Storm-time geomagnetic disturbances induce significant geoelectric fields within the Earth that can adversely affect the operation of electric power grids. The recently completed magnetotelluric survey supported by the NSF EarthScope program (2006-2018) has produced a large public archive of impedance tensors across much of the continental United States (US). In this work, the EarthScope tensors are convolved with long time series of geomagnetic field variation recorded at USGS observatories to obtain estimated time series of historical geoelectric fields. Integrating these geoelectric fields across power transmission lines results in time series of geomagnetically induced voltages on each power line. These voltages are analyzed statistically to construct hazard maps of the maximum voltages that could be realized in transmission lines across the US for an extreme, once in one hundred-year, geomagnetic storm. In combination with grounding resistance data and network topology, these voltage estimates can be utilized by power companies to estimate extreme geomagnetically-induced currents within their networks. These voltage estimates can provide information on which power lines and substations are most vulnerable to geomagnetic storms and can guide power companies in assessments of where to install additional protections within their grid.