The Siemens 3RV2321-4NC20 is a SIRIUS circuit breaker designed explicitly for starter combinations — meaning it's sized and coordinated for motor branch-circuit protection, not just general distribution. That's the spec the panel designer cares about for selectivity and SCCR compliance.
That means the fuse coordination is flat across the voltage band; you don't need to swap fuses when the line voltage changes within that range.
The 45 mm width occupies one standard module slot on the rail. Clearance requirements: 50 mm upward and downward, 30 mm at the side, zero forward and backward. Any mounting position allowed. These clearances are tight enough for dense panels but the 50 mm vertical gap matters when routing busbars or trunking above and below the breaker.