The minimum size wire that engineers spec for industrial and commercial installations is #12 and it usually is stranded not solid wire.
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It depends on whether the building classification is combustible or non combustible. On a new commercial building wiring is usually designed by an electrical engineer. In this case the wiring is done to their specifications. On a rewire of a commercial building BX cable can be used behind wall board. The reasoning is that most commercial buildings these days use steel studs. Dragging ordinary NMD wire through these types of studs can cut the outer sheath. In some jurisdictions steel stud bushings have to be used when a steel stud is hole punched. By using this type of wiring cable, rigid conduit or EMT does not have to be used.
To answer this question a voltage and amperage must be stated. W = Amps x Volts.
A 600 MCM copper conductor with a insulation factor of 75 or 90 degrees C is rated for 420 and 455 amps respectively. To reduce the wire size from a single conductor, a parallel feed of 3/0 can be used.
Depends on the state you live in and the local regulations.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.