My Friend is refinishing his basement (1100SF). He currently has a 200Amp Service, but the panel does not have enough Spaces. We want to put in a 100 Amp sub panel in the Basement to include:

50Amp- 5 Ton air conditioning
20 Amp- Dedicated Bathroom Circuit
15 Amp- Lighting Circuit
15 Amp- Receps
15 Amp- Receps
15 Amp- Bedroom

My Main question is what size should I Use for the feeders?
According to NEC table 310.15(B)(6) I could use #4's for the Hots and neutral, but I have also read other places that you cannot use this table And need to use table 310.16

If I can use table 310.15(B)(6) I was planning on running 3 #4 (CU), and 1# 8 (CU)

Voltage Drop will not be an issue, the run will be less than 20'.

It's a bit confusing, the table references. #2 copper is rated for 100amps, but on service wire #4 copper is rated for 100 amps. I would use the #2, even if it is overkill.
Hopefully some others will chime in this thread with suggestions and comments. And you know to separate the neutral and the ground at the sub panel, correct? And you are correct that voltage drop won't be an issue in that short run.

So do I need to use 310.16 or 310.15B6? I don't think I need to go overkill as long as it is safe and within code.

Table 310.15-B-6 is only for "main power feed between the main disconnect and the panelboard" that serves all loads associated with the dwelling.

Any branch circuit leaving the man panel, which includes a feeder to another panelboard, must use Table 310.16.

Minimum #3 copper , either THHN/THWN or XHHW insulation, as listed in the 75 Deg C column, is allowed.

Use of the 90 Deg C column is not allowed unless the equipment is rated 90 Deg C, and this is special order, so rarely seen.

If all you need is more spaces you could switch out about 7 breakers for twin or slim line (what ever your brand calls them) and eliminate the need for a sub panel?

TK, Thank you very much for clearing that up.

hkstroud, most of the breakers in there are the tandem breakers. There are only three unused spaces 2 of which will be taken up by the 100 Amp Breaker

Do they make #3/3 wire ?