Fiber Optic Conduit Pull Boxes

By Fiber Instrument Sales, Inc. on December 18, 2013

Pull boxes serve several functions for above and below ground fiber optic installations. As the name suggests, fiber optic conduit pull boxes are used primarily to facilitate cable pulling through conduits and raceways. Pull boxes, installed at strategic points in the cable run, significantly reduce stress on cable as it is pulled through walls, partitions, tunnels and so forth.

Telecommunication Pullboxes Standard
This article addresses pull boxes in terms of their primary use, which is to facilitate cable installation. However, it should be noted that pull boxes are also used to gain access to cable for splicing and testing, to store slack cable, and to house in line equipment. Pull boxes are available in many sizes, depending on their intended use and the space available for mounting the boxes.

Once installed, pull boxes continue to be of benefit as cables are added to meet future needs. In addition to pull boxes, when planning your installation be sure to choose conduit that is large enough to allow for the correct Conduit Fill Ratio. The Conduit Fill Ratio represents the volume of installed cable vs. empty space within a conduit.

Adhering to this standard helps minimize the pulling force required to run cable, allows sufficient room for expansion and contraction of the cable, and makes allowance for future cable upgrades. The proper Conduit Fill Ratio means that no more than 40% of the space within a conduit is filled with cabling. Furniture paths can be filled only up to 33 percent. These ratios apply to both fiber optic and copper cables.

Locating Pull Boxes

To minimize cable tension, pull boxes should be located so that cable is not pulled as a continuous run through more than two 90° bends or their equivalent, e.g. four 45° bends, or one 90°  bend and two 45° bends, and so forth. If your bends total more than 180 degrees, install additional pull boxes as required.

If three or more 90° bends are unavoidable and additional pull boxes cannot be installed (as may be the case in an existing installation), install the cable from the box closest to the center of the run. Unspool the cable and lay it on the ground near the box in a figure-eight pattern. This will make it easier to pull the cable, since the figure-eight pattern neutralizes the curve in the cable that was created by the cable spool.

In a straight horizontal or vertical run, cable will penetrate the pull box from two opposite sides of the box. If a pull box is to facilitate a change of direction in a cable run, the cable will penetrate the pull box on two adjacent sides.

Installing Fiber Optic Cable

Typically, cables are pulled through conduits and raceways using cable lubricant and a pull line. Choose a lubricant that is safe for the type of cable you are pulling.

Use a swivel eye to connect the pull line to the cable's strength member. Do not connect the pull line to the cable jacket. As a rule of thumb, do not exceed a pulling force of 300 pounds (indoor cables) or 600 pounds (outdoor cables). However, it is best to check cable specifications. A tension meter can be used to ensure that you do not exceed the specified pulling force. This meter is vital if you are using power pullers.

Underground Fiber Optic Installation

Smaller "handhole" pull boxes are often used for underground cable. Handholes are usually protected by a removable or hinged metal cover.