Underground duct sizing, arrangement and coordination for power cables, based on the Project Design Underground Duct Selection guide (BS 7671, ENA TS 12-24, IET GN6, DNO and IDNO practice). Work in runs: for each run, choose its authority, cables per duct and arrangement (flat, trefoil, quad or grid), then add the cables. Each run is spread across as many ducts as the cables need, with the fill shown. You can use the tool to size the ducts, to find the cover depth, the spacing, the colour, and the BS 7671 depth and grouping factors, and to draw a true coordination section. Free to run. The free version is limited to one run in the trench, and a sample trench with three runs is shown. Engineering Toolkit subscribers can add more runs to the same trench, save the report to a project, export editable Excel, Word, comma-separated values (CSV) and text files, print the report or save it as a portable document format (PDF) file, and open the full underground coordination guidance.
Largest duct ID0 mm
Total ducts0
Min cover depth0 mm
Duct spacing0 mm
Depth factor Cd1.00
Grouping factor Cg1.00
Duct colour-
Recommended ducts: -
v3.14.1 · Verified
BS 7671:2018+A4:2026, Appendix 4 (Ref Method D)
- 1,993scenarios enumerated in each tier on 3.9.0 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
- October 2026
- Data basis
Show the full record
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
- Duct fill and duct sizes are independently verified against calculations derived from stored network duct tables and cable diameters. Version 3.14.1 was last validated on 18/10/2026, passing 98 automated checks, including 28 fault-injection tests.
Scope and limits
- 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 changedOnly wording is corrected: UK Power Networks ECS 02-0019 v15.0 Appendix A.3 lists 95, 185 and 300 mm2 11 kV single cores only as three per duct. Free visitors are no longer referred to My Projects. The UK Power Networks note no longer describes 150 mm CHECK figures as sizes applied. Duct sizes and results are unchanged. Re-check saved reports.
- Only wording is corrected: UK Power Networks ECS 02-0019 v15.0 Appendix A.3 lists 95, 185 and 300 mm2 11 kV single cores only as three per duct. Free visitors are no longer referred to My Projects. The UK Power Networks note no longer describes 150 mm CHECK figures as sizes applied. Duct sizes and results are unchanged. Re-check saved reports.
- The tool is now PD Cable Duct. One run in the trench is free; more runs come with the Engineering Toolkit. On UK Power Networks routes, low voltage mains above 35 mm2 need 125 mm ducts (ECS 02-0019 v15.0, EDS 08-2100 v11.2). Automatic depth of lay can be deeper and depth factor (Cd) lower. Re-check saved reports.
- Minimum cover is lower in 70 of 4,867 test cases and higher in 44. A lower minimum cover gives a shallower automatic depth of lay and higher depth factor (Cd). New NOTE and CHECK lines name conditions not checked. No bending radius or pulling tension is shown at 11 kV or 33 kV. The ENA ER G87 building entry sentence under the Cable handling table is shown at low voltage only, where it applies. The printed report and exports now include the network operator lines shown in the tool. Duct sizes, counts and grouping factors are unchanged. Re-check saved reports.
- On UK Power Networks 33 kV runs set to "up to 500 mm2", 630 mm2 cables now get 225 mm, not 150 mm (ECS 02-0019 v10.0 Appendix A.4 gives 190 mm). Cover is measured to the highest duct. In 4,066 test cases, 69 designs now fail on cover; depth factor (Cd) is lower in 431. Re-check saved reports.
- FP400 and FP500 now count as armoured: the bend radius is calculated at 6x the overall diameter, up from 4x for 18 sizes (Institution of Engineering and Technology On-Site Guide, BS 7671:2018+A4:2026, Table D5). Prysmian's FP400 8D (8x) is given beside the 6x and takes precedence. For shaped conductors, check the data sheet (Cable handling note). Duct sizes are unchanged.
- UK Power Networks 33 kV runs can be set to three single-core cables per duct, with a 150 or 190 mm minimum by conductor size (ECS 02-0019 v10.0 Appendix A.4). UK Power Networks 50 mm service and 125 mm 11 kV ducts are labelled Project Design (PD) practice. Where the choice is not made, no duct size or result changed.
- UK Power Networks low voltage mains in 100 mm ducts now get an amber check: EDS 08-0143 v2.0, for supplies above 100 A per phase, requires at least 125 mm under a hard standing (clause 5.2.1). The check line states which document versions were read. No duct size or grouping factor changed.
- Some grouping factor (Cg) values fall, because the clearance is now measured between duct outsides and every working duct in the trench is counted. Re-check private-route runs where cables share ducts: a duct is now added where one was over 45 % full. FP600 cables now count as armoured, with bend radii and fill from the corrected shared cable list.
- The printed report is now readable, apart from the drawing labels. Pages are no longer shrunk to one A4 sheet: in the reports checked, text outside the drawings measures 8 pt or more, and pages are numbered. Every check line shown in the tool is now printed. The calculations, the wording shown in the tool and the exports are unchanged.
- The ENA ER G87 lines now give G87's recommendation on where the duct should terminate, the UK Power Networks standard is cited as ECS 02-0019, and the single-core lines cite BS 7671 Reg 521.5, not 523.6. The green, amber and grey are now darker, for a contrast above 4.5:1 with white text (Web Content Accessibility Guidelines 2.2, level AA). The calculations 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, and the Trench positioning and Duct section (detail) views are chosen from tabs. The calculations, every result and the wording shown in the tool 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. For example, it now states that an adoptable route always includes at least one spare duct and that 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 buried ratings at 2.5 K.m/W, and the Table 4B3 factors for other resistivities are to be applied in the cable sizing. Where a service-size run shares a trench with a larger cable, the notes give the larger cable's minimum spacing for the whole trench. 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
- Circuits, not cables or cores, are now counted for the grouping factor, so a trefoil or quad set counts as one circuit and a single-core CPC as none. A spare, spacing or depth left blank is now asked for, with no figure assumed. In a mixed trench, each authority's own minimum cover is printed and checked.
- 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
