Reduce voltage on PSU from 52V to 12V
You would be better off to distribute the 48V-52V to the motherboards individually (about 3A per wire), and use a point-of-load DC-DC converter (there are many available commercially, and they aren't all that expensive) on each one to produce the 12V @ @13A that the motherboard needs.

This not only reduces the wiring losses, it also makes it easier to work on one motherboard at a time without shutting down the entire farm.