The type of generator used depends again on the head and flow rate. Vertical configurations are typically used for large hydropower facilities with low head and high flows. Horizontal configurations able to sustain greater RPM are typically favored for high head low flow circumstances common to run of river style power generation. Typically generators produce three phase power to feed directly into a three-phase grid connection. Single phase alternatives allow for feed to the grid where three-phase grid extension is economically prohibitive.