But the connections will be rated 60 deg C in those sizes so you have to use the 60 degree ampacities column. Whatever derating of the conductors that might be required can be subtracted from the 90 deg C column, but you cannot exceed the 60 deg C rating. You have to use the 60 degree rating for circuits rated 100A or less or for #14AWG through 1 AWG. See NEC 110.14(C)(1).I see comments about using #1, #2 and #3 wiring here which I find confusing when, according to the NEC 310-16, the allowable ampacities for #4 and #6 THHN are 95 amps and 75 amps respectively.
Based on the chart below, the #4 AWG coming from the panel is well within the range of 125% of circuit size (70*1.25 = 87.5 amps). And so is the #6 AWG between the VMC and the RPC, considering the VMC's rating of 44 FLA (44*1.25 = 55 amps). And knowing that the RPC and VMC are the only two loads on this circuit, if I combine the RPC's idling 20 amps and the VMC 44 FLA, I'm still at 64 amps (if that is the correct math).
So considering all that, and knowing that the manufacturer of my RPC said "the RPC can peak at 70 amps during startup, but it will never draw above 70 amps, even under load," why would I need to use #1, #2 or #3 wiring? This seems pretty straight forward to me, but I'm not an electrician and I'm always willing to learn from people with more experience than me. Am I missing something here?