor counts circuits, not cables or cores: each cable line has a cores forming one circuit basis, so a trefoil or quad set is one circuit and a single-core CPC is none, and the record states how the count was derived and labels Table 4C3 as an approximation for single-core sets. A private small-power draw-in takes the 50 mm minimum and the design note says which minimum applied and why. The voltage is a chosen input, a blank spare, spacing or depth field gets a request instead of a figure, a mixed trench prints and checks each authority's own minimum cover, the private basis names the 45% fill limit, the pulling tension cites BS EN 50565-1 with its 1000 N cap and drops the unsourced aluminium figure, the 900 mm G87 bend prints only with a DNO or IDNO authority, and the soft ground and clearance figures state their sources
- Single-core bonding note and basis line now cite BS 7671 Reg 523.201 (armoured single-core cables); the earlier reference to Reg 524.201 was a wrong clause number. No size, count or factor changed
- The wording was revised in plain UK English: minimum replaces floor, limit replaces ceiling, plurals written out, and the duct with the most circuits is named as such, and the on-screen text has no dashes. No size or factor changed
- The recommended duct schedule stays in view in a bar that follows you down the page while you edit the runs and the trench settings. Labels on the coordination drawing and the duct section have a white background so they read clearly over the fills and dimension lines
- 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 report goes by email for free users; the printed report is part of the Engineering Toolkit. Every answer stays free in the tool; subscribers are unchanged
- Subscriber-facing copy states what the Engineering Toolkit adds rather than what the free preview lacks
- The fill gauge now means one thing everywhere: the true space factor fill of the duct against the 45% limit, with the bar scaled to that limit. Adding a small CPC moves it by the cable's own area instead of jumping from 40% to 96%. Spare capacity is provided as whole spare ducts, and a design note says so
- Sizing aligned to the PD underground duct guide and validated against it on 122 scenarios: the space factor limit is 45% (was 40), spare capacity buys whole spare ducts rather than space inside each duct, and a private route is never more stringent than the DNO equivalent for the same cables
- A single large cable (SWA 240 mm² and up) no longer tiles into two ducts: a duct holding one cable is judged by the 1.5 x diameter clearance rule, not by packing capacity
- Shared duct sizing rebuilt on real cable areas: a CPC shares the duct with its circuit cable on Auto, so a distribution cable and its CPC read one duct plus the spare, and a duct chosen by hand that is too small now fails clearly
- A multicore cable sharing a run with a single-core cable counts as one cable, not as its individual cores, so an SWA and its CPC need two working ducts rather than five
- Results and schedule tables scroll inside their card on a phone
- New cable handling table: each cable's minimum bending radius (On-Site Guide Appendix E) and maximum pulling tension, in the tool and in the report. Manufacturer and DNO data remain the final authority
- Duct to duct spacing defaults to 250 mm, and a one-cable-per-duct run reports its true fill instead of 100%
- Each cable has its own reference and description, and a saved duct report can be re-opened to edit
- Runs take two independent choices, cables per duct and arrangement (flat, trefoil, quad or grid), for any cable type, so a compact duct bank saves trench space. Grouping derating is applied and explained per BS 7671 Table 4C3
- Formation reworked to cables per duct, the fill gauge tracks the cables you add, and DNO ducts always include a spare
- Cover is measured to the crown and depth of lay to the cable centre, so they read as different figures; DNO and IDNO ducts follow the adoptable rule of one cable per duct with a spare added
- Duct-first redesign: add a duct, choose its size and authority, then drop cables into it, with a live fill gauge, saved projects, and the grouping factor taken from the real number of circuits in each duct
- New grouped formation so several small cables run in one duct
- Fixed the results and drawings freezing when a preferred duct size was set together with grouped multicore
- Preferred duct size (private) now sets the minimum size for every private duct, and each drawing refreshes when cable quantity changes
- The multicore quad arrangement is now applied, preferred duct size and spare duct controls were added, and the drawing text is larger with service labels under the dots
- Word and PDF embed the duct section drawing, single-core ducts grow with cable size, and depth of lay moves the ducts in both drawings
- Authority is sticky per cable, the trench drawing auto-scales, and the duct section gains cover detail and intra-group spacing
- Single-core formation (trefoil, quad, flat) with IET GN6 guidance, authority per cable for mixed runs, and editable spacing and depth of lay with the BS 7671 grouping and depth factors as a design record
- Free PDF fixed so the branding no longer shrinks the page
- Small multicore cables share one distribution duct, with sensible minimum sizes
- Sizes a full duct schedule per cable type, with single-core trefoil arrangements
- Rebuilt on the underground duct selection guide: DNO and IDNO against private, with an NJUG trench diagram
- First release: underground duct sizing with NJUG coordination guidance
