(2*L * R *I)/1000 particular formula is essentially derived from:
2 * L ; Hot and return as was said
R/1000; is from the table where R is ohms/1000 ft, thus (R/1000)/1000 is R
You have to use the table for aluminum and for the specific wire gage.
P = V * I works when the loads are resistive and thus V is an RMS value and I is an RMS value and it would be equlivelent to DC.
Remember that the NEC also classifies loads as Continuous such as space heating and they will be derated.
The service entrance calcs will be quite different. You wont have the AC and heat on at the same time, for instance. Your home stove won't be on for more than 3 hours at a time and would not be continuous, but in a restauraunt it would be.
The NEC stuff is all based on self-heating, so type of cable, in conduit, ambiant, motor, how long it's going to be on all enter in. Tolerable drops are on the order of 3 to 5% max depending on use and ratings.