Hopefully to add to what luxO says, all homes (US, CA) are wired with 2 phase power, but I think electricians just talk about "220 volt" and never worry about describing the phase in home wiring. As he stated, your home has essentially the secondary of a transformer with a center tap, which is connected to neutral. 220 volt outlets are wired across the entire secondary and ignore the neutral. 120 volt outlets are wired between neutral and either hot lead. Thus, the 120 volt outlets wired to hot #1 are technically 180 degrees out of phase from the 120 volt outlets wired to hot #2 - but who cares at home. Either of these 120 vac outlets are called single phase - they have one hot wire, one neutral (which is actually at ground potential in the service entrance) and the ground. The ground never carries current unless there's a fault (a hot wire touches the chassis), then it carries a current to trip the breaker.
Panel breakers may have ratings on them (i.e. 20 amps), but in my experience, they can trip unpredictably at modest currents. Sometimes they seem to trip for no apparent reason well within their rating. Sometimes you run what you think is close to rating and nothing happens. Time may be a factor.
Audio amps will typically draw about twice their audio power rating from the AC line if operated at absolute full power with sine waves. Some designs today may have better efficiency. With music, a nice rule of thumb is that the AC draw will about equal the audio power amp rating. Check out this current draw info from QSC:
http://www.qscaudio.com/support/library/techaids/techaids.htm
Their Powerlight PL1.0 map is rated at 200 watts/ch at 8 ohms. QSC recommends using a 1/8 power rating on pink noise as approximating music requirements. Their rating is 2.8 amperes - 336 watts at 120 volts. With full power sine waves, the draw is 984 watts, 8.2 amperes! But like they say, you can never do this in the real world with real audio signals, even heavy ones.
Mackie SRM450 powered speakers recommend no more that 5 units per 15 amp circuit for normal use, which calculates at a current draw of 3A (360W) from the wall. The internal audio amps are rated at about 350 Watts audio power.
As to low voltage problems, there's only so much variation to design for and keep the costs down. I am told by sources familiar with a certain famous line of mixers made near here that 105 VAC is about as low as they design for, then the regulation drops out, headroom is reduced, and much lower and other things stop working right.
I think if you want to measure current, use an inductive-type clamp-on non-destructive probe for your DMM. They might be expensive, but much safer and just as accurate. You don't really want to mess with safely making a box with a line current shunt in it to use on the road. It could be a technical and legal disaster if the slightest thing went wrong with your shunt. I don't know about the line voltage conditioners, but you can't make up power out of thin air. I suspect they have multiple taps on their transformers which electronically are switched as needed. If the source voltage goes down, the conditioner must switch to a higher voltage tap to keep the voltage up. This must be heavy and expensive. And you do use big, heavy gauge power cables, right? A couple hundred feet of 16/3 power cord could have significant voltage drop under heavy load.
Like LuxO says, E=IR (voltage =currentxresistance for AC circuits if you use the RMS (root-mean-square) value of the AC voltage and current. We will ignore power factor for now, where the voltage and current shift phase due to capacitive or inductive load, however, at least audio amp manufacturer, QSC, touts a design with PFC, power factor correction.
BTW, most digital meters today don't care much about whether you set it to AC or DC, and in fact, you can use the different settings to crudely separate the AC/DC component measurement. But please don't measure voltage with the current setting!!
Now, is a Moderator going to move this discussion yet?
