For amperage capacity I found this; I am not certain how accurate it is across various manufacturers however.
Overall, I'm not too keen on these. I have never used them because: I have seen the 'Punch-Down' style of tap loosen over time causing a poor connection. My concern too is that it nicks the wire during insertion which may over time result in fractures in the wire from vibration/movement. As I have never used them, this concern of mine may be completely unfounded.
I am also rather careful when I use spades. I have had to replace some due to the female side loosening resulting in heating and charring. As such I use them sparingly and up-size them when I do.