What the ratings mean for fit
The ABB S203P-K6 is a 3-pole molded case circuit breaker with a K-curve trip characteristic, rated 63 A continuous and 10 kA interrupting capacity at 480Y/277 VAC. The K curve means the magnetic trip is calibrated between 8 and 12 times the rated current — tighter than a D curve but wider than a C curve — so it handles moderate inrush from motor loads or transformers without nuisance tripping, yet still clears a hard fault fast. The 10 kA SCCR at 480Y/277 VAC tells you it can safely interrupt a fault up to that level on a 277/480 V wye system; if your available fault current is higher, you need an upstream current-limiting device to coordinate with this breaker.