30A is the maximum amperage fuse that the holders can accept. Cartridge fuses up to and including 30A are one diameter and length. Fuses above 30A to 60A inclusive are a different, larger size. These adapters allow you to put a 30A-class fuse in a 60A-class fuseholder for cases where a 60A-class fuse body is hard to find/unavailable in the amperage rating you want to use. Your 20A fuse will work fine.
My above statement stands provided you're adapting fuses in the same voltage class. For instance, fuses for 600v are longer in length (to provide a longer fusing length to quench the arc at higher voltages) and won't fit in a 250v class fuse holder. You can adapt fuse diameter from larger to smaller, provided you are staying in the same voltage class. There are different adapters, rated for the voltage class of the fuses you're adapting.
