The Siemens SIRIUS 3RV2321-4DC10-ZX95 is a circuit breaker designed for starter combinations — meaning it integrates directly into a motor starter assembly, combining short-circuit and overload protection in one unit that coordinates with a contactor.
The 63 A at 400 V (gL/gG) rating is the maximum fuse backup current — it tells you the upstream fuse can be sized up to 63 A without coordination issues, which is typical for a 15–20 hp motor at 400 V. The interrupting capacity at 400 V is 25 kA, enough for most industrial panel fault levels. For 240 V circuits the interrupting rating jumps to 100 kA, covering high-fault installations. The screw terminals accept 2x (1–2.5 mm²) or 2x (2.5–10 mm²) solid or stranded wire, so it handles standard motor power wiring without adapters. Clearance distances: 50 mm upward and downward, 30 mm to the side, zero forward or backward — plan the enclosure depth accordingly.