The code doesn't forbid it, but in the long run copper is the only way to go. It has a much longer "life cycle" than aluminum.
It has a 1.6 times ampacity advantage, it is easier to install because you can twist it tighter, bend it further and pull it harder than it's alum counterpart. It is resistant to corrosion, has the highest conductivity rating of all engineering materials. It is more resistant to nicks and mechanical abuse.
Copper connections also run cooler which definatly pro;ongs there life, unlike aluminum. When and if you see old alum conductors check the connections. Normaly they are loose and completly oxidized.
It is more cost effective to run copper for commercial and industrial applications.
