Im installing a 100 amp sub panel about 3ft from my main panel the panel is to power my new kitchen which is my second kitchen I have another kitchen in the basement.I have a 200 Amp main service panel wire size is the question everything I find is very confusing, #4 copper or #2 al copper very difficult to find in my area which is up state NY

I would take a wild guess, and say that is because local authorities have prevailed on the home centers to not sell the stuff. that would be to force installation of high-amp devices towards licensed and bonded electricians who won't screw up the job.

you can't mess that up. there is a reason the electrical code is owned by the national fire prevention association. the reason is.... uh... ahhhhh... gee, I dunno.

I would be very surprised if local authorities could tell a retailer what they may or may not sell. Restrain of trade is a federal offense methinks.
According to NEC 310.15{B}{6} [conductor sizing] 100 amp service or feeder rating {amps} You can pull either #2 aluminum or copper clad aluminum; or #4 copper. Remember you have to pull 2 hots a neutral and a ground. The ground wire can be sized one size small than the other three conductors, if that helps. I think this is what you are after. I would also size the PVC sch 40 @ 1.25" that you would have install to protect the conductors. 3 feet or 53 feet it still requires protection.
I wonder aloud why you are not using SER cable to make up the connections. Type XHHW-2 or THHN/THWN-2 SER 3conductor Al with bare ground size 2-2-2-4 AWG rated @ 100 amps in dwelling would work as well. Might be somewhat more difficult to work with but an option.

This message has been edited. Last edited by: CommonwealthSparky,

Adding of course you are confident in your electrical abilities and able to pull the necessary permits and inspections. Good luck.

One thing to keep in mind is that the neutral is not connected to ground in any downstream panel. Look it up in NEC code, Article 384-24, grounding panel boards. The panel box should be grounded, but the neutral should not be connected to ground. A great many nonprofessional electricians do not know this. The only place the neutral is grounded is at the service entrance. Basically it says the terminal bar, that is the grounding point, shall be bonded to the cabinet or panel board frame and shall not be connected to the neutral bar in other than service equipment. Service equipment is the main box where the power enters the house.