The electrical service voltage used when you create a project in SolarDesignTool is 240V. It's not possible to change this value or design 480V/three-phase systems at this time. 

While we'd like to support a wider range of voltage options in the future, right now our development roadmap is focused on equipping our users with tools for designing and permitting residential-scale, single-phase systems primarily in the United States. 

By sticking with one grid voltage, we're better able to keep improving popular features like automatic component sizing, code compliance validation and single-line diagrams, and to develop new options currently in the works.