Took me awhile to understand that the breaker protects the wire. Then you can understand fuses!Yes 15 amp is the most common but on a occasion you will find a 20 amp that someone has put in thinking it would help them to keep it from tripping. I once had a guy that kept tripping a 20 amp breaker and wanted me to install a 30 so it wouldnt trip I told him it wasnt happening unless he wanted to change the wire size also.