How to size a wiring duct by fill ratio and heat
A wiring duct is not packed to the brim - it is sized by a permissible fill ratio and the heat it must dissipate. This guide shows how to work out cross section, a 60 to 70 percent fill ratio and spare capacity for later additions.
View wiring ductsWhat does permissible fill ratio mean?
The fill ratio describes how much of the free duct cross section is taken up by cables. In practice a wiring duct is filled to only 60 to 70 percent, so cables can be laid in, the cover closes securely and heat can escape.
An overfilled duct pushes the cover open, pinches conductors and traps loss heat. The remaining 30 to 40 percent is not waste - it is working room for installation and reserve for future additions.
- Use the cable outer diameter, not the copper cross section, for the calculation.
- Round bundles never pack to 100 percent - air gaps always remain.
- Plan at least 30 percent spare capacity for extensions.
- The cover must close without force and without conductors bulging out.
Why is heat dissipation decisive?
Every current-carrying conductor generates loss heat. When many cables lie tightly bundled in the duct they can no longer cool one another - the current rating drops and must be reduced by a grouping factor.
Under IEC 60364‑5‑52 and DIN VDE 0298‑4, bundled cables call for derating factors: from around nine grouped conductors the factor is about 0.7, meaning the permissible current rating falls to roughly 70 percent. A packed duct can raise conductor temperature by 10 to 20 kelvin.
How to choose cross section and duct type
Wiring ducts come in stepped widths and heights, typically from 25 x 25 mm to 100 x 100 mm. To select one, add the outer cross sections of all cables and divide by a target fill ratio of about 0.65.
- Route power and control cables in separate ducts to reduce interference coupling.
- Choose a duct height so conductors do not protrude past the fingers.
- With heavy loading go wider and shallower rather than narrow and tall - better cooling.
- Use covers, dividers and branches from the same duct system.
- Select halogen-free, flame-retardant PVC or PC/ABS to meet the fire requirement.
Frequently asked questions
How full can a wiring duct be?
In practice 60 to 70 percent of the clear cross section. Beyond that the cover no longer closes cleanly, heat dissipation suffers and there is no reserve left for extensions.
Why must I derate current for grouping?
Tightly bundled conductors can no longer cool one another. IEC 60364‑5‑52 and DIN VDE 0298‑4 therefore apply a derating factor that cuts the permissible load to 70 percent or less for many cables.
Slotted or solid-wall duct?
Slotted ducts improve convection and heat dissipation and make it easier to break out conductors. Solid-wall ducts look tidier and are fine at low occupancy.
How much spare capacity should I plan?
At least 30 percent free cross section, so later extensions are possible without swapping the duct and adding new conductors is not a struggle.
Looking for the right wiring duct?
We supply slotted and solid-wall ducts in all common grid sizes including covers and accessories - halogen-free and flame-retardant.
All grid sizes
From 25 x 25 to 100 x 100 mm in stock.
Good heat dissipation
Slotted ducts for loaded enclosures.
Flame-retardant
Halogen-free materials for fire safety.
Expert advice
We help with cross section and fill ratio.