The rule of thumb
Under tension: radius ≥ 20 × OD · At rest: radius ≥ 10 × OD
When a cable is being pulled, the load presses the fibers against the inside of the bend, so it needs a gentler curve. Once installed and relaxed, it can sit at about half that radius. These multipliers are the Fiber Optic Association's general guidance for when you do not have the manufacturer's numbers.
Worked example
A 144-count loose tube cable with a 0.61 in (15.5 mm) OD. While pulling: 20 × 0.61 = 12.2 in radius, so any sheave, corner or quadrant block must be at least about 24 in across. Once installed: 10 × 0.61 = 6.1 in radius in the vault or the slack loop.
The datasheet wins. Manufacturers often publish their own figures, and they do not always match 20× and 10×. Corning, for example, lists its ALTOS 144-fiber cable with a bend diameter of 474 mm loaded and 316 mm unloaded, which is a radius of about 15× and 10× the OD. Read whether the sheet gives radius or diameter before you use the number.
Bend-insensitive fiber (G.657)
The bend radius of a cable protects the whole cable during pulling. The bend performance of the fiber itself is set by ITU-T G.657, which defines fibers that can be bent much tighter than standard G.652 fiber without much loss. That is what makes tight routing in MDUs and small closures possible.
| Category | Bend radius | Max loss |
|---|---|---|
| G.657.A1 | 10 mm | 0.75 dB |
| G.657.A2 | 7.5 mm | 0.5 dB |
| G.657.B3 | 5 mm | 0.15 dB |
A1 and A2 fibers are fully compatible with standard single-mode fiber and splice to it normally. Bend-insensitive fiber still has a limit, and the cable around it still has its own bend radius.
Signs a bend is costing you
- Loss at 1550 or 1625 nm much higher than at 1310 nm on the same event.
- An OTDR event that appears at a closure, a vault or a corner where there is no splice.
- A visual fault locator glowing through the jacket or buffer at a tight spot.