6

On larger projects we tend to buy CAT6 in drums of 1000ft, these are then run to the locations as required and RJ-45s placed on the ends by the electrician, this all works fine.

Were now starting to use devices that have POE (power over Ethernet), and I was wondering: for POE to work do the RJ-45s have to be wired in a particular / different way to how they normally would?

  • What cable are you planning to use? CAT 5e or CAT 6? – Prasanna Apr 12 '15 at 19:48
  • Its usually Cat6, but from time to time Cat5e – sam Apr 12 '15 at 19:49
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    CAT 6 uses 23AWG size wires - hence can carry voltages to long distances than others - please perform voltage drop calculations before you implement your design – Prasanna Apr 12 '15 at 19:51
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    I strongly recommend that you stick to a specific standard of wiring - be it power wiring or not. – Prasanna Apr 12 '15 at 20:05
11

From experience (and I mean I have spent weeks figuring out why some PoE cables don't work and others do) I can say this: Ensure that your use RJ45 type B wiring and not type A for PoE. In some cases I have not been able to get type A to work. The power is transmitted, the powered device comes on, but there's no communication with the device, not even on layer2 (powered device MAC address is not visible on the switch).

So, use type B.

On a short cable (a few meters) it makes no difference, but on 70m+ it does. Using CAT5/5e or CAT6/6a cable, both cause problems on type A but not type B.

Type A and B (for CAT5 and 6)

Although this is not strictly a wiring issue, to further explain the problems that can be experienced with CAT6 cable over long length (50m+), eejim writes at https://community.ubnt.com/t5/UniFi-Wireless/Unexpected-PoE-Cable-length-limitations-with-UniFi-Switch-16/m-p/1918103/highlight/true#M226319:

"CAT6/6A cables are being made with 26 gauge cable.

The biggest problem with 10Gb ethernet is crosstalk between the wires/pairs in the cable. There's just one way to lessen this - move the wires physically farther apart. One way the cable manufacturers have found to accomplish this while not making the cable too large - there is actually a spec for the maximum diameter of ethernet cable - is to make the copper wire itself smaller, so the insulation is thicker, so there's more space between conductors. This actually works (and they love it because there's less copper which saves them money, but they can charge more for the cable because, well, it's spec'd higher, right?) , but it causes the problem you are seeing - the smaller conductor has higher resistance and therefore more voltage drop than the standard CAT5e 24 gauge wire. So with longer CAT6 cables the PoE limitation needs to be accounted for. This is why I never use Cat6/6A cables except for very specific things, and never ever for PoE.

Change out your cable to CAT5e and the problems will go away. And at gigabit speeds it will work just fine up to 100 meters, even at 24V PoE."

Having taken note of eejim's comments, I have checked with suppliers and there is a better quality CAT6/CAT6A available that uses 23 gauge wire, which will not suffer from these issues when used for POE purposes.

5

As others have commented/answered, the "wire layout" (i.e. the color code) doesn't matter, the cable type (5/5e/6) determines signal properties (i.e. how far the signal can travel and at what "speed", etc.) so as long as the cable is POE capable (any 5+ cable, or CAT3 if less power is needed) and the switch/device is POE compliant (802.3af-at), the "color code" of the wire is more for any one coming up behind you to know your scheme (i.e. if it's wired all wonky, you might not be able to quickly tell if one cable is a pass-through or crossover ... not fun when 1 cable can stretch 200+ feet and there are 200 cables to dig through)

FYI: The "standards" that most use are the TIA/EIA 568 A/B termination standards.

4

Based on the standards diagram posted in another answer and sourced from Wikipedia, the POE issue with crossover cables comes if the device using POE is using the Mode A standard - which puts biased power over the data pins. If the data pins reverse then the DC voltage gets to the device in reverse polarity, and will not work.

If the wiring is standard straight-through cable, then there should be no issue; and also if the device uses the Mode B standard then the power is sent over other conductors which do not cross over, even if inside a crossover cable.

3

Of course, the wiring scheme is different.

This are the following schemes for Power Over Ethernet (POE) wiring
enter image description here

I have taken this from Wikipedia link here

Note: Please remember that the maximum voltage allowed is 50V and current is .36A which is 18W. So the load you are connecting to should not be drawing more power than this recommended current / voltage levels.

  • 2
    I think the question is asking if they have to be connected differently; not if the signal on the wire is different. In other words, a 568-A on 1 end and a 568-B on another results in a X-over, doesn't matter if it's CAT5/6 or POE, that's why they are a standard, the cable just defines the signal that can be carried over it (you could essentially have the same color code on both ends and it will still work even though it's "wired differently") – txtechhelp Apr 12 '15 at 19:55
  • You are probably correct. But to me a cross over connection or a straight connection shall be decided depending upon what devices you connect on both ends. I read the question as POE specific... :-( – Prasanna Apr 12 '15 at 20:01
  • 50V*0.36A = 18W and not 36W – phuclv Sep 7 '18 at 15:18
2

No; POE does not require any different or special cabling.

  • Upvoted because this is the most direct answer to the OP. – Wranorn Feb 1 '20 at 7:43
0

If you're talking about the male end (The cable), then no. The devices I've used PoE used the same type of pinout as the ones that had their own power supply. It should be noted that this MIGHT depend on the speed in use, as not all pins are active until you get to gigabit ethernet.

What goes on on the female end (the device), I do not know, but I believe the device has a certain amount of resistance or capacitance between a set of pins, as a PoE-providing switch can detect whether or not it should supply PoE or not.

0

As long as you keep both ends same, it doesn't really matter how you align the wires. You can align them any way you want, but you also have to do this on the other end (if you aren't making a cross-cable).

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    I had this mindset too - until I handed over the project to another team - its complete chaos now - everyone in my team have used different types of connections to make things work - some have crimped only 4 wires and remaining are taped and kept out :-( – Prasanna Apr 12 '15 at 19:58
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    Actually, if you care about signal integrity, it does matter. The twisted pairs need to be kept paired, but other than that, you can do them in any order as long as it's the same on both sides of the cable. – nw. Feb 2 '18 at 18:37
  • Downvoting... Not only keep the twisted pairs paired, but for best result, you need to use proper wiring as shown in LifeBoys answer. Some other variation would work, but above all, each twisted pair should be used for an Ethernet pair... see: pinoutguide.com/Net/ethernet1000baset_pinout.shtml – user1532080 Sep 7 '18 at 13:55

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