Q: When you are calculating the fill ratio of a conduit, how do innerducts fit into the calculation? My colleagues and I are working on a design which specifies that multiple innerducts and a fiber-optic cable be installed in a single conduit. The fiber-optic cable will not be installed within an innerduct.
Does this violate the National Electrical Code`s (nec`s) fill ratio? We know how to do the math, so whether it works mathematically is not the issue. But should we incorporate the innerduct into the formula, and, if so, how?
Should the total conduit size be reduced by the total innerduct calculation and the fill ratio be calculated based only on the remaining number? Or should that number be subdivided into at least three to four subsections, since the available space will not be in a single common area?
We cannot find a discussion of the role of innerduct in these calculations either in the nec or bicsi`s Cabling Installation Manual. What do you think?
LaDonna Hagler
Southwestern Bell
Dallas, TX
A: According to nec Section 770-5 (FPN), you are not installing innerduct. You are installing listed optical-fiber raceway, as defined in nec Section 770-5 Exception. While the difference may seem to be semantics, a listed optical-fiber raceway is not required to be installed in accordance with nec Chapter 3. "Installed in accordance with Chapter 3" would require that the number of conductors in a single raceway not exceed that permitted by the percentage fill in Table 1, Chapter 9.
According to nec Section 300-1 (a) Exception No. 4: "Only those sections referenced in Article 770 shall apply to optical fiber cables." According to nec Section 770-52, installations in raceway shall comply with Section 300-17, which states: "The number and size of conductors in any raceway shall not be more than will permit dissipation of the heat and ready installation or withdrawal of the conductors without damage to the conductors or to their insulation."
So, I conclude that you can install a listed optical-fiber cable in the conduit, along with a number of listed optical-fiber raceways as long as the cable can be removed without damage to the cable.
Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: [email protected].