How to size cable trunking and keep the fill ratio right
A wiring duct must leave room for conductors, future additions and heat dissipation. This guide shows how to calculate the cross-section, why a fill ratio of 60 to 70 % is sensible and how spare capacity protects your installation.
View wiring trunkingWhy does the fill ratio of a cable duct matter?
The fill ratio describes how much of the internal duct cross-section is taken up by conductors and cables. A proven rule of thumb is a fill ratio of 60 to 70 %, so air can circulate between the conductors and later additions stay possible.
An overfilled duct traps heat, makes assembly harder and can prevent the lid from closing. When a duct is packed to the edge, the conductors press against the comb fingers and the insulation may suffer over time.
How do I calculate the required cross-section?
Add up the cross-sectional areas of all conductors including insulation and divide the sum by the desired fill ratio. At 70 % fill the rule is: required duct cross-section = conductor sum divided by 0.7.
- Use the conductor area including insulation, not just the copper cross-section.
- Round cables need more room than the pure area value because gaps remain between them.
- Calculate generously for bundles and thick control cables.
- Always round up to the next standard size.
How does the duct ensure heat dissipation?
Tightly bundled conductors release their power losses less easily. Under the installation methods of DIN VDE 0298‑4, the current-carrying capacity must be derated when conductors are grouped, because they heat each other up.
A moderate fill ratio keeps air gaps between the conductors open and so limits the temperature rise. In densely packed bundles the core temperature can climb by several degrees, which noticeably lowers the permissible continuous current rating.
Frequently asked questions
How full can a cable trunk be?
A fill ratio of 60 to 70 % of the inner cross-section is the guideline. That leaves room for spare capacity, lets the lid close cleanly and allows heat to escape.
Why should I plan spare capacity?
Installations are nearly always extended in service. With 20 to 30 % spare you can pull extra conductors without replacing the whole duct.
Does heat in the duct really matter?
Yes. Densely packed conductors heat each other up, and DIN VDE 0298‑4 requires derating the current capacity for grouped cables. A moderate fill keeps the temperature lower.
How do I calculate the duct size?
Add up the cross-sections of all conductors including insulation and divide the sum by 0.7. Then round up to the next standard size.
Looking for the right wiring duct?
We supply wiring and cable ducts in all common sizes including lids and accessories - for clean wiring with enough spare capacity.
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Advice on fill ratio, spare capacity and cross-section.
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Enough air gaps for reliable heat dissipation.