The vast majority of permanent sensor and actuator nodes within a building automation environment are connected to mains electricity and seldom require significant bandwidth. Therefore, it is a good choice to set up a Narrowband Power Line Communication (NB-PLC) network over the electrical cabling as an alternative to dedicated wiring or using wireless communication on the often saturated ISM radio bands. The NB-PLC technology has many similarities with Wireless Sensor and Actuator Networks (WSAN), allowing reuse of several protocols and implementations. In this paper we present an open-source NB-PLC system based on Contiki, a popular IoT operating system focused on WSANs. This solution is aimed primarily at building automation purposes such as smart lighting, but is applicable to a variety of lightweight smart systems connected to mains electricity.