Can you use cat 5/6 ethernet cable to power devices?
Can I use the 4 pairs on the ethernet cable to supply power?

Sure. They're just wires.

Can I use 6 wires to vcc and 2 to ground?

Sure. They're just wires.

Can I supply different voltages on the same cable?

Sure. They're just wires (and you have 8 of them). For DC power purposes, the twisting doesn't really make a difference.
Things to note:

If you're planning to use more than about 1 amp per wire, make sure to find out what size of wires you have, so you can look up the resistance and maximum current. See here: Current over CAT6 ethernet cable

If you have a single centralized power source, then it may be a good idea to put a separate fuse on each power wire (at the end with the power source). Not ground wires.

If you do this, you obviously can't use the same wires for Ethernet. 100Mbps Ethernet only needs 4 wires though, so you can use the other 4 for power.

There is a standard system for this called PoE (Power over Ethernet). With PoE, you can use the same wires for Ethernet. This uses 48V and can transmit up to 60W of power on each cable (but it also depends on the power source and receiver). If you decide to use PoE, you can use standard equipment instead of building your own. Devices which send PoE power are called PoE injectors and devices which receive the power are called PoE splitters.

If you use normal Ethernet connectors (i.e. RJ45 connectors) then some moron will come along and plug in an Ethernet device, which is a short circuit. You should be prepared for that to happen. Maybe you can use different connectors or at least use big red warning labels. (thanks to awjlogan in comments)


If you are planning to install these permanently, you also need to check what your building code says about low-voltage wiring. There may be additional requirements, such as keeping the low-voltage wiring separated from the mains wiring.