Hi
I have a 200 amp main panel, but it is a small 24 slot panel. It is full and I need to add three more circuits. I want to run a 100 amp main lug subpanel. The plan is to move the clothes dryer breakers to the new sub and use the existing location in the main panel to install a 100amp breaker to supply the subpanel. The sub is going to be about 30 feet from the main. My first question is what size wire do I need to do this. I have been told everything from 6-3g to 2-2-6-8 The heaviest single load I forsee being on this subpanel is the clothes dryer My second question is since they make 200 amp panels with anywhere from 24 to 42 slots (mine being the 24 slot) should I have any concern that I maybe overloading this panel? I can't see a size marking on the SE cable but it looks to be at least .25 inch thick

You may be able to install twin breakers in the main panel, and say forget the sub.

Twin breakers will help you make some room for the new circuits you plan to install.

Post your service panel information, and we can tell you if installing twins are possible.

Also, only you will no if you are overloading the existing service rated panel, by doing a demand load calculation.

Are your appliances gas or electric? Do you have any heavy players like an electric furnace, tankless water heater, etc..

Cable size 120 mm2 with matching lug

#2 ser is best for this. Easy to use and priced right. Around here the inspector doesn't flinch at a 100 amp breaker, though I've been told the 2008 NEC has it at 75 (?). Use antioxidant and you'll be good.

cable size 120 mm2 with matching lug

Judging from his post, unless otherwise stated we have to assume the OP is in the US or Canada.