My best guess, given insufficient information, is that you should use type SER cable or individual conductors in conduit, using #3 copper for the hots, #3 copper for the neutral, and #8 copper for the EGC. Depending upon your specific circumstances, you may need to use anything from a cable with 3#1 conductors and 1#6 conductor, to a cable with 4-4-4-6, to individual wires in conduit. Still others permit non-metallic cables, but do not permit the use of 310.15(B)(6). Other regions permit you to use table 310.15(B)(6) for residential _feeders_. Some regions _require_ that all wires be run in conduit, and prohibit the use of non-metallic cables. You must also consider the rules and interpretations of your local code enforcement authority. You must also consider the temperature rating of the insulation, the wiring method used, and the temperature ratings of the terminations. The ampacity of a conductor is _not_ just based upon its size. But even here there are details to be considered. Once you pick the 'ampacity' of feeder that you need, you then need to select the correct wire size to use. If you sit down, and calculate out _now_ what size AC you would require (how many 'tons' of cooling), and then get the circuit ratings for AC systems of that size, then you could calculate out how much of a feeder you really will need. There is a _very_ good chance that this subpanel and feeder would be 'overkill', and that a 60A subpanel would be sufficient, but if you include the central AC the 60A subpanel becomes questionable. If you choose the latter approach, then just install a 100A subpanel. The big unknown is the size of the central air unit, and you never specified how large the 240V heater is. _Or_ you need to simply oversize the feeder to the subpanel to take care of all reasonably possible future uses. What you need to do is a 'load calculation' (also called a feeder calculation or a demand calculation) to see how large a feeder to this subpanel is necessary. There are almost certainly details that you don't even know how to ask about. The answer to your individual questions will probably not tell the whole story. Electrical installations are all about the details.
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