What is the peak voltage of a 120 vac supply?

"It is about 170 volt." was the given answer to this question.

Incorrect. This would be the correct answer if question were "What is the 0-to-peak voltage of a 120 volt AC circuit?" The "120" represents the root-mean-square (rms) value of the voltage. To get the peak value of the sine wave for any such rms figure, you multiply the rms figure by the square root of 2 (~1.41), i.e. 120 x 1.41 = 170.

The PEAK-TO-PEAK voltage of the sine wave is simply twice the 0-to-peak value. Hence, the correct answer, for PEAK-TO-PEAK, is 170 x 2 = 340 volts.

So, the correct answer to the question is 340 volts.