Abstract: An adapter is for mounting an electrical switching apparatus, such as a circuit breaker, on the back panel of an enclosure, such as a switchgear cabinet. The back panel has at least one aperture. The adapter includes a first end portion coupled at or about the aperture of the back panel, an engaging portion engaging and securing the circuit breaker to the adapter, a fastener, and a second end portion disposed distal from the first end portion and including an elongated slot. The elongated slot receives the fastener in order to couple the adapter to the back panel when the fastener is fastened, while enabling adjustment of the adapter and the circuit breaker coupled thereto, with respect to the back panel, when the fastener is unfastened. The first end portion, the engaging portion and the second end portion form a single piece adapter. An enclosure assembly is also disclosed.