Main breaker panel as subpanel
I want to install a 12 circuit 60 amp subpanel in my attached garage. Both for convenience and the 6 circuit rule I need a disconnect. The easiest would be to buy a main breaker panel and use it as a sub. How do I separate the ground and neutral busses? My two service entrance 200 amp Siemens panels (400 amp service) have the ground bus attached to the metal casing, and the neutral busses are attached to a metal slot that's touching the casing. I bought a Cutler Hammer subpanel for my basement workshop and it's ground is separated by a plastic riser.
Is there some way to separate the ground and neutral with a main breaker panel?
Could I buy a 2 circuit subpanel with 60 amp breaker, and feed a main lugs subpanel from that, as a disconnect?
Are there 60 amp knife type disconnects that I could use to feed a main lugs subpanel? I have Siemens, Cutler Hammer, Square D, and (yuk) Federal Pioneer Stab-Lok available.