A monitoring device is for a block of building material. The monitoring device may include an electric supply line configured to be buried in the block of building material and having a flexible main cable, and flexible jumper cables coupled to the flexible main cable and extending outwardly. The monitoring device may include sensor devices configured to be buried in the block of building material and coupled to respective ones of the flexible jumper cables. Each sensor device may include a primary inductor coupled to the electric supply line at a position based upon peaks of a stationary waveform when the electric supply line is alternating current (AC) powered, and a monitoring circuit. The monitoring circuit may include an integrated sensor, and a secondary inductor magnetically coupled to the primary inductor and configured to supply the integrated sensor, and communicate through the electric supply line.