Murray QO replacement breakers are UL listed for use in Square D QO brand load centers and are ideal for replacement of existing breakers or if expansion circuits are required. These 3/4-Inch breaker snap on to the QO type bus bars. Murray strongly recommends against the use of "used" breakers. The installation of used breakers in a Murray panel will void the warranty on the panel. Murray does not sell used breakers and has not approved any 3rd party sellers to do so.