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Is it possible to use a UTP cable for:

  • 12V DC (~30W),
  • 100BASE-TX Ethernet, and
  • A single phone line,

at the same time? 100BASE-TX should give me two spare pairs, can I use one pair for the phone line and one for the power?

My plan is to supply a (rather old) VoIP-capable board with power, internet and a hook it up to a single phone. Can I do all that with a single cable? If yes, how do I do the wiring (i.e. which are the spare pairs in 100BASE-TX Nevermind. Found that on Wikipedia)?

To clarify, I would only run this over a short distance, 1-2m.

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IEEE 802.3af POE has two modes, mode A sends power on the "data pairs" while mode B sends power on the "spare pairs". However that is not very relevant as it doesn't sound like you are planning to use that. It sounds more like you are planning a "Ghetto POE" type setup.

I wouldn't worry too much about mixing POTs and Ethernet on the same cable, especially if the POTs link is only local. I also wouldn't worry about CAT5 vs CAT5E on a link this short.

The bigger problem would be current on the DC power lines. 35W at 12V is nearly 3A. Apparently the safe current limit on the conductors is 350ma so if you used one conductor for power and one for ground you would be running nearly 8 times the acceptable current on each conductor. Even if you used the whole cable for power and ground you would still be running at over double the acceptable current.

  • 802.3bt Type 3 allows 0.6A per pair, Type 4 nearly 1A. Assuming cheapo chinese cables, i wouldn't like to go much over 0.5A per pair. So, a higher voltage is required, like 48V. That'l give 0.625A, which is probably okayish. On the output-side, a buck converter like aliexpress.com/item/x/32646322126.html (with additional heatsink!) could give the req'd 12V/3A. – Themroc Dec 29 '18 at 21:33
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It should be possible (in theory), as long as your PoE is using 802.3af mode A. Mode A uses the data pairs to provide power. If however your PoE is using mode B, then it is sending power over the unused pairs and will not work.

In practical terms, a cable where there are more twists per pair, or even better a separator between the pairs, would be preferred to reduce the chance of cross talk.

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First, Category 5 cable is no longer a recognized cable per the ANSI/TIA/EIA-568B Commercial Building Telecommunications Standard:

Recognized Categories Categories 1, 2, 4, and 5 are not recognized as part of the standard and therefore transmission parameters are not listed. The only recognized categories are 3,5e and 6. Category 6 was ratified mid 2002, and the specifications for it can be found in Addendum 1.

Second, sharing ethernet and PSTN in the same 4-pair cable sheath is non-standard not not allowed. You are allowed to share different, compatible applications in a 25-pair binder group, but there are restrictions that preclude what you propose:

Signals with significantly different power levels should not share the same binder group.

  • Great add. I should have also pointed out it is not standards based, however the OP was asking if it was possible. I would also note that while Cat5 is no longer a recognized cable, that doesn't make it cease to exist in the real world and most organizations will not just rip and replace their cabling because it is no longer recognized. – YLearn Feb 18 '15 at 15:46
  • @YLearn, I understand your point, but, at 1 to 2 meters, I don't see not ripping and replacing to use a recognized cable. The replacement for Category 5 was approved in 1999; in the vernacular of the Millennials: Category 5 is sooo last century. – Ron Maupin Feb 18 '15 at 16:12
  • Exactly. I have some Cat5 lying around and was planning to re-use it for this project. Also, I don't care too much about standard compliance here since this is more of a hobby project. Still, great info (but technically @YLearn's answer answers my question.) – Attila O. Feb 18 '15 at 16:13
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I just wanted to add a quick note to answer one of your last remaining questions:

Standard Fast Ethernet (100BASE-TX) calls to use Pair 2 and 3, which correlate to pins 1&2 and pins 3&6, respectively (the yellow and green wires, + their white stripped counterparts).

Pair 1 and 4 are unused (the blue and brown wires, + their white counter part).

Pair 1 was specifically skipped in the Ethernet world because the RJ11 (standard phone cable) used it for the phone line. More details here.

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