Q: eia/tia-569, Table 4.4-1, "Conduit Sizing," indicates cable fill, but nowhere in the standard is there an indication of the basis of the table for design. The Telecommunications Distribution Methods Manual of bicsi (Tampa, FL), in Issue 7, Chapter 4, page 41, repeats this table and provides notes that indicate the presumption of a design basis. Where does the 15% reduction per bend discussed here originate? It is not from nfpa-70 or eia/tia-569. I find the calculation in Note 2 suspect. It comes up with a similar answer to that found by using nfpa 70-1996, Chapter 9, Table 4, in the section for "Electrical Metallic Tubing," under the column headed "2 Wires, 31% Sq. In."
What do you think? Does the conduit-fill limit specified in the National Electrical Code (nec) apply to cabling referenced in Chapter 8, even though Sections 90-3 and 300-1 (a), Exception 5, appear to indicate otherwise? And if so, why is bicsi using the 40% conduit-fill limit?
Tim Quiring
Indiana University
Bloomington, IN
A: The basis for the conduit-fill limit is that it worked for power cable, so it will work for telecommunications cable as well. I suspect that the 15% reduction per bend originated when a group of building industry consultants got together in a Lexington, KY, tobacco barn in the 1970s, but I will have to confirm this with a few of those who may have been there. The "tobacco-barn science" involved a 100-foot length of conduit with a 90o bend at each end. The test was conducted by pulling the cable until it broke and recording the pulling tension--data from which many of today`s conduit-fill and pulling-tension guidelines were later derived.
The answer to your question about cabling in Chapter 8 is: "No." Chapter 8 covers communications systems and is independent of the other chapters in the nec, except where they`re specifically referenced. So why is bicsi us-ing the 40% figure? See my comment on the basis for the conduit-fill limit, above.