It looks like some kind of industrial grade cable used for high amp applications. Each of those wires is used to carry one phase of current on a three-phase power supply. Probably went to a motor or something like that. Often, the cable is aluminum or nickel, but it can be copper. The insulation can also sometimes be pretty difficult to remove, but if it is copper in there, definitely worth it. But, like I said, that is just what it looks like to me based on what you said, what I can see, and my previous experience. Someone else here may have a different statement about what you've got there. Hope this helps!