Stranded-conductor factors from OSG Tables E5/E6. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This calculator works out the minimum trunking size for a bundle of single-core cables using the cable and trunking factors from the IET On-Site Guide. UK electricians use it to avoid the two classic containment mistakes: trunking stuffed beyond its space factor, which overheats cables and makes future work impossible, and oversized trunking that wastes money and wall space.
The method is the standard factor comparison from OSG Tables E5 and E6. Each cable size has a factor representing the space it occupies; each trunking size has a capacity factor. Add up the factors for every cable in the run and the trunking's factor must equal or exceed the total. Enter how many singles of each size you are pulling and the tool does the lookup.
Σ(cable count × cable factor) ≤ trunking factor
Each stranded single-core cable size has a factor from OSG Table E5 — 8.6 for 1.5mm², 12.6 for 2.5mm², up to 47.8 for 16mm². Multiply the number of cables of each size by its factor and sum the results to get the total cable factor for the run. Each trunking size has a capacity factor from OSG Table E6 — for example 767 for 50 × 37.5mm up to 4252 for 100 × 100mm — and the smallest trunking whose factor meets or exceeds the total is the minimum size. The factors build in the space factor, so no further allowance is needed.
Enter the number of single-core cables of each size in the run, from 1.5mm² to 16mm². Count every conductor — lines, neutrals and CPCs all occupy space.
The tool multiplies each count by its Table E5 factor and totals them, giving the combined factor the trunking must accommodate.
The smallest standard trunking from Table E6 whose factor covers the total is shown, from 50 × 37.5mm up to 100 × 100mm. If the total exceeds the largest size, the tool tells you to split the run.
The factors used are for stranded conductors, from Tables E5 and E6 of the On-Site Guide, and they already embody the space factor for trunking — you do not add a further percentage on top. Solid conductors and cable types not covered by the table need their own factors.
Sizing at the limit is rarely wise. Trunking runs acquire extra circuits over their life, and a run sized with headroom costs little more now and saves replacing containment later. Also remember that grouping many circuits in shared containment derates the cables themselves — check the grouping factor in the cable sizing calculation, not just the physical fit.
Bends, sets and fittings reduce usable space and make pulling harder; long runs with multiple bends may justify a size up regardless of the factor arithmetic. Guidance only — verify against the On-Site Guide and BS 7671 for the actual installation.
Stranded-conductor factors from OSG Tables E5/E6. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
The space factor is the proportion of the trunking's cross-section that cables may occupy, and the On-Site Guide factor tables are constructed so it is respected automatically. Using the E5 cable factors against the E6 trunking factors is the practical way to comply without calculating areas yourself.
Multiply the number of cables of each size by that size's factor from OSG Table E5, add the results, then pick the smallest trunking in Table E6 whose factor equals or exceeds the total. For example, twenty 2.5mm² singles give 20 × 12.6 = 252, comfortably within 50 × 37.5mm trunking at 767.
Yes. Every conductor in the trunking occupies space, so count lines, neutrals and CPCs individually at their actual sizes. Missing the CPCs is a common reason a calculated fill turns out optimistic on site.
Fill and derating are separate checks. The fill calculation only ensures the cables physically fit within the space factor; bunching many circuits together in trunking also applies a grouping correction factor to their current-carrying capacity. Run the cable sizing calculator with the grouping set to match what shares the containment.
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