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When a PoE cable is used to transmit power, it uses two wires for DC+ and two wires for DC-. In contrast, a simple light bulb powered by a battery circuit uses just one DC+ wire and one DC- wire. Why two wires for each polarity in PoE? Is it because the gauge of the wires are very thin and may not reliably transfer power over larger distances?

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This is because the wires are used for power AND for data at the same time. The power is carried common mode in each pair, one pair for the supply and one for the return, while the data is carried differentially. There is also the added benefit of doubling the current carrying capacity and halving the resistive loss. Using a relatively high voltage also helps send the power a long distance.

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    \$\begingroup\$ Also necessary to prevent damage. Each twisted pair of the ethernet, goes into a transformer, which would be a short circuit to DC if power was applied across one of the twisted pairs. As they are transformer isolated, you can safely apply DC between pairs. \$\endgroup\$ – Henry Crun Jul 20 '18 at 0:54
  • \$\begingroup\$ The transformer is required per the Ethernet standard to isolate the ends of the link, PoE simply uses the center taps to inject the power on to the pairs in common mode. \$\endgroup\$ – alex.forencich Jul 20 '18 at 0:58
  • \$\begingroup\$ Indeed. My point is that if you put DC on the pair, and then plugged in an (existing) ethernet device, it would be short circuited by the transformer. They had to do it this way - DC across one/some of the pairs was never an option they could adopt. \$\endgroup\$ – Henry Crun Jul 20 '18 at 1:04
  • \$\begingroup\$ @alex.forencich - I don't think the reason has to do with it sharing the wires for power and data. You are talking about Mode A PoE there. What about Mode B PoE then? It uses dedicated wires for power (4,5+;7,8-) and dedicated wires for data. Why two lines for DC+ in this case? The latter part of your answer may explain that. \$\endgroup\$ – inquisitive Jul 20 '18 at 19:12
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    \$\begingroup\$ Gigabit uses all four pairs, so that means sharing power and data. Even if only one end is gigabit and no data is sent down the wires, the receiver will still be connected so it still has to be electrically compatible. \$\endgroup\$ – alex.forencich Jul 21 '18 at 1:57

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