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[802.3_4PPOE] about the 12.5 Ohms






In TIA 568-C.2, it is specified that for channels, of categories 3, 5e, 6, and 6A, the DC loop resistance shall not exceed 25 Ohms. 


For horizontal cable, the one way DC resistance shall not exceed 9.38 Ohms per 100 meters, for all categories.


For patch cord cable, it’s 14 Ohms per 100 meters.


For connectors, it’s a maximum of 0.2 Ohms for categories 5e, 6, and 6A, but it’s .3 Ohms for cat 3.  A maximum of 4 connectors is allowed.


If you add up the parts, they add to less than 12.5 Ohms one way, therefore less than 25 Ohms loop.  I haven’t lived long enough to remember how the 25 Ohms was established.  I would guess it was established quickly, and knowingly with some unallocated margin, since it was not felt to be a highly critical parameter at the time. 


I would recommend accounting for only, the maximum DC resistance that can arise from the sum of the parts.  If you want to account for the 25 Ohms, you can, it will just result in some extra robustness in your system, and potentially unnecessary extra cost.