I would like to run two 100 Mbit Ethernets including a common PoE over a single cable which will be about 3-5m long. Since 100 Mbit needs 4 wires I will need 8 plus some wires for the PoE which is at least 10 or better 12 wires altogether. I wont mind having even more wires (14, 16...).

Do I need a twisted pair for such cable length? Are there any standards for cables (twisted/non-twisted) with more than 8 wires that UTP has? The environment for the setup is a passenger car.


The reason I want to do this is that I am synchronizing 2 devices with Ethernet and am not able to put a switch close to them so I need to run 2 Ethernet lines down to the switch. Also, the 2 devices can't do PoE by themselves - I will inject and split the power from the cable manually by hand (and lead it to DC inputs on those devices for power). The cable I intend will have 2 RJ-45 connectors on each side plus the power lines.

  • \$\begingroup\$ Can you explain why you need two Ethernet lines? If you just need more bandwidth, can you use 1000Mb Ethernet instead? \$\endgroup\$
    – The Photon
    Commented Jul 19, 2015 at 17:37
  • \$\begingroup\$ I believe that PoE is... over ethernet, so you do not need any more wires for that. \$\endgroup\$ Commented Jul 19, 2015 at 17:41
  • \$\begingroup\$ @VladimirCravero there are two different standards of 802.3af. Mode A uses the same wires for power and data, so you could fit two 100Mb PoE links through a single patch cable. Mode B on the other hand uses two pairs for power and two for data. There is an instructable on making an Ethernet Splitter (using two pairs for one port and two for the other). \$\endgroup\$ Commented Jul 19, 2015 at 17:53
  • \$\begingroup\$ @ThePhoton: I am synchronizing two devices over Ethernet and am not able to put a switch close to them, so I need to run two Ethernet lines down to the switch. The devices are 100 Mbit only, gigabit is not a solution for me here. at Vladimir Cravero: The end devices cant do PoE, I will be injecting and splitting the power manually by hand, but I only want to run the power in the same cable. \$\endgroup\$
    – Kozuch
    Commented Jul 19, 2015 at 17:57
  • \$\begingroup\$ Some versions of 100Mb ethernet are touchy about sharing the other unused pairs with anything else. I once had endless trouble when I used a spare pair for a digital phone extension, but couldn't say if another 100Mb ethernet would cause trouble. Also - this was about 15 years ago ... \$\endgroup\$
    – brhans
    Commented Jul 20, 2015 at 12:43

1 Answer 1


Firstly you will need to check your end devices are 802.3af mode b compliant, otherwise you can't inject the power onto a discrete pair, but instead if they are mode a, you need to put the power onto the data pair which is usually a function of the switch. With regards to UTP. It is possible you might be able to use non twisted cables if the distance is short, your devices will put up with a lot of dropped packets, sub 10Mbit speeds are ok and you don't have too much issue with external EMF sources. But I would be much more inclined to simply run two shielded UTP cables.

  • \$\begingroup\$ My devices can not do PoE at all (no 802.3af mode A or B) - I will just transfer the power and take care of it myself. \$\endgroup\$
    – Kozuch
    Commented Jul 19, 2015 at 21:02
  • \$\begingroup\$ Ok. in that case be careful of the cable resistance too. I assume your run will be short but some cheap cables are made with inferior cores. So you could drop a few volts over the cable. \$\endgroup\$
    – BenG
    Commented Jul 20, 2015 at 0:45

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