How about the power unit first? The first item in the design is the size of the core. We must determine the output VA (volts x amps) then add them together to get the total output VA (volt-amps). The next step to determine the type of steel to use (for the core) and determine the core size that will support the VA requirement. Edcor only uses M6 grain oriented silicone steel so we can make a reasonable chart to determine core size vs. VA. Before all of this, we determined what flux density we will use on the transformer. The flux on M6 is acceptable from 0 gauss to about 17500 gauss. We have chosen to use about 14000 gauss (1.4 Tesla). The higher the gauss the higher the operating temperature and frequency limitations (example 50 Hz or 60 Hz). We try to work with 50 Hz to satisfy most of the world. If it works at 50 Hz it will work at 60 Hz. If it is designed at 60 Hz it may not work at 50 Hz.