Author Options:

Use of Ethernet cable for 100m power and RS485 transmission is OK? Answered

Hello Friends,

I want to use Ethernet CAT 5e cable for Power transmission and RS485 transmission.
But I am not sure this suits to my application or not.
I have 5 different load which requires 20mA normally and 1A instantaneous current.
I need to transmit 5V via Ethernet Cable as load accepts 5V.  If not possible to transmit then I may use 12V.
Distance I need is 100meter. Ethernet Cable is ok for RS485 transmission over this range with 5v/12v power?

CAT 5e seems better solution for this case or not?


To do POE on CAT5, you really HAVE to run with "high voltage", say 24V supplies, and use switch mode supplies. The 1A isn't too hard, if its brief, and you can feed it from substantial capacitors

Hi Steve,

Can I use 12V ? As load accepts 5V only if there is voltage drop around 1.2V at 100m I think it will be OK.

You can try a 5V linear regulator, and few thousans uF cap What will happen otherwise is that the 1A pulse will drop the supply at the far end of the cable so low the circuit will drop out and reset. The resistance of Cat 5 is something like 10 ohms/ 100 metres. A 1 A pulse will be an effective short circuit !

Hi Steve,

Yes I am thinking of transmitting 12V over CAT 5 cable and will put small PCB at every sensor with 7805 and Capacitor. By this I think 1A peak of microsecond will not effect other circuitry.

BIG fat cap, and you'll probably be OK, if this 1A PRF is low enough.


4 years ago

The remote wireless radios I used to help set up used POE , (power over Ethernet) . They came with power injectors, adapters that connected to the cable and to power adapters. We were putting 18 volts through it with no problems. I don't know what the max capacity of the cable is but it can handle low voltage, low amperage pretty well.