David82:
I want to run 120V AC through a CAT5 cable.One twisted pair will be hot, another twisted pair will be neutral. The cable may be 6ft to 100ft. At the end will be a 12V, 7A power supply. Will it all melt to the ground? If so, Why? I can already draw 12V, 7A DC through a 6ft length using one twisted pair just fine..
The question can't really be answered unless you say what current will be flowing in the wires. Then you would have to research the wire size of CAT 5 cable and see what current it can handle. Just because you say at one end will be a 7 A power supply doesn't say what current will really be drawn at the end. That's just the Max capability of the PS.
Off the cuff - I wouldn't do it.