What this RCBO is and what the ratings mean for fit
The Siemens 5SU1653-7KK20 is a 2-pole (1P+N) residual current circuit breaker with overcurrent protection — an RCBO — from the SENTRON 5SU1 series. It combines a 20 A rated current (at 30 °C ambient) with a Type A residual-current sensor and a C-curve magnetic trip, all in a 36 mm wide (2 width units) package that snaps onto DIN rail. The 4.5 kA breaking capacity per EN 60898 covers the vast majority of residential and light commercial distribution boards; for industrial panels with higher prospective fault current, you'd step up to an IEC 60947-2 device. The Type A trip means it catches pulsating DC fault currents from single-phase rectifiers or switching supplies — common in modern loads like LED drivers, VFDs, and IT equipment — not just pure sinusoidal AC leakage.
Derating reality — the 20 A rating is only at 30 °C
The 20 A rated current is the headline number, but it only holds at 30 °C ambient inside the distribution board. At 40 °C it derates to 18.8 A; at 50 °C it drops to 17.4 A; at 55 °C — the upper end of the operating range — it's 16.2 A. If the panel runs hot (stacked breakers, poor ventilation, or high ambient), size the upstream load accordingly. The C-curve characteristic (magnetic trip between 5 and 10 times In) suits motor and lighting circuits with moderate inrush, not purely resistive loads where a B curve would do. The 6.6 W maximum power dissipation per pole matters when grouping multiple RCBOs in a small enclosure — heat buildup accelerates the derating curve.
Mounting and wiring — what fits in the panel
The 5SU1653-7KK20 occupies 36 mm of DIN rail (2 width units) with a depth of 77 mm and an installation depth of 70 mm behind the panel face. The terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm² — that's enough for the 20 A circuit and then some, but the larger clamp range also means you can loop through a heavier feeder. Supply can enter from either top or bottom. The IP20 rating holds only when the distribution board is installed and conductors are connected; this is a panel-internal device, not a standalone enclosure. Mounting position is any — no restriction on horizontal or vertical orientation.