Re: Ideal wiring situation
I have done it successfully all 3 ways that you suggest. For industrial applications I normally use a transformer (240/120V) in the panel to supply computer power and power up small power supplies.
You mention ''step down transformer'' , that implies AC output and would not be acceptable for the 24/48VDC supply. I think what you really mean is power supply. Also, most power supplies today are universal, including computer power supplies, they will run on about 100 - 240 VAC. Some have automatic input voltage detection, some require flipping a switch, and others require wiring to the proper terminals for the input voltage.
On my lathe and mill, I power the VFD from the breaker panel directly, and plug in the controls to a 120V outlet on the wall.
On the other side of the shop I ran 4 wires to another machine and split the power in the cabinet.
A 40 amp breaker is acceptable for a #8 wire.
Jim Dawson
Sandy, Oregon, USA