Cable Containment (CMS) Capacity Calculator
Size 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.
How Cable Containment is Sized by Fill
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 conduit/duct (GN1 Amendment 3, Appendix A). 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.
Trunking, Duct, Tray, Ladder & Basket in One Tool
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.
A Printable Calculation Record
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.