Distributed calculation of an economic dispatch can be used for efficient resource allocation in a power distribution system with local generation. The lossless dispatch can be implemented in a fully decentralized way, however, because the impact on real losses in a power distribution systems cannot be neglected, this kind of calculations might lead to suboptimal solutions. This work proposes a semi-decentralized algorithm that solves the economic dispatch with transmission losses. The algorithm was implemented in a multiagent system (MAS) with JASON programming language, using a Contract Net Protocol (CNP) to adjust power output of each generator. CNP algorithm implemented has a good behavior under unconstrained conditions, but further researches are needed to calculate the optimum if capacity constraints are included.