Arduino relay power over Ethernet cable?

Hi. I have an Arduino setup with 2 individual relay boards controlling two 240v power outlets. The relays are housed separately from the Arduino and I have connected the two via a standard ethernet cable. The relays I have are both 12v powered and switched via a 5v Arduino digital pin.

I have currently wired the following.

Orange and orange/white ---> 12v+ (from a 12v plug transformer)
Blue and blue/white ---> GROUND (connected to both plug transformer and Arduino GROUND)
Brown ---> Switched +5v from Arduino to relay 1.
Brown/white ---> Switched +5v from Arduino to relay 2.

SITUATION.
One of these relays for some reason isn't working properly. The board is receiving the right power but the relay itself is not clicking/switching. Maybe sticky contacts. The whole thing is already wired and the unused cables of the network cable trimmed so I cannot use them. 

MY QUESTION
I have a spare single 5v powered relay board which I could replace the faulty 12v with, but with no wires left to connect it, could I pinch one from the orange pair that I am sending 12v through to the first relay and use that for the 5v to the replacement one? In a less round about summary... can I send 12v+ through a single strand of ethernet cable with a run of only about 30cm?

Thanks, Steve.

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steviecuk (author) 1 month ago

I'm more concerned about it bursting into flames to be honest. I can deal with any drops in current/voltage its the safety element I want to make sure is ok. 12v through one strand of cable. It is just a random network cable so don't know if its 5/5e/6. :-|

It's difficult to determine the safety aspect without knowing how much current is being drawn as well as the properties of your cable, but as you're only driving relays there's a good chance it will be ok.

It's always better to be safe rather than sorry, so check the datasheet for the relays you're using and determine what their maximum current draw is. I'd expect the insulation on the Ethernet cable to have some kind of markings on it to determine the spec as well.

Note that PoE (Power over Ethernet) via CAT5 can happily supply at least 350mA over a twisted pair, so if your relay draws less than 100mA you should be fine for a single core. If in doubt just put it all together, run it for many hours (never leaving it unattended) and keep an eye on the temperature. If the cable gets warm you have a safety issue.

Depending on the lenght of yor cables I would check if the relays still get enough voltage to switch.
Ethernet cable is great for signals, not so much for actually getting power from point a to point b.
For short distances a single wire will be enough for your needs but if talk several meters you might have a problem.

steviecuk (author)  Downunder35m1 month ago

The cable length is literally only about 20-30cm so tiny.

+1. Cat 5 cable is resistive stuff. 0.025 Ohms/foot, 0.085/metre x 2 (two wires), 0.05 Ohms per foot, 0.170 Ohm/metre

Due to the very short length of your cable run, it probably will work... but consider that there are different types of Ethernet cable.

Forget about using plain CAT5, you want to be using at least CAT5e (enhanced CAT5), but CAT6 would be even better. CAT5e typically uses 24-26 AWG wire, whereas CAT6 typically uses 22-24 AWG, so has thicker strands.

If you're experiencing issues I would use a multi-meter to check for voltage drop on the Ethernet cable.