What year did 115 volts changed to 120 volts?

Since about the middle of the 20th Century, 120VAC has been the nominal voltage for the United States and Canada. However, the voltage provided at the wall can be between 114 and 126VAC as a 5% margin above or below is deemed acceptable for general use. If one needs more accurate regulation, one can purchase a "line stabilizer" that has tighter tolerances.

Household appliances often list 110, 115, 120 or 125VAC as their input voltages but as long as they have either a NEMA1-15P (two-bladed) or NEMA 5-15P (two-bladed with a ground pin) plug, they will work in the modern North American outlet.