The Siemens 5TT5801-8 is a SENTRON Insta contactor — a 1-module-width (18 mm) DIN-rail component with one normally-open and one normally-closed contact, rated for 20 A at 230 V AC and controlled by a 24 V AC coil. This is the 0/1-automatic variant, meaning it can be wired for either momentary or maintained switching depending on the control circuit design.
The 20 A operational current at AC rated value is the continuous thermal current (Ith) for the main contacts — it handles resistive and moderate inductive loads up to that level. For incandescent lamp loads, the switching capacity is 1,000 W, which covers most single-lighting-circuit applications in residential or light commercial panels. The 24 V AC control coil is standard for building-management systems; verify your control transformer supplies AC, not DC, because the coil impedance and dropout characteristics differ.
Depth is 71 mm, height 90 mm — fits standard distribution boards and sub-distribution panels. Supplementary devices (auxiliary contact blocks, shunt trips, etc.) can be added on the side or top via the product extension interface.