A modular electrical wiring system, methods and components thereof, for connecting electrical devices to structural wiring. The system may include a base modular unit having a first electrical connection feature and one or more dependent modular units each having an electrical device, a first electrical connection feature and second electrical connection feature electrically and mechanically interconnectable with a first electrical connection feature of another modular unit. The modular units are may be assembled sequentially along a first axis by being rapidly interconnectable with adjacent modular units by relative movement therebetween in a direction perpendicular to the first axis. When modular units are connected in a mechanical series, electrical devices in the modular units may be functionally electrically in series or parallel, depending on the internal wiring of the upstream modular unit. Modular devices may have a switch to provide a choice between a series or parallel connection.