Note : The Main Unit and 4-Relay of the Stackable Power Meter need to be used together. The Main Unit is connected to at least one 4-Relay in order to function Max. 20A/Gang.Each channel can support maximum current of 20A, no fear of high power load Communicated by RS-485.The SPM-Main communicates with the SPM-4Relays through RS-485, which allows for multi-terminal and stable long-distance data transmission Overload Protection.Once the overload protection is enabled, it can protect your devices from burning out.