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I know that we (typically) get 120 V AC in USA, but what does the waveform look like? Can anyone post a pic? How many phases are there, and what's the peak voltage?

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  • \$\begingroup\$ Welcome to EE.SE! Some generic help: en.wikipedia.org/wiki/Three-phase_electric_power . However, given the lack of waveforms on en.wikipedia.org/wiki/Mains_electricity , I can't fault this simple question. \$\endgroup\$ – user2943160 Sep 6 '16 at 3:21
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    \$\begingroup\$ This is an interesting question, because it's supposed to be a sine, but in reality it's anything but, with all sorts of harmonics and the peak chopped off due to lots of garbage switch mode power supplies without any power factor correction. I don't know if anyone's done a good writeup on what AC mains actually looks like, in the real world. \$\endgroup\$ – Connor Wolf Sep 6 '16 at 3:23
  • \$\begingroup\$ On this page, there's even an image of the three-phase waveform: en.wikipedia.org/wiki/Three-phase \$\endgroup\$ – ilkkachu Sep 7 '16 at 19:26
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I took some oscilloscope captures of the mains waveform in my house in Houston, Texas. The house is over 50 years old and the wiring is definitely not recent (no grounding). We have LED bulbs on dimmers that weren't designed for them as well as the usual collection of electronics with switching power supplies. This is definitely not ideal!

First, here's a large-scale view of the sine wave with measurements. Normal household outlets have only one phase. I see 110 Vrms with a peak of about 150 V, which is on the low end of the nominal range. Distortions are visible near the high and low peaks.

Large-scale U.S. AC mains waveform

Here's an FFT, which shows third harmonic (180 Hz) distortion at roughly -30 dB relative to the fundamental. The next-largest harmonic is the seventh (420 Hz) at about -45 dB relative to the fundamental. The other harmonics are in the -60 to -70 dB range.

U.S. AC mains FFT

Here's a zoomed-in capture of the distortion, showing variations of perhaps 5V from an ideal sine wave.

Zoomed-in distortion

UPDATE: I took some scope captures of the mains voltage in the lab in Sugar Land, Texas, where I work. The building was built a few years ago in a rapidly-developing commercial area, so I'd assume the infrastructure is up to date. Nearby loads include lots of test equipment and LED lights. There's an ATE test floor on the other side of the building, but I doubt it's a huge factor.

Here's the full waveform. It's monotonic and the distortions are much less obvious. Note that the voltage here is 122 Vrms with a peak of about 165 V, slightly above the nominal range. Both this and the household mains show a peak voltage that's lower than what we'd expect for a pure sine wave with the measured RMS value.

U.S. AC mains waveform from an electronics lab

Here's the FFT. The harmonics look very different from the household mains. Surprisingly, the next-largest peaks after the fundamental have about the same magnitudes as before. The other harmonics are worse than the household mains.

U.S. AC mains FFT from an electronics lab

Finally, here's a zoomed-in view of the positive peak. The flattened top is much more obvious here.

U.S. AC mains peak from an electronics lab

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    \$\begingroup\$ Interesting. I'm guessing you have something in the house that has a Thyristor or SCR that's switching on and causing the notches. \$\endgroup\$ – Tom Carpenter Sep 7 '16 at 1:30
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AC power is nominally a 60 Hz sine wave, but it may be somewhat distorted, depending on loading. We normally speak of the Root Mean Square (RMS) voltage of AC. The peak voltage is about 1.4 times the RMS value.

Normal domestic power is single phase, but the overall power distribution system is three-phase.

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    \$\begingroup\$ if you add the clarification that 'somewhat distorted' usually means with the top shaved off, due to rectifier/capacitor loads conducting, then I'll upvote. \$\endgroup\$ – Neil_UK Sep 6 '16 at 6:06

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