I've been using coax cables with BNC connectors for over 40 years. I use them a lot for power connections to DC power supplies (convenient with a BNC female to dual banana plug adapter), but I wondered what I should limit the current to. Since I couldn't find any usable information on the web (it's probably out there somewhere; I just didn't find it), I made some measurements last weekend. Results in attached file (also at http://code.google.com/p/hobbyutil/). Bottom line: I'll run my cables all day at 5 A, occasionally go up to 7.5 A, and once in a while when needed I might go to 10 A.