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Minimum fiber bend radius is generally 10 x outer diameter of cable, unless specified differently by the manufacturer.

For a tight-buffered indoor fiber breakout cable with 8 or 12 fiber strands, does that actually apply to the outer diameter of the entire cable (which could be like 12mm or more)? Being a tight-buffered cable makes it more or less the same mechanics as in a single strand, and should place the same strain on the individual fiber, thus requiring larger minimum bend radius.

I have never seen a fiber manufacturer list a minimum bend radius specifically for the combined cable specifically.

Sorry if this seems obvious or dumb, and never thought about it before but actually I am not clear on what stable data to adopt.

What is the accurate data here?

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Apply the minimum bend radius to the outer sheath. A sheath containing 12 fibers would have a minimum bend radius of 10 times the outer sheath diameter. It is the same idea as a copper cable (4 times the diameter of the outer sheath). You cannot measure the minimum bend radius of a bundle from the individual components in the bundle.

Once the fibers are split out form the bundle sheath, then the minimum bend radius of each fiber is 10 times the outer diameter of the fiber sheath of the individual fibers.

Always err on the side of caution because any bending causes micro cracks that degrade the fiber.


Edit to clear confusion:

The measurement of the cable diameter is taken from the cable outer sheath, and that is what you multiply by the minimum bend radius value for the cable to get the minimum bend radius for the cable. The minimum bend radius of the cable is the inside radius of the cable measured from the inside the bend of the outer sheath of the cable. The outer sheath of the cable is both outside and inside the bend, but it is the outer sheath inside the bend that counts for the bend radius, not the outer sheath on the outside of the bend.

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The minimum bend radius always applies to the inner radius of a cable, in reference to 10x the diameter of the outer sheath. It's about the inner strands, the outer ones being bent around a larger radius.

(Strands are usually twisted into a helix, making the effective bend radius of single strands larger than the inner radius, but all that depends on the construction of a cable and is documented in its datasheet - to quote Ron M.: always err on the side of caution.)

Note that the minimum bend radius refers to unloaded handling or deployment. When a cable is pulled with considerable force, up to and approaching the maximum tensile strength, the minimum bend radius is much larger, often 15-20 times the diameter.

See Wikipedia: Bend radius for details.

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  • Now I have one answer saying it is the inner strands, the other says it is the outer diameter of the entire cable ... what is the correct one?
    – nepdev
    Mar 10, 2022 at 15:15
  • I've only ever seen references to the inside radius - I have just added a reference to the answer.
    – Zac67
    Mar 10, 2022 at 15:50
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    @nepdev, you are confusing two different things. The inside bend radius is based on the outer diameter of the cable sheath. You measure the outer sheath diameter (~.25" for copper cable) and multiply that by the minimum bend radius (4 for copper or 10 for fiber, absent any different information from the cable manufacturer), and that will be the minimum inside bend radius of the cable. For example, copper cable would need to be able to wrap around a 2" diameter (1" radius) circle at the default minimum for a .25" copper cable. That means the inside radius of the cable. bend.
    – Ron Maupin
    Mar 11, 2022 at 2:22

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