Final answer:
The voltage selector on a power supply in the United States should be set to 120 volts, as this is the standard electrical outlet voltage.
Step-by-step explanation:
In the United States, a voltage selector for a power supply should be set to 120 volts. This is because the standard electrical outlet voltage in the US is 120 volts. Ensure that your electricity device matches with this voltage to avoid any electrical failure or harm. Voltage can be found labeled on the power supply of the equipment or within user manuals.
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