A trend is emerging towards the activation of distribution networks, which have only been passive until now. The main purpose is to use the existing infrastructure more efficiently. Due to the vast number of low and medium voltage networks this can only be achieved with autonomous decentralized grid monitoring and usage management systems. The most sophisticated task is the autonomous determination of remaining transfer capability. The authors wish to contribute a novel approach for solving this problem based on the online approximation of operational constraints in the domain of complex nodal power.