Re: Buck-boost transformer sizing question
Assuming the incoming supply is adequate the voltage will not change from no load to full load.
Size the transformer to the maximum load you expect, when the load is less the voltage will still remain the same.
I have to ask why you would bother to boost 208 to 220, typically 208 will run a 220 motor fine.
Most systems work best if they are kept simple rather than made complicated; therefore simplicity should be a key goal in design and unnecessary complexity should be avoided.