I'm looking to control an automotive relay over a cat 5e cable. The run is about 1500 feet. Would there be a way to set up some capacitor resistor coupling at the end to give the relay the initial boost in amps for it to charge or could I do something complete different.

The set up is, 800 feet of run into the woods to a tank where 4 float contact switches are inside. There's also an LED light that uses one pair which isn't a problem ( it flashes when another tank, which isn't in the woods, is full). Of the 4 floats, they use a pair for the power and then individual wires for the signal line out from the float.

The relay I currently am using is a solid state coil and of course it won't energize. However, the pairs still have continuity. Although the relay is 12v, would only having 10.8 volts not make it trip?

Considering I'm using cat 5e, could it be possible to set up some form of networking switch or a raspberry pi..

Anything..as long as I can monitor the level of the tank from the Sugar Shack. Hopefully I can use the cat5e.

  • \$\begingroup\$ Do you have another power source at the location of the relay? \$\endgroup\$
    – DoxyLover
    Commented Mar 22, 2017 at 1:29
  • \$\begingroup\$ The answer is: PoE - Power over Ethernet. \$\endgroup\$ Commented Mar 22, 2017 at 7:32
  • \$\begingroup\$ @JakubRakus PoE is specced for 300ft max, not 1500ft. \$\endgroup\$
    – marcelm
    Commented Mar 22, 2017 at 19:33
  • \$\begingroup\$ Yes, I have additional power sources where the relay is an could add more. \$\endgroup\$
    – Eric Peck
    Commented Mar 23, 2017 at 2:50

1 Answer 1


The line is too long for Ethernet function. Nominally, Cat5 twisted pair is good for 200 meters, about 650 feet.

1500 feet of cable, means that your thin wire has a significant resistance. A typical 12V relay takes 120 mA of coil current, means about 100 ohms of coil resistance. The 3000 feet (1500 out, 1500 back) of #24 wire in a cat5 pair has 77 ohms of resistance. So, delivering 12V to the relay means you have to apply $$12V \times 177/100 = 21V $$ at the base, in order to get the right power in the woods. It varies, of course, depending on the relay.

Something similar to PoE (power over Ethernet) can safely supply regulated 12V with a buck converter to drive several such relays. It takes a safe power supply (48V max, 350 mA max), at the 'home' base, driving one pair, into the woods where a buck converter regulates it down to 12V. You'd get 5 watts or so, and the relay coil uses less than a third of that.

I'm guessing uses for the extra 12V power can be found...


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.