We recommend using a 40A breaker for the Dura i, particularly for the 6k inverter. The breaker size can vary depending on the inverter's start up current, but 40A is suitable for the highest kVA model (6k).
For reference: 3.6k = 25A 4.6k = 32A 5k = 32A 6k = 40A.
You can calculate the appropriate breaker size using this formula: Inverter Capacity × 1.5 / 230.