Size conduit, trunking, duct, cable tray, ladder and basket from a cable schedule. Add each cable and quantity, choose the containment type, and the tool sums the required cross-sectional area (or cable width), applies the space factor and future spare capacity, and picks the smallest standard size that passes — with a printable calculation report. A first-pass design aid based on IET Guidance Note 1, not a substitute for a full design.
This is a desktop tool — the cable schedule is wide, so on a small screen it scrolls sideways. Every value is indicative; verify cable dimensions against manufacturer data and capacities against IET GN1 before issue.
This calculator is a first-pass containment-fill aid based on IET Guidance Note 1 Amendment 3, Appendix A (45% trunking / 35% duct space factor). Cable outer diameters and cross-sectional areas are an indicative subset of Draka / Amtech catalogue data, and containment capacities follow the manufacturer catalogues — every value marked VERIFY must be confirmed against current manufacturer data and your own copy of GN1 before a calculation is issued. The design remains the responsibility of the engineer.
It is the maximum proportion of a trunking or conduit's internal cross-section that the cables may occupy — 45 per cent for trunking, 35 per cent for conduit and duct, from IET Guidance Note 1 Appendix A. The other 55 per cent is not wasted: round cables cannot tile a rectangle solidly, cables need to be drawn in without damage, and the air gap carries heat away. So a trunking that looks half empty on a drawing is at its permitted fill.
No. A space factor is a fill limit for an enclosed system. Open containment is normally sized on the width the cables occupy when laid in a single layer — total width for cables laid touching, or the width plus the clear spacing for cables laid 0.5, 1 or 2 diameters apart. This calculator uses the width method for open containment and switches to an area calculation only when you tell it the cables are grouped rather than laid out in a row.
There is no figure in BS 7671 — it is a project decision, and 20 to 30 per cent is common on a riser or a main route where cables are likely to be added. The calculator reserves your percentage before it tests each size, so the recommended size has the spare already set aside rather than being filled to its limit on day one. Check the employer's requirements or the specification: many name a figure.
Only under conditions. BS 7671 528.1 says Band I circuits (ELV, data, signalling) and Band II circuits (LV power) must not share a wiring system unless every Band I cable is insulated for the highest voltage present, or the two bands are separated by a partition or run in separate compartments. Cables of a fire safety service are a separate requirement again — Section 560 requires them to be kept apart from other circuits. The calculator flags both when it sees the categories mixed.
Yes, and this is the step most often missed. The number of circuits sharing the containment sets the BS 7671 grouping factor Cg from Table 4C1, which reduces every cable's current-carrying capacity — nine circuits bunched in trunking keeps only half. The calculator reports the Cg your fill implies so you can carry it into the cable sizing calculation. Spacing the cables by more than twice their diameter removes the factor entirely (Table 4C1 Note 2).
Not by percentage fill. IET Guidance Note 1 Appendix A gives every cable a dimensionless cable factor and every conduit size a conduit factor; the smallest conduit whose factor is at least the sum of the cable factors is the answer. The method is about the ease of drawing the cables in rather than whether they geometrically fit, which is why it depends on the run — GN1 uses Tables A1 and A2 for a straight run of 3 metres or less, and Tables A3 and A4, with lower figures, for anything longer or with bends in it. No space factor applies on top; the factors already include that allowance.
Because they change the answer. A conduit factor falls as a run gets longer and falls again with every bend, since both make the cables harder to pull in. Twelve cables that go comfortably into a 20 mm conduit over two metres straight may need 25 mm over four metres with two bends. GN1 also stops tabulating altogether past a point — three bends beyond five metres, or four bends beyond two and a half — which is the standard telling you to fit a draw-in box and treat each section as its own run. The calculator will not extrapolate past the table.
No — treat every value as indicative and confirm it before issue. The outer diameters, cross-sections and weights are catalogue figures for common Draka and Amtech cables and will differ between manufacturers and between production runs of the same cable. The containment capacities are likewise indicative. Check the actual cables and products specified for your project against their current datasheets; the printed report carries a VERIFY register listing exactly which values need confirming.
For enclosed containment — trunking and duct — BS 7671 and IET Guidance Note 1 limit how much of the internal cross-section the cables may occupy. The classic rule is a 45% space factor for trunking and 35% for duct (GN1 Appendix A). Conduit is the exception and is sized a different way entirely — see below. This tool sums the cross-sectional area of every cable (πd²/4 for each conductor, times quantity), applies the space factor and a future spare capacity allowance, then finds the smallest standard size whose usable area still clears the total. For open containment — tray, ladder and basket — it instead sums the cable widths laid touching or spaced (0.5D, 1D, 2D) against the usable tray width, or the bunched cross-section when cables are grouped.
Ask for a conduit and the calculator switches method entirely. UK practice (IET Guidance Note 1, Appendix A) does not size conduit on a percentage fill — it gives every cable a dimensionless cable factor and every conduit size a conduit factor, and the conduit passes when the factors add up to no more than its own. The point is not whether the cables fit, but whether you can pull them in: GN1 says so plainly — "only the ease of drawing-in is taken into account".
That is why the tool asks for the run length and the number of bends. A 20 mm conduit that passes over 2 m straight can fail over 4 m with two bends, because GN1 uses one pair of tables for straight runs up to 3 m (A1 and A2) and a different pair — with lower figures — for longer runs or any run with bends (A3 and A4). The same cable has a different factor in each, so the printed report names which tables were used, the run length, the bend count and the conduit factor it read. No space factor applies to conduit: the GN1 factors already allow for drawing the cables in, and adding one on top would count it twice.
Where GN1 has nothing to say, neither does the calculator. Its cable factors cover single-core thermoplastic and thermosetting conductors from 1 to 25 mm², so the cable list narrows to those in conduit mode, with a box to enter a factor by hand from manufacturer data for anything else. Flexible and oval conduit are not in GN1's tables at all and come back for review rather than being guessed at. Push a run past what Table A4 tabulates — more than four bends, or a long run with three — and the tool asks for a draw-in box and two shorter runs instead of extrapolating, which is exactly what the standard intends.
Switch the containment type and the calculator loads the right method and the right standard size range — trunking widths and heights, circular duct bores by product family, and tray / ladder / basket widths by duty class. Each candidate size is listed with its usable capacity, the resulting utilisation percentage and a clear PASS / FAIL, so you can see not just the smallest size that works but how much headroom every size leaves. A built-in cable catalogue covers common LSOH, XLPE/SWA, PVC, flexible, fire-resistant and data cables, or you can enter a custom outer diameter for anything else.
Print / Save as PDF produces a submission-style report — project header, the full cable schedule with per-cable areas, the containment method and space factor, the size check for every candidate, the recommended size and a VERIFY register. For a full containment support network to BS 8519, size the brackets and supports with our MEP bracket calculator, and for whole-installation cable sizing see the BS 7671 cable sizing calculator. Need a full pack produced and checked? See our ProDesign cable calculation service.
Containment fill and the 45% space factor explains where the limit comes from, why a passing trunking still looks half empty, and how the fill you choose feeds straight back into the cable size through the grouping factor. See also cable derating factors for how Cg combines with the ambient and thermal-insulation factors, and BS 8519 fire-rated containment supports for holding the containment up.