pancho puts it well.. a safetly feature
the rails have a max of 19 amps normally its an itel standard..
cpu and one high end grafix card two rails.. two 15 amp ones would do..
cpu and two high end grafix cards three rails at 15 amp would do..
cpu and three high end grafix cards.. 4 rails at 15 amp would do..
cpu and 4 high end grafix cards 5 rails at 15 amps would do..
one rail for the cpu and one rail for each high end grafix card..
the average 1200 watt psu would have 6 rails at 19 amp.. somewhat overkill unless u wanna run 5 2900xt cards and two cpus.. he he
rails u dont use are wasted..
for comparisons sake the 3870 draws about 8 amps.. the old 2900xt double that..