I know this is not the right place to post and I truly apologize. If someone can send me in the right direction I would really appreciate it or maybe help me. I want to run POE over a distance of between 1 kilometer to 12 kilometer (I know this is very long). Would POE work for me or am I wasting my time researching this. What I want to do is have a single power and communication point to connect Arduino's every 100 meter on a single cable (CAT5 or similar) and communicate over I2C to a single master Arduino unit. I am stuck at this. Thanks again for hearing me out, pointing me in the right direction or answering my question for me. And sorry for my horrible english :-)
there is a dedicated Arduino forum, http://arduino.stackexchange.com/ which you may prefer.
Arduino uno draws around 45 mA. Cat5e is 0.188ohm/meter, so the cable has a resistance of 376 ohm (one kilometer power, one kilometer ground). Hm, this is not that impossible after all :) you lose like 17V on 1 km of cable.
However if you have an arduino at every 100 meter, that means 120 Arduino, a solid 5.4A current draw. Maximum allowed current per conductor is 0.577A on a cat5 cable, so this won't work on 5V.
POE can make this a little easier, as it runs at up to 48V. By increasing the voltage, it is possible to use thinner wires, since less current at higher voltage carries the same power. PoE can wire 60W of power over 100 meters (with serious losses due to the thin copper wires in the Cat5e cable). So I would say, although the idea is insane (no offense :)) but actually it is possible to make it working.
However you can't do I2C on such a long cable. You have to go for at least RS485, and use it at a very low speed, say, 300 BPS if you want to have 12km.
Is there a chance to learn the problem you're trying to solve? There could have been much better solutions than 12km of wire. That is a serious cost in copper, not to mention the cost of a good PoE adapters. For a 12km long wire, you have to have very serious overvoltage protection, too. Probably a radio link and a battery-operated device would be a better choice, no?