I just built a system using a Carbide 500R case, which I think is the same as the 400R except White color.
I had to orient my power supply so the 24 pin and 8 pin cables were toward the motherboard so they would reach, routing them on the back side of the motherboard.
I have a Gigabyte GAZ77X UD5H motherboard. The 8 pin connector was long enough for this motherboard. I recently built another system using an ASRock Z77 Professional motherboard, in a different case, and I don't think the 8 pin connector would reach this board, in the Carbide case, with using an extender cable.