I am going to use this passive PoE injector cable set to power a Raspberry Pi model B.
The Raspberry Pi model B consumes between 700mA-1000mA (mine will also be powering the RPi Camera Module). Using this cable set, I plan to power the Raspberry Pi and provide Ethernet data at the same time. My questions are:
For a 60 meter CAT5 cable, how much DC voltage should I supply to account for voltage drop and still meet the needs of the Raspberry Pi?
For a 60 meter CAT5 cable, and let's say a 6V DC power supply - the above passive injector cable will split the current between pins 7 and 8 (meaning each of the two 24 AWG wires will carry between 350mA-500mA). Does this breach the zone of how much current the CAT5 cable can safely carry? I don't want to start an electrical fire.
Answer
According to a reference from the Handbook of Electronic Tables and Formulas for American Wire Gauge a conservative estimate for 24AWG power transmission is 0.577 amps. Current-wise, I'd expect you to be fine. I'd still recommend testing it and monitoring the temperature for a while to make sure. If you've got that cable in a tight bundle of some kind, it will get warmer than if it was in free space. On the bright side, wire that small won't take long to reach its ultimate temperature!
As for voltage drop, according to this AWG table, 60 meters of 24AWG has a resistance of about 5 ohms. 500 mA will drop 2.5V over that distance. You also have to consider that the current has a return path, so that's 2.5V drop in each of the positive and negative legs, 5V drop total.
If, on the other hand, the draw is on the low end of your spec, you'll only get 3.5V drop total. That means that the voltage on your load (the Raspberry Pi) will vary by 1.5V depending on how much current it's drawing. That's quite some variation, and you'll need to make sure it can handle that.
If it was me, I'd come up with a different plan. You're running really close to the edge of what's reasonable.
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