The Siemens 5SJ3120-7RC is a 1-pole miniature circuit breaker from the 5SJ3 Retail line, rated 20 A with a C characteristic and 4.5 kA breaking capacity. The C-curve trips between 5× and 10× rated current, making it the standard choice for general-purpose loads like lighting circuits, small motor feeders, and control transformers where moderate inrush is expected. The 4.5 kA breaking capacity covers most residential and light commercial panelboards — check the prospective fault current at the installation point if feeding from a larger transformer.
Snap-mounts onto standard 35 mm DIN rail per EN 60715. The 1-pole width (1 module at 17.5 mm) fits tightly in a crowded panel — watch the thermal derating when packing multiple breakers side-by-side at rated load; the manufacturer's derating curve should be applied above three adjacent loaded units. Terminals accept up to 25 mm² conductor for the load side.