PSAI got a nice refresher course in power usage today. I was running 4x750 ti cards flashed with an unlocked TDP VBIOS, using powered USB risers, connected to a single 4-connector Molex cable. One unlocked card can pull 65.5w from the PCI express slot. Typical molex connectors have a max power handling of 187w (11A). With 4 unlocked cards, I was shoving 262w@12v down one cable. The end result:
http://m.imgur.com/vDqGNmh,787qociEven at the default TDP of 38.5w (4 cards @ 154w), that's still dangerously close to the limit. Try to connect as few cards as possible to each power lead, or use a higher rated connector designed for more power, like the 6-pin or 8-pin PEG connectors.