The derating tables are there so that engineering will not have to be done for each specific case.
In many cases more wires can be sefely run without derating.
Consider a 100amp service with only 50amp 240v breakers all using #6 wire (I think that is a correct size) and the #6 wires run in conduit.
2 circuits or a dozen circuits can only draw 100 amps nd any number of circuits produes the same amount of heat.
The temperature in the conduit is determined not by the number of wires or circuits but by the diameter of the conduit. Larger conduits will have a lower internl temperature. Make the conduit large enough and you do not need to derate.
Most houses around here have the majority of circuits running between studs above the main panel. 40 or 60 current carrying wires in an insulated space. Perhaps engineering or temperature measurments should be taken there.
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The 50 motor problem. I would use sub panels.
