Fiber Optic Cable Bend Radius or Diameter

All fiber optic cables have specifications that must not be exceeded during installation to prevent irreparable damage to the cable. This includes pulling tension, minimum bend radius or diameter and crush loads. Installers must understand these specifications and know how to install cables without damaging them.

Not following bend radius guidelines can lead to cable damage. If the cable is damaged in installation, the manufacturer's warranty is voided. Here is what one manufacturer's warranty says: "This warranty does not apply to normal wear and tear or damage caused by negligence, lack of maintenance, accident, abnormal operation, improper installation or service, unauthorized repair, fire, floods, and acts of God."

Bend Radius or Diameter?
Bending of a fiber optic cable can damage the cable if the curvature of the bend is too small. Damage may not always be obvious, like a kink in the cable, but may include broken fibers, fibers with higher loss due to stress and cable structural damage that may lead to reliability problems.

Note: The common term for the curvature of the cable is "bend radius" but sometimes "bend diameter" may be more useful. For example when a cable is bent around a corner, bend radius may be appropriate, but if the cable is used with pulleys or capstans during pulling, then left stored in loops, the diameter of the pulley, capstan or storage loop may be more descriptive. Thus we will define and use both terms.
radius or diameter
The diameter of a circle is the total width across the center and the radius is the distance from the center to the circumference.

Diameter = 2 X Radius

Typical Specifications

The normal recommendation for fiber optic cable is the minimum bend radius under tension during pulling is 20 times the diameter of the cable (d). When not under tension (after installation), the minimum recommended long term bend radius is 10 times the cable diameter.

Note: Some cables have different specifications, e.g. the specified minimum bend radius is 15 times the cable diameter instead of 20 while being pulled and 10 times under no tension after installation. Some cables are 15 times both under tension and after pulling. Always check with the manufacturer of the cable you are installing to ensure you have the proper spec.

For the purposes of this explanation we will use 20 X while being pulled and 10 X after installation.

Refer to the diagram below: A cable under tension (top) and after installation (bottom).

R = bend radius

D = bend diameter

d = cable diameter.
Fiber Optic Cable Bend Radius

If the bend radius spec is 20 times the cable diameter during installation but we're referring to the diameter of a pulley or a capstan, the diameter of the pulley or capstan should be 40 times the cable diameter, since the diameter of a circle is twice the radius.

Likewise, is the bend radius is 10 times the diameter of the cable  after installation, storage loops should be no smaller in diameter than 20 times the diameter of the cable.

Note: Always check the cable specifications for cables you are installing as some cables such as the high fiber count cables have different bend radius specifications from regular cables!

Bend radius example:

A cable 13mm (0.5") diameter (d) would have a minimum bend radius under tension of 20 X 13mm = 260mm (20 x 0.5" = 10")

If you are pulling this cable over a pulley, that pulley should have a minimum radius of 260mm/10" or a diameter of 520mm/20" - don't get radius and diameter mixed up!

Pulling Cable

bend radius

Example of cable bend radius violations:
(1) the cable is being pulled out of the conduit up out of the manhole at an extreme angle, exceeding the cable bend radius where it exits the conduit.
(2) the pulley on the truck used to turn the cable toward the capstan pulling it is 5-6 times too small for the cable - the red dotted circle shows the proper diameter.
We should also add that having the capstan at a right angle to the cable direction is wrong - the truck and the capstan should be aligned with the cable pull direction to avoid use of the pulley altogether.

pulling cable
When pulling underground cable in conduit, the cable needs protection at the end of the conduit and at the mouth of the manhole or handhole. This can be done with several techniques, e.g. sheaves, quadrants or flexible ducts.

pulling cable

More on pulling cable in conduit underground 

Underground Cable Storage - Manholes and Handholes

After pulling cable, excess cable must be stored, usually in manholes or handholes. Those should be large enough to allow the cable to be stored with loops larger than the recommended bend diameter.

manhole loops
Right and wrong ways to manage manholes or handholes

Aerial Cable Installation

There are also bend radius issues with aerial installation, depending on whether the installation is lashed, ADSS or figure 8 cable and the installation is done with the moving reel or stationery reel method. The size of pulleys, cable in storage loops, points of transition up or down the pole, etc. can involve bends in the cable that require careful monitoring.

Aerial Cable Storage

After cable installation and splicing, aerial cable is typically stored in service loops  lashed to the messenger. The ends of the loops must be kept within specs for bend radius, often using plastic "snowshoes" that are then suspended from the messenger.

aerial closure 
Aerial closure properly attached to messenger and cables

aerial service loops
Two "snowshoes" for service loops, one has not been secured to the messenger and is hanging down, stressing the cable beyond its bend radius limits.

Premises Cabling Installation

Premises cable also has issues with bend radius as cables may be installed below floors, above ceilings and are routed around many obstacles. Many premises installations use small diameter orange duct or cable trays to prevent bending the cable or other damage to the cable after installation. Hardware for mounting storage loops on the wall are also recommended.


Proper premises cabling installation using cable trays and innerduct.

More on Outside Plant Construction and Installation

Table of Contents: The FOA Reference Guide To Fiber Optics


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