Nominal mains voltage (North America)

Edison's original DC distribution ran at 110 V because that was what his carbon-filament lamps liked, and the number survived the switch to alternating current by inertia. It drifted upward over the decades — 110, then 115, then 117 — as utilities raised voltage to squeeze more power through existing copper, and settled at a nominal 120 V. Residential service arrives as 240 V centre-tapped, giving two 120 V legs for lighting and receptacles and the full 240 V for ranges, dryers and air conditioning.

ANSI C84.1 defines the tolerance rather than a single number: Range A is 114 to 126 V at the utilisation point, so any calculation that treats 120 V as exact is already working to about ±5%. That band is why the National Electrical Code caps branch-circuit voltage drop at 3% and total feeder-plus-branch drop at 5%, and why motors carry a nameplate utilisation voltage of 115 V rather than 120.