Issue link: https://www.e-digitaleditions.com/i/1327492
January/February/March 2021 I 31 TABLE 3: Ampacity table. FIGURE 1: Temperature rise over ambient at 1A/pair for a 91-cable bundle. 0 5 10 15 20 25 Temperature Rise (C) Category 5e (24 AWG) Category 6 (23 AWG) Category 6A (23 AWG) Category 6 (22 AWG) Category 6A (23 AWG) with Discontinuous Shield AWG Number of 4-Pair Cables in a Bundle 1 2-7 8-19 20-37 38-61 62-91 92-192 Temperature Rating Temperature Rating Temperature Rating Temperature Rating Temperature Rating Temperature Rating Temperature Rating 26 24 23 22 NOTE 1: For bundle sizes over 192 cables, or for conductor sizes smaller than 26 AWG, ampacities shall be permitted to be detemined by qualified personnel under engineering supervision. NOTE 2: Where only half of the conductors in each cable are carrying current, the values in the table shall be permitted to be increased by a factor of 1.4. 60°C 1 2 2.5 3 75°C 1 2 2.5 3 90°C 1 2 2.5 3 60°C 1 1 1.2 1.4 75°C 1 1.4 1.5 1.8 90°C 1 1.6 1.7 2.1 60°C 0.7 0.8 0.8 1 75°C 0.8 1 1.1 1.2 90°C 1 1.1 1.2 1.4 60°C 0.5 0.6 0.6 0.7 75°C 0.6 0.7 0.7 0.9 90°C 0.7 0.9 0.9 1.1 60°C 0.4 0.5 0.5 0.6 75°C 0.5 0.6 0.7 0.8 90°C 0.6 0.7 0.8 0.9 60°C 0.4 0.4 0.5 0.6 75°C 0.5 0.5 0.7 0.8 90°C 0.6 0.6 0.8 0.9 60°C NA 0.3 0.4 0.5 75°C NA 0.4 0.5 0.6 90°C NA 0.5 0.6 0.7 Ampacities of Each Conductor in Ampires in 4-Pair Class 2 or Class 3 Based on Copper Conductors at an Ambient Temperature of 30°C [86°F] with All Conductors in All Cables Carrying Current. 60°C [140°F], 75°C [167°F], and 90°C [194°F] Rated Cables As seen in Figure 1, the UL study showed that 22 AWG Cat 6 cable and 23 AWG Cat 6A cable had lower heat rise than other cable options. The results of the study led to revisions of the National Electric Code (NEC) in 2017 that were the first to address PoE heat rise and safety in cabling. The revisions offered two alternatives to address heat rise in high-powered (60W or more) PoE applications. The first option recommended in the revision is the use of an ampacity table, which dictates how much power can be put through the infrastructure. It requires the user to consider factors, such as gauge size, temperature rating, and the number of cables in a bundle; then calculate the amperage and wattage that can be run safely over the cable. For example, Table 3 indicates that 24 AWG cable with a 60°C temperature rating is allowed to run 0.4A per conductor (80W) up to a 91-cable bundle in an enclosed space; by comparison, a cable rated to 90°C can support up to 0.6A at 120W. Many designers and installers using the table may find it too complex and time consuming—requiring, among other things, knowledge of the cable type, how densely the cable is packed, its temperature rating, and its gauge size—to determine what amperage to run over PoE. Additionally, according to the table, applications outside of ampacity table parameters require additional consideration by "qualified personnel or engineering supervision." Temperature Rise Over Ambient at 1A/Pair for a 91 Cable Bundle