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?
Electronic – Why does PoE use duplicate wires for power
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Related Solutions
Very likely, the PoE injector has a current limit circuit to deal with a cable or device fault. For example, this Wikipedia article claims 350mA / 600mA limits for 802.3af and 802.3at Moreover, it seems that the limits are negotiated, so you could be set to an even lower limit.
The limiter would be there to protect the power supply and to ensure the continued operation of other devices that are powered by the same injector. And, more importantly, to protect against unsafe short circuit currents in the cabling which could, in the extreme, lead to a fire.
I think this document helps Introduction Basic principles of PoE Power over Ethernet (Page 4)
Spare-pair power feed
Spare pair power feeds utilize wire pairs that are not otherwise used, the voltage being placed directly on the spare pairs 4/5 (+) and 7/8 (-). This method can be used only on networks with eight-wire cabling. This does not apply to Gigabit Ethernet, because here all eight wires are used for signal transmission and there are therefore no spares.
Phantom power feed
A phantom power feed has the voltage coupled to wire pairs 1/2 ( ) and 3/6 (+). This method can be used for networks with four-wire or eight-wire cabling.
So you can check with a voltmeter, if there is a voltage in pins 4/5 - 7/8 then the method used is Spare-pair power feed.
Best Answer
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.