The way I understand it:
20/20 means that the panel is built & listed for 20 branch circuit poles. (20 single poles or 10 double poles, or any combo to 20)
This also indicates that "full size breakers are "only" allowed in this panel.
24/40 means 24 full size breakers (24 poles)
or by installing listed "twins" or "piggy back" a maximum of 40 poles. The panel buss is supposed to be manufactured with a means to prevent the insertion of "twins" in excess of the "listed" quantity.
John
