This disclosure describes insulated glass units (IGUs) that incorporate electrochromic devices. More specifically, this disclosure focuses on different configurations available for providing an electrical connection to the interior region of an IGU. In many cases, an IGU includes two panes separated by a spacer. The spacer defines an interior region of the IGU and an exterior region of the IGU. Often, the electrochromic device positioned on the pane does not extend past the spacer, and some electrical connection must be provided to supply power from the exterior of the IGU to the electrochromic device on the interior of the IGU. In some embodiments, the spacer includes one or more holes (e.g, channels, mouse holes, other holes, etc.) through which an electrical connection (e.g., wires, busbar leads, etc.) may pass to provide power to the electrochromic device.