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SESK: Stuff Everyone Should Know · sesk.info/elec-peak-voltage-120

A 120 volt AC outlet reaches a peak of about what voltage?

120 V rms peaks near 170 V.

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  1. 120 volts
  2. 170 volts ✔
  3. 240 volts

Why: The 120 V figure is an rms (effective) value, equal to the DC voltage that would heat a resistor the same. The waveform actually swings higher, to about 1.41 times 120.

Huh, didn't know that: OpenStax notes most of the world uses 240 V household power instead of 120 V.

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Which is an electrician's favorite band? AC/DC.
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References

Answer: 170 volts. The 120 V figure is an rms (effective) value, equal to the DC voltage that would heat a resistor the same. The waveform actually swings higher, to about 1.41 times 120.

  1. OpenStax University Physics Vol. 2, 15.1 AC Sources openstax.org/books/university-physics-volume-2/pages/15-1-ac-sources
    with a frequency of 60 or 50 Hz and an amplitude of 170 or 311 V
    Checked 2026-10-10.
  2. OpenStax College Physics 2e, 20.5 AC vs DC openstax.org/books/college-physics-2e/pages/20-5-alternating-current-versus-direct-current
    the 120-V AC (240 V in most parts of the world) we use in homes and on the job
    Checked 2026-10-10.

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