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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.

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3 Answers 3

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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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I can't comment on your answer but have to give you an advice, hope somebody turns it into a comment. When powering things over long lines, it's best to put more volts into it and put a DC-DC at the end where the device is, setting it to 5 volts (or having a fixed-voltage DC-DC). LM2976 or XL6009 converters are the best bet, since they're switching-mode and thus much more efficient. In the end, you have less current running through your wires and constant voltage powering the device, with no need to worry about all the things that could happen be the current high or voltage unstable. I have a couple of routers powered like that and currently am in the process of making a big system with 30 devices powered like this =) I'm sure that's your best bet in this situation.

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  • \$\begingroup\$ What is a LM2976? Doesn't this do the trick What would be wrong with that? ebay.com/itm/like/… "Micro USB Active POE Splitter Power 48V to 5V 2.4A for Raspberry pi Board" \$\endgroup\$
    – Ted Taylor of Life
    Commented Aug 29, 2016 at 17:19
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    \$\begingroup\$ @Ted Hi! Sorry, that meant to be a LM2576/LM2596 - modules like this one. Yours will also work, since it has a DC-DC inside, which I can understand from the description. It claims it uses "Active POE", so you might need a special POE injector compatible with Active POE. \$\endgroup\$
    – Arya Voronova
    Commented Aug 30, 2016 at 15:21
  • \$\begingroup\$ @ Арсений Пичугин hello, that makes more sense. That is what I got when I looked for it. Lol I will test the claims and see if it lives up to the hype. Thanks again! \$\endgroup\$
    – Ted Taylor of Life
    Commented Aug 30, 2016 at 15:28
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    \$\begingroup\$ @Ted XL6009 modules are much better, that I can tell you - those cheap LM2956 converters often have underrated components and might have a fake LM2956 =( XL6009 modules can give much more current and are more efficient. \$\endgroup\$
    – Arya Voronova
    Commented Aug 30, 2016 at 15:44
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    \$\begingroup\$ @Ted AFAIK if it claims it uses XL6009 it should be a legit IC - XL6009 is an IC from a Chinese manufacturer, so it's more safe from counterfeits IMO =) The modules look essentially the same as the one I linked. \$\endgroup\$
    – Arya Voronova
    Commented Aug 31, 2016 at 0:01
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I powered a raspberry pi with an old printer power adapter I believe it was around 1.7amp approx 28 volts. Then I used a LM2956 module on the other side to reduce the voltage to 5v. You probably will want to use both blue and brown so you have 4 wires transmitting power. It worked fine providing power and internet. Very reliable it ran for 4 years outdoors in Alaska 365 days a year. The Cat-5 was at least 50 meters or more.

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    \$\begingroup\$ Your answer just states what you did and asserts it works. Fine. But you could make it much better if you explained why you made these choices and how safe it is (elaborate around current in the wires, AWG, voltage drop, efficiency, and stuff like this). \$\endgroup\$
    – dim
    Commented Mar 18, 2019 at 9:38

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