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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Four-Pair Implementations
In the AT standard, Section 33.1.4.1 titled “Type 2 cabling requirement” says:
…Under worst-case conditions, Type 2 operation requires a 10°C reduction in the maximum ambient operating temperature of the cable when all cable pairs are energized at ICable (see Table 33-1), or a 5°C reduction in the maximum ambient operating temperature of the cable when half of the cable pairs are energized at ICable.
So though the AT standard left a “hook” for four-pair operation, that is all it did. Market forces are however already at work and we have UPOE, HDBaseT among others, driving a lot of power into the cables. UPOE for example is basing its abilities on the temperature data collected by ISO/TIA as described in Fig. 13.17 ...

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