I experienced melted cables myself, but it was due to poor wiring. Are you sure you didn't accidentally use bad wiring to get that melted cable, for example poorly connected +12V?
Don't skimp on cables. Don't use a 6/8pin GPU power adapter from SATA or Molex. Even 2xMolex to PCIE isn't a smart idea.
Of course if you have a crappy PSU it only makes things even worse.

Use only dedicated 6/8pin PCIE cables and I prefer only power supplies of the top brand with enough juice.

Smoke detector alarm should be your second line of defense. Better safe than sorry, as they say.