I hate to be the spoil sport here but the code is wrong.
The whole derating issue assumes all conductors will be on separate circuits so the heat adds. If they are on the same circuit it actually subtracts where they are paralleled.
If the total amps is 15 and it is spread out across 1 pair of conductors "in" and 5 "out" pairs of conductors the heat generated in the insulation is ~60% of what it would be with a single Romex in and out. Resistance of the "outs" is 1/5th of a single pair and total heat is still I2R.
The only way this current could add is if this was a series loop and full current comes in and out 3 times.
