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A US wall outlet is called 120 V AC. About what peak voltage does it actually swing to?

About 170 V; 120 V is the RMS value, and peak is about 1.41 times that.

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  1. 120 V
  2. About 170 V ✔
  3. About 240 V

Why: AC voltages are quoted as RMS, the steady DC value that would heat a resistor the same. The peak is the square root of 2 times the RMS, so 120 V RMS swings about 170 V each way.

Huh, didn't know that: RMS stands for root mean square: square the wave, average it, then take the square root.

The dad joke
My outlet says 120 volts, but on its good days it peaks at 170. Must be all the alternating moods.
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References

Answer: About 170 V. AC voltages are quoted as RMS, the steady DC value that would heat a resistor the same. The peak is the square root of 2 times the RMS, so 120 V RMS swings about 170 V each way.

  1. OpenStax College Physics 2e, 20.5 Alternating Current versus Direct Current openstax.org/books/college-physics-2e/pages/20-5-alternating-current-versus-direct-current
    This means that the AC voltage swings from 170 V to -170 V
    Checked 2026-10-10.

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