A type of AC power supplied by electrical companies and found in many commercial and industrial organizations. In most household wiring there are two 110 (to 120) volt legs 180 degrees out of phase with each other. Together they provide 220 (240) volts (the difference between the two), which is what your household dryer or stove uses to operate. Three phase is similar; as you’d expect, there are three hot wires and a common neutral. However, in three-phase power the hot lines are each only 120 degrees out of phase with each other. Any one of them may be 110 volts when referenced to ground, but depending upon how they are wired with respect to neutral, may only have 208 or so volts between them. This is because in a three phase system each leg operates 120 degrees (not 180) out of phase from each of the other two. The result of measuring the ‘difference’ between two 110 volt signals 120 degrees out of phase with each other is 208 volts.
What does this mean in practice? Using transformers, there are many different ways to arrive at the various voltages used in commercial installations. You may see 240, 208, 440, 480, etc. In facilities supplied with three-phase power the standard wall electrical outlets will behave as normal. The only worry you have if your band happens to play in one of these places is if you have equipment that runs on 220-volt power. Almost no audio equipment I can think of does, but there are many lighting dimmer packs that do. Fortunately, most of those are quite capable of operating at 208 or 220 volts and on two phases of a three phase supply (remember each phase is still the standard 110 or 120 volts). A three phase source can be a huge benefit because you can now run all of your lighting off of two phases while your audio gear can run from the third phase. Having your audio gear separated from the lighting like this can dramatically reduce hums and buzzes in your system. If you are building a studio in a commercial space you should inquire about three-phase power for the same reason. You can operate all the lighting and HVAC off of two phases and put your delicate electronic equipment on the third phase. The only caveat here is that the three phases must be load balanced, meaning they have to each have approximately the same amount of current draw. If you don’t do this you’ll have worse problems than you would with standard wiring, so be careful and consult with a qualified electrician.