Issue link: https://www.e-digitaleditions.com/i/1327492
32 I ICT TODAY FIGURE 2: Cable marked as LP-rated. Temperature (°C (°F)) Maximum horizontal unscreened cable length (m) Maximum horizontal screened cable length (m) Length de-rating (m) (unscreened) Length de-rating (m) (screened) 20 (68) 90.0 90.0 0 0 25(77) 89.0 89.5 1.0 0.5 30 (86) 87.0 88.5 3.0 1.5 35 (95) 85.5 87.7 4.5 2.3 40 (104) 84.0 87.0 6.0 3.0 45 (113) 81.7 86.5 8.3 3.5 50 (122) 79.5 85.5 10.5 4.5 55 (131) 77.2 84.7 12.8 5.3 60 (140) 75.0 83.0 15.0 6.0 NOTE - This table assumes that the channel includes 10 meters of patch and equipment cords at 20°C. The second, much simpler and less time consuming, option recommended in the 2017 NEC revision is to specify an LP-listed cable. This is a cable that has been tested by a third party (usually UL) to determine maximum PoE amperage and wattage per application. According to UL, the LP rating indicates that the cable has been "evaluated to carry the marked current in any reasonable worst-case installation without exceeding the temperature rating of the cables." A cable listed "LP 0.6A," for example, can support up to 120W in any bundle size in any installation environment without exceeding its temperature rating. This bears repeating: There is no restriction on bundle size for LP cable; so, in another example, an LP 0.7 cable rated for 140W can run 140W regardless of the number of cables in that bundle. TABLE 4: TIA guidance on length de-rating for various temperatures. For those designers and installers who may not want to use an LP-rated cable but want to reach the temper- atures specified in the ampacity table, they can refer to the bundling guidelines specified in TIA-TSB-184-A, Guidelines for Supporting Power Delivery Over Balanced Twisted-Pair Cabling. There will be cases where it may not be possible to break a bundle into smaller ones for applications delivering large currents, in which case the designer/installer should seriously consider using the LP-rated cables. It is critically important to consider the impact of heat rise on the electrical performance of a system. At higher temperature levels, attenuation can become a concern. Attenuation (insertion loss) is the primary variable at elevated temperatures; other electrical characteristics are minimally affected if at all. The TIA provides guidance on length de-rating for various temperatures (Table 4). This guidance applies to elevated temperatures over the entire 90-m link. Increased heat due to large bundles is not normally seen on the entire length. Like a plenum or riser rating on a cable, an LP-rated cable must be marked as such (Figure 2). Among other benefits, this enables installers and inspectors on a brownfield installation to determine quickly and easily what cable is installed and wattage can be run.