BigJohn;
Glad that we all were able to assist with the Buck/Boost Transformer sizing criteria.
As JBD described above, to figure the capacity of the Secondary Windings - in Amperes, divide the Transformer's rated Volt-Amps by the Secondary Voltage.
Using JBD's example Transformer above, these are the results:
500 VA Transformer - Secondary = 12/24V
* Secondary setup in Parallel for _12V_:
500 / 12 = 41.67 Amps maximum load Current through Transformer.
* Secondary setup in Series for _24V_:
500 / 24 = 20.84 Amps maximum load Current through Transformer.
Determine the "Per-Line" Amperes, which the Load Device will require, by dividing the Apparent Power (Volt-Amps) by the Voltage; or, if available, just use the nameplate Amperes of the Device.
If there is any doubt per these figures, feel free to reply as necessary! We are all here to assist.
Scott