In one embodiment, a device in a computer network monitors an alternating-current (AC) waveform of an electrical power source at the device, where the power source is part of a polyphase power source system. Once the device determines a particular phase of the polyphase power source system at the device, then the device joins a directed acyclic graph (DAG) specific to the particular phase. In another embodiment, a device detects a time of a zero crossing of the AC waveform, and may then determine a particular phase of the polyphase power source system at the device based on the time of the zero crossing relative to a corresponding location within a frequency hopping superframe of the computer network.