The United States standardized around a 110-to-120 volt electrical grid system primarily due to early historical precedent set by Thomas Edison during the late nineteenth century. His early direct current incandescent lighting systems operated safely at 110 volts, which was considered an optimal compromise at the time for incandescent bulb filament durability and indoor safety. When alternating current systems were subsequently adopted, the 110-to-120 volt standard remained deeply entrenched across North American residential infrastructure.