not much diff than r10 says;
on a tangent:
one 3ph had amped improperly, but only found under load moment, the voltage at 2 were fine and the third leg way short...
customer said his 'electrician' did repair on nearby mast for wires going to unit 20 ft away...
building had good source, good size, except when checking mast -to drooping wires- to into edge mast to unit... ONE WIRE SIZE BELOW CODE and twisted, not clamped under tape, one leg was cooking-out the connection.
three clamps, it ran fine- but loaded/ not code- what a wiring...!
undersized was removed and replaced the next day
I'm sure no where near the 'bad' that's out there.