Conduit & Trunking Fill Factors Explained
Fill factors keep containment from being overstuffed — so cables pull through without damage and can shed heat properly.
Cram too many cables into a conduit or trunking and two things go wrong: the cables are hard to draw in without stripping insulation, and they can’t dissipate heat, so their current rating drops. The IET On-Site Guide and Guidance Note 1 give a simple unit system that tells you how much you can safely install.
This guide explains the cable-factor and conduit-factor method for round conduit, the 45% space factor used for trunking, why you derate for bends and long runs, and a worked example so you can size containment with confidence rather than guesswork.
Key takeaways
- Fill factors stop containment being overfilled, protecting cables during installation and letting them run cool.
- For conduit, the IET uses a unit method: total the cable factors and compare against the conduit factor.
- For trunking, the accepted maximum space factor is 45% of the internal cross-sectional area.
- Conduit factors reduce as runs get longer or include more bends, so the same conduit holds fewer cables.
- If the cables don’t fit, go up a containment size or add a draw-in box to break a long or bendy run.
Why fill is limited
There are two independent reasons to limit how full containment gets. The first is mechanical: cables have to be pulled through, and an overfilled conduit means high friction, snagged cables and damaged insulation — the very thing containment is meant to prevent. The second is thermal: bunched cables can’t lose heat as easily, so grouping reduces their current-carrying capacity.
The fill rules keep both problems in check without you having to calculate heat dissipation from first principles every time. Stay within the published factors or space factor and the installation will pull in cleanly and run within its thermal limits.
The conduit unit method
For round conduit the IET uses a system of unit factors. Each cable of a given size has a cable factor — a number representing the space it needs — and each conduit of a given size and run length has a conduit factor. You add up the cable factors for every cable you want to install, and as long as that total is less than or equal to the conduit factor, the cables will fit and draw in.
The clever part is that the conduit factor already accounts for the run: there are separate factor tables for short straight runs and for longer runs with bends. So you pick the conduit factor that matches your actual route, total your cable factors, and compare the two numbers directly.
Two tables, not one
Use the short straight-run cable and conduit factors only for simple, short drops. For anything with bends or appreciable length, switch to the longer-run tables — the factors are smaller, so the same conduit legitimately holds fewer cables.
The 45% trunking space factor
Trunking is sized by space factor rather than unit factors. The accepted maximum is 45% — the total cross-sectional area of all the cables (including their insulation) should not exceed 45% of the internal cross-sectional area of the trunking. The remaining 55% is air gaps, which is what lets you lay cables in and take them out again and helps them run cool.
In practice you can use the same unit-factor approach for trunking too: the IET publishes cable factors and trunking factors that already build in the 45% space factor, so you total the cable factors and check against the trunking factor exactly as you do for conduit.
| Containment | Basis of limit | Rule of thumb |
|---|---|---|
| Round conduit | Unit factors (cable vs conduit factor) | Sum of cable factors ≤ conduit factor |
| Trunking | Space factor | Cable area ≤ 45% of internal area |
| Both | Derated for route | Fewer cables as bends and length increase |
Derating for bends and length
The longer a run and the more bends it contains, the harder cables are to pull and the more friction builds up. That is why the conduit factor falls as you move from a short straight run to a long, bendy one — the tables effectively derate the conduit’s capacity to reflect the extra difficulty. Two right-angle bends over a long run can drop the usable factor substantially.
The practical limit is usually taken as no more than the equivalent of a couple of bends between draw-in points; beyond that you fit an inspection box or pull box to break the run into shorter sections. Each section is then treated as a short run again, restoring the higher conduit factor.
Worked example
Suppose you want to run eight 2.5 mm² single cables in conduit over a straight 3 m run. Look up the cable factor for 2.5 mm² singles and multiply by eight to get the total cable factor. Then look up the conduit factor for the size you propose over a short straight run.
If the total cable factor comes to, say, 240 units and a 20 mm conduit offers a factor of around 460 on a short run, the cables fit with room to spare. Add two bends and a longer route, though, and the 20 mm conduit factor might fall below 240 — at which point you either step up to 25 mm conduit or add a draw-in box to shorten the run.
Let the tables carry it
TradePlanr’s trunking fill calculator totals the cable factors and checks them against the containment factor for your run, and the cable sizing calculator confirms the conductors themselves are right before you commit to a size.
Frequently asked questions
What is the maximum fill for trunking?
The accepted maximum space factor for trunking is 45% — the total cross-sectional area of the cables, including insulation, should not exceed 45% of the internal area of the trunking. The remaining space allows cables to be installed and removed and helps them dissipate heat.
Why do more bends mean fewer cables in a conduit?
Bends and length increase the friction of pulling cables through, so the IET conduit factor tables give lower factors for longer, bendier runs. A lower conduit factor means the total cable factor you can install is smaller, so fewer cables fit the same conduit.
What do I do if the cables won’t fit?
Either step up to the next containment size, which raises the conduit or trunking factor, or add a draw-in box to break a long or bendy run into shorter sections. Each shorter section is treated as a short run again, which restores a higher usable factor.
