Normally, 5P20 is more than suffcient for most protection applications. Past that, it's hard to say without knowing exactly what you're using it for. As you may know, 5P20 means that the CTs will remain accurate within 5% up to 20 times nominal current. You of course specify a burden (?VA) to go with that rating to determine the overall performance of the CT.
Inorder to decide on the selection of CT for protection application, we first need to determine the maximum value of fault current available at the location, then we have to take this maximum value and divide by the rating of the CT the result will determine the multiplication factor, then we have to choose the accuracy depending on our application
eg. if we have maximum fault current available as 15 kA, and our CT current rating is 1000/5 then 15000/1000 =15.
therefore a CT XP20 is sufficient, X is determined based on the accuracy requirement eg 5 , 10 etc.
