Circulator or circulating pumps belong to the centrifugal pumps category and incorporate a metal volute, direct drive motor and wet-rotor cartridge design. Unlike traditional pumps, which are sized to overcome both the internal friction of water against the pipe walls and external factor of elevation height, circulating pumps are installed in pressurized systems and are sized accounting only for internal pressure drop. Powered by an external source, such as an aquastat or a switching relay, circulators have either terminal box (hardwired) connections or come with a power cord with a plug.