Does the 3125 use a motor-generator set, like the
larger
 models?  Do you have the right power to run the MG?  I know
 the MGs on the 3145 were oversized, so maybe the same unit
 could also run nthe 3158, and demanded an INSANE current
 surge when spinning up.  Also, the running current, even at
 idle, was pretty crazy, somewhere around 60 A at 208 V 3-phase. 
No, the 3125 does not. Power consumption for a model 125 system is
actually not bad for a mainframe of the era.
Generally, big iron collectors are not to concerned about big copper.
--
Will