Even if the attic space is not used for air return, you may still need plenum-rated cables. When it gets hot in the summer, all the cables laid in the attic have to work at elevated temperatures. Unfortunately, the old category 3 PVC- insulated cables suffer from excessive signal attenuation at elevated temperatures. Do not use category 3 PVC cable at temperatures greater than 40 °C or 104 °F, a temperature easily attained in an enclosed attic. Instead, specify a less temperature-dependent cable, such as a FEP, PTFE, or PFA plenum-rated cable, or any category 5e or better cable.
With category 5e or better cables, the performance doesn't degrade as severely as with category 3 PVC, but the cable attenuation must still be de-rated to account for the increased resistance of the copper conductors at elevated temperature. See further information in Section 8.6, "Category-3 UTP at Elevated Temperature."
POINT TO REMEMBER
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
Time-Domain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. Two-Port Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )