Thank you, LK. Let me clarify.
The existing wiring is more than adequate. The house is only 10 years old. The purpose of the subpanel was more to save money running the needed wire back to the main panel since copper is so expensive.
The subpanel will have an 8/3 feed that already exists (no new load). From what I have read it seems the total load calculation can be approximated by ~3 watts (VA) per square foot of new area. Since these are bedrooms, there are no big appliance/motor loads.
In doing the NEC method it seems all I need is ~11 amps@240 volts. I agree I may need many individual circuits (smoke, GFI etc) but the total load this subpanel would need to supply would be significantly less than 40 amps@240 volts I have available... correct?.
Here's the calculation:
General Light and Receptacle Load
600 (2@300)sqft x 3VA = 1800VA
Total General Load Less Demand
sum of above less T220.11 demand
sum of above = 1800VA
first 3000 at 100 % = 3000
next 117,000 at 35% = -420
remainder at 25% = 0
sum less demand = 2580VA
Computed Amperage
2580 divided by 240 = 11Amps
