According to the answer from Sir Steven ,
I think there may be a point need to be confirmed :
About the Rb , Sir Steven said : Rb = rated resistive burden (20 VA in your case)
and what i understand is Burden = Isn^2 *Rb
In this case Isn=1A, so the Burden =1^2*Rb, Rb is equal to the value of rated burden .
when the Isn=5A, the Burden=5^2*Rb , the Rb = burden/ 25
Please have a check .
Thank you very much .