Bend Radii  

Conduit Fill 

Many novices to Telecommunications Distribution Design believe that the space factor rules that apply to power cables in conduits and cable containment systems also apply to telecommunications cables. Wrong! 

Most Electrical Engineers make serious and costly mistakes when attempting to size conduits for telecommunication cables.

Space factors for power cable are based on reducing the inductive heating effect that occurs when power cables are bulked closely together. 

Conduit fill factors for telecommunication cables are based on the maximum allowable pull that should be exerted on a telecommunications cable during installation (e.g. a Cat5e four pair UTP cable must not be pulled more than 25Ibs force ).

The BICSI DD102 course covers these complex calculations in detail and explains with design exercises how to apply the three default fill levels for conduits and calculate the de-rating factors on the bends. One can also refer to the BICSI Telecommunications Distribution & Method Manual (TDMM) for a basic guide which is shown in various tables.

Calculations for riser sleeves and slots against floor area served are also shown in tables of the BICSI TDMM.

Minimum Bend Radii

The easy way to remember these are as the following :

  • 1 inch
  • 2 inch
  • x 4
  • x 6
  • x 8
  • x 10
  • x 15
  • x 20

the question I now pose is match the following items to the above ? 

  • Four pair UTP Cat5e cable.
  • Conduits less than 2 inch diameter.
  • Conduits greater than 2 inch diameter.
  • Telecommunications Bonding Backbone (TBB).
  • 2 & 4 fibre distribution cable under load.
  • 2 & 4 fibre distribution cable relaxed.
  • External fibre cable under load.
  • External fibre cable relaxed.
  • Internal fibre cable relaxed.
  • Internal fibre cable relaxed.

Students who have attended the BICSI design courses will never forget the answers !

We provide a Trayfill Autocalculator spreadsheet which should assist in your projects when calculating tray size and weights.


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