Why conduit and trunking size matters
Conduit is more than a route for cables. Its internal area decides how many cables fit, how much heat can escape and how easily a cable can be pulled or replaced later. Choose it too small and you will struggle to install the cables, overheat them, or breach the fill limits in your wiring regulations.
A well-sized conduit also supports the cable. The right diameter keeps the cable within its permitted bend radius and leaves enough room for a draw wire, so the installation is quicker and the cable is far less likely to be damaged.
Good conduit design is therefore a balance: large enough for the cables and the heat they produce, small enough to remain practical and economical to install.
This guide covers fill factor, derating and a practical method for choosing conduit and trunking size for a given set of cables.
Fill factor: the rule that drives the choice
Fill factor is the ratio of the total cross-sectional area of the cables to the internal cross-sectional area of the conduit or trunking. It is expressed as a percentage and kept below a limit so that cables can be pulled without damage and heat can dissipate.
A common design rule is to keep the fill of a conduit below roughly 40% when three or more cables share it, with lower limits for two cables and a higher limit only for a single cable. Always confirm the exact figure against your standard, because IEC 60364-5-52 and national regulations differ in detail.
How fill factor is calculated
Take the outer diameter of each cable, work out its area, add the areas together, then divide by the internal area of the conduit. The result is the fill percentage.
What matters is the internal area, not the nominal trade size. Two conduits of the same nominal size from different ranges can have slightly different internal diameters, so check the manufacturer's data before you commit to a size.
The 40% figure is not arbitrary. It leaves roughly half the cross-section free for air and for the friction of pulling, which keeps both heating and installation under control.
Derating when cables share a conduit
Bundling cables together traps heat and lowers the current each cable can carry. Standards handle this with derating, or grouping, factors. A cable rated at 27 A on its own may only be good for around 20 A when it runs alongside several others in the same conduit.
The more cables and the tighter the bundle, the stronger the derating. Enclosed conduit derates more than an open cable tray. If the conduit passes through a hot roof space or a thermally insulated wall, apply the relevant ambient-temperature and thermal-insulation factors as well.
Grouping factors are published as tables indexed by how the cables are arranged — bunched in a conduit, in a single layer on a tray, or touching in free air. Use the row that matches your reference method, not the nearest convenient number.
Voltage drop is affected too. A smaller derated cable has more resistance, so a long run that passed the heating check may still fail on voltage drop. Test both before you settle on a cable size, then size the conduit.
Step by step: sizing a conduit
The sequence below keeps the cable decision ahead of the conduit decision, which is the order that avoids rework.
- Note the number, type and overall diameter of the cables that will run in the conduit.
- Apply the derating factors first and re-check the cable current rating; the cable itself may need to grow.
- Calculate the total cable area using π × (d ÷ 2)² for each overall diameter d.
- Divide the total cable area by the maximum fill factor for your cable count to get the minimum internal area.
- Convert that area back to a diameter and round up to the next standard conduit size.
- Add spare capacity where future circuits are likely, then check bend radii and pull points.
Worked example: three circuits in one conduit
Suppose three cables with overall diameters of 12 mm, 10 mm and 8 mm share a conduit at a 40% fill limit.
Areas: 113 mm² + 79 mm² + 50 mm² = 242 mm². The required internal area is 242 ÷ 0.40 = 605 mm². The matching internal diameter is √(4 × 605 ÷ π) ≈ 28 mm, so choose the next standard size, normally 32 mm conduit.
Notice how quickly the answer moves. Adding a fourth cable of similar size would push the requirement beyond 32 mm, which is why it pays to count the cables carefully at the start.
The table below gives a feel for how much usable area common conduit sizes provide at a 40% fill.
| Conduit trade size | Typical internal diameter | Usable area at 40% fill |
|---|---|---|
| 20 mm | ≈ 17 mm | ≈ 91 mm² |
| 25 mm | ≈ 21 mm | ≈ 139 mm² |
| 32 mm | ≈ 27 mm | ≈ 229 mm² |
| 40 mm | ≈ 35 mm | ≈ 385 mm² |
Use these as a quick guide only. For a real design, take the internal diameter from the manufacturer's data for the exact product.
Trunking and cable tray: when to switch
Trunking is the better choice when the cable count is high or likely to grow, because you can add circuits without re-pulling the entire run. Cable tray suits longer, more accessible routes where heat can escape more freely.
For enclosed trunking, use a fill limit around 45% or lower and keep a spare compartment for future use. For tray, follow the standard's grouping factors rather than a simple area ratio.
Cost matters here. A larger conduit or run of trunking is cheap compared with the labour of re-pulling a cable, so where growth is plausible, leave the extra room.
Conduit sizing checklist
- Cable types, diameters and insulation confirmed.
- Derating factors applied before the conduit is sized.
- Fill factor within the limit for the cable count.
- Conduit diameter rounded up to a standard size.
- Bend radius and number of bends within the pull limits.
- Spare capacity allowed for future circuits.
- Ambient temperature and thermal insulation considered.
- Fixing and support spacing within the manufacturer's data.
Conduit, cables and the drawing
Conduit size depends on decisions made elsewhere: the cable list, the circuit layout and the derating from shared routes. When those change, the conduit has to change with them.
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FAQ
What fill factor should I use for conduit?
Common practice keeps conduit fill below about 40% for three or more cables, with lower limits for two. Check your national wiring regulation for the exact figure.
Why do I apply derating before sizing the conduit?
Grouping lowers the current a cable can carry, so it may need a larger cross-section. That changes the cable diameter and therefore the conduit size.
When should I use trunking instead of conduit?
Trunking suits high or growing cable counts because circuits can be added without re-pulling the run. Conduit is usually better for short, fixed routes.
Does cable tray use the same fill rule as conduit?
Not exactly. Tray is open, so heat escapes more freely. Follow the standard's grouping factors rather than a simple area-ratio rule.