For my boat automation system I plan to use RJ45 sockets and cat 5 cable for interconnecting the master and the nodes.

Some nodes will have their own 12 V supply for larger loads, but ones that are just for switches and other transducers (sensors) will need to have 12 V delivered to them. According to the cat5 specification the cores are AWG 24, which means that it has a nominal maximum current carrying capacity of about 0.5 A (short term peak 3.5 A).

Ethernet 10/100 uses only 4 cores. 1,2,3, and 6. This leaves 4,5,7 and 8 free for other things (because of the way the cable is wired those are both 2 pairs of twisted cable, so no sharing of twisted pairs either).

CAT 5 Data Pins

CAT 5 Data Pins

As you can see we now have spit out all the pins required for data;

CAT 5 Data Pins & RS485 extracted

CAT 5 Data Pins & RS485 extracted

If we use pins 4 & 5 for RS 485 they are one of the two spare twisted pairs,

CAT 5 Data Pins, RS485, & Power extracted

CAT 5 Data Pins, RS485, & Power extracted

And we can use Pins 7 & 8 for the  power lines. If we had two of these, we could use them as a kind of POE injector, and extractor (If RS485 was not required you could double the current carrying capacity by using Pin 4 as an additional 12 V and pin 6 as an additional ground.

RJ45 to RJ45 straight adapter

RJ45 to RJ45 straight adapter

By wiring two RJ45 sockets together you can make a RJ45 to RJ45 straight adapter, but this is not much use for RS485 or Power.

RJ45 to RJ45 straight adapter, with RS485 and Power extracted

RJ45 to RJ45 straight adapter, with RS485 and Power extracted

I plan to use RJ45 to RJ45 straight adapter, with RS485 and Power extracted as the biases for my data and communications, as it allows me the passive provision to add an Ethernet 10/100 device at each end of the run.