S203-K50: 50 A K-Curve Supplemental Protector for 480Y/277 V Panels
The ABB S203-K50 is a 3-pole miniature circuit breaker from the S200 series, rated 50 A with a K trip curve. It's built as a UL 1077 supplemental protector for 480Y/277 V AC circuits, with a 6 kA interrupting capacity. The K-curve means it trips magnetically between 8 and 12 times rated current — designed for loads with moderate inrush like motors, transformers, or solenoids where a C-curve might nuisance-trip but a D-curve is too slow.
What the Ratings Mean for Your BOM
The 50 A rating is the continuous current it carries without tripping in free air at 30 °C. The 480Y/277 V AC rating confirms it's rated for line-to-line 480 V and line-to-neutral 277 V wye systems — common in North American commercial and light industrial panels. The 6 kA interrupting rating means it can safely clear a fault up to 6,000 A at rated voltage; if your available fault current exceeds that, you'll need a current-limiting fuse upstream or a higher-rated breaker. The K-curve is the key selection parameter: it's the middle ground between C and D curves, with magnetic trip at 8–12× In, so it holds through motor start-up but still protects against hard shorts.
Deployment & Integration
Snaps onto standard DIN rail — no tools needed for mounting, which speeds panel assembly and replacement. The S200 series footprint is compact, so it fits tightly in multi-pole groups. Use it as a supplemental protector in branch circuits downstream of a main breaker or fused disconnect, typically in control panels, HVAC units, or machine-tool enclosures where UL 1077 listing is accepted.