The Siemens 5SL6120-7BB is a SENTRON miniature circuit breaker (MCB) — a 1-pole, C-curve device rated 20 A at 30 °C, with a 6 kA breaking capacity certified to both EN 60898 and IEC 60947-2. Single-phase operation is rated 250 V AC; multi-phase (line-to-line) goes up to 440 V AC. That covers standard North American 120/240 V split-phase panels and European 230/400 V three-phase distribution boards.
Where it fits — panel and environment
Width is 18 mm — one modular unit on a standard DIN rail. Depth is 76 mm (70 mm installation depth), so it sits flush in a shallow enclosure without crowding the wiring gutter. Mounting position is unrestricted; you can lay the panel on its back for commissioning without worrying about orientation. Pollution degree 2 (normal, non-conductive) and overvoltage category III (distribution-level) are standard for residential and light commercial infrastructure.
Thermal derating — the real-world current
At 40 °C it derates to 18.48 A, at 45 °C to 17.6 A, and at 55 °C to 15.92 A.