- Jan 1, 1970
Think about it, the one line that will most definitely get loaded is
the one that also just happens to supply the most current, the +5v line.
Undoubtedly the supplies are designed so the load of a motherboard (and
maybe some minor peripheral) is enough so the supply can run. There
is no need for a built in load resistor because the supplies are intended
for a specific appplication, that will be enough of a load.
In practice, the worst outcome I've seen from huge load mismatches on
ATX PSU's is that the unused rail goes to an extreme of of its
regulation range - eg; 6A on the 12V rail might bump the (unused) 5V
rail up to 5.15V, for example, or heavily loading the 5V rail might
drop an unused 12V rail to 11.5V, etc.