According to AWG standards 18 AWG wire is only 1 mm thick in diameter. Going by your pictures it certainly seems to be at least that thickness. The problem is 18 AWG wire is only rated for 10 A max for single core wire, which is 120 W at 12 V and much less for multi stand wire. A GPU dual mining can easily pull 150 W+ from the VGA connector. I had a similar issue with the same type of dual molex to 8-pin connector. Now I only use 18 AWG splitters for 6-pin risers and 16 AWG for 8-pin PCI-E connectors.
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