Hi Stan:
I installed dedicated 110v 20A and 220V 30A circuits into my shack about a year ago.
The 110V circuit uses standard 20A 12-2 with bare ground Yellow Romex cable feeding a standard 20A duplex outlet that will accept a 110V 20A plug that has 1 horizontal blade and 1 vertical blade (standard 110V 15A outlets will only accept 15A plugs with 2 vertical blades). This circuit is fed through a single 20A GFI breaker in the service panel.
The 220V circuit uses standard 30A 10-3 with bare ground Orange Romex cable feeding a single 20A round outlet that will accept a 220V 20A plug using straight blades instead of the industrial-type TwistLok plugs. This circuit is fed through a double 20A GFI breaker in the service panel. This circuit was wired with the 30A Orange Romex in case I want to upgrade it in the future with 30A GFI & outlet to handle the Henry 2K Classic floor-model amp currently in storage.
One thing to watch out for: assuming you are considering a linear amp, what power feed configuration is the amp set up for? Some older amps are set up to run on a 2-wire 220V circuit, 2 hot wires with no white neutral wire, such as Yaesu FL2100 and Kenwood TL922. This works fine until you want to install an option board like a soft-start-up board that runs on 110V & needs the white wire, so you really should just go ahead and set up the circuit as a 110/220 feed at whatever Amp rating you need.
The attached NEMA configuration chart may be useful, my 110V 20A circuit is a NEMA 5-20R, my present 220V 20A circuit is NEMA 14-20R and my future 220V 30A would be a 14-30R. While I was at it, I put in a 220V 50A outlet on the garage workbench for a future welder (old habits die hard 😀 ) configured as NEMA 14-50R which is the same as the large motorhome RV's.
Hope this helps, & Best 73.
Larry Sheridan K4LES
307-630-5697