What the ratings mean for fit
The S201-K3 is a 1-pole miniature circuit breaker rated 3 A continuous at 277 V AC, with a K-curve trip characteristic and 6 kA interrupting capacity. It's a UL 1077 supplemental protector — meaning it's listed for branch-circuit back-up protection inside equipment, not as a service-entrance main breaker. The K-curve (10–14x In magnetic trip) is the key spec: it's designed to ride through the high inrush of motor starters, transformers, and solenoid valves without nuisance tripping, while still clearing a hard short. At 3 A, this is sized for a small motor or control transformer secondary, not a feeder. The 6 kA SCCR at 277 V tells you it can safely interrupt a fault up to that level without welding contacts or cascading upstream — adequate for most panelboards behind a 10 kA main, but verify coordination if the available fault current at the panel exceeds that.
Where it lands in the panel
Snaps onto standard DIN rail (35 mm) in any enclosure. Single-pole width means it takes one module slot — 17.5 mm — so it fits tight layouts. The S200 series shares a common busbar and accessory footprint across the line, so you can mix K-curve and other curves on the same rail without extra adapters. Terminal torque is typically 2–2.5 Nm; strip length around 10 mm for the cage clamp.