Abstract: A customizable, modular circuit breaker housing assembly having an optimized compact form factor for use with electrical and telecommunications equipment. The housing assembly is comprised of an interlocked arrangement of individual housing portions coupled at one end to an end housing portion, and to an end panel portion at an opposite end. The end housing portion and individual housing portions include a load bus chamber and a line bus chamber for receiving load and line bus members, and one or more alarm contact member receptacles for receiving at least one alarm contact member. The line bus member and the alarm contact members may be provided as individually segmented members. In other embodiments, the individual housing portions may include pass-through slots for inserting a common line bus bar, pass-through apertures for common contact members, or both. The line and load bus members are formed of extruded metallic members having substantially the same cross-sectional surface.