It's best to size load by Watts (or Volt-Amps). The NEC has some minimums that must be included that often push homes over 100 Amps. I'm assuming electric applinaces.
220.82(B)(1) = 3VA per square foot. Assume 1,500 sf home = 4,500VA
220.82(B)(2) = 4,500VA (2 kitchen circuits + 1 laundry at 1,500VA each)
220.82(B)(3) = 5,000VA clothes dryer; (220.54)
= 4,500VA water heater;
= 8,000VA range. (220.55)
220.82(B)(4) = any other motors like pumps.
220.82(B) = 4,500 + 4,500 + 5,000 + 4,500 + 8,000 = 26,500VA
= 40% anything above 10,000VA = 16,600VA = 69.2 Amps @ 240V
220.82(C) = Add A/C nameplate. 2-ton is about 17.6 Amps
69.2A + 17.6A = 86.8 Amps @ 240 Volts. This is the bare minimum for a 1,500sf house. And other known loads connected need to be added. I'd suggest new homes get a 200A service size.
The 80% rule is from 210.20. It is often misunderstood, even by inspectors and electricians. Loads are separated into two categories, continuous and noncontinuous loads. All breakers and fuses are rated for 100% of noncontinuous loads + 125% of continuous loads.
The inverse of 125% is 80%. So a 100A panel can only hold 80A of continuous loads. Section 100 defines continuous load as any load that runs at peak amperage for 3 hours or more. This is different than being "on" for 3 hours. I contacted a kiln manufacturer when I was green because their listed amperage was 49.5A on a 50A outlet. I thought I knew more than them and pointed out the 80% rule, kilns run all night, well more than 3 hours. They pointed out to me that the kiln cycles on and off to maintain the temperature inside. The peak amperage is not continuous. I had a "duh" moment.
There's not a lot of stuff that runs at peak amperage for 3 hours or more.