Hints and Tips - How to Specify an RS Pro TransformerFollow article
1) Primary Voltage?
This is usually mains voltage i.e 230/240V ac, but it might be 110V, 120V, 400V, 415V, 440V or something else.
2) Secondary Voltage?
A large range available here. Remember if you can't find the voltage you want consider using a transformer with 2 secondaries of half the voltage:
e.g. You want 40V secondary-you could also use 2x20V secondaries wired in series.
3) VA Rating?
If you don't know this you will need to know the current you require (Amps)? If so multiply that with the secondary voltage to give a VA rating. VA=Volts x Amps
If you know your LOAD in Watts multiply this by 1.5 to get the VA figure.