Acceptable voltage drop limit for 24v dc solar power system

I don't think there are requirements for max voltage drop. There are recommendations from many sources. Typically for AC, 5% max from Source to load is accepted. Hence the stated 2% for MCC bus feeder and 3% for bus to Motor...I know mathematically, this is really 4.94%.

The other thing to consider is your regulated source. For AC this could be the grid, or a Generator voltage regulator or a Load Tap Changer. I would assume the grid and voltage regulator to be digital with fast response, whereas a LTC is much slower to respond to voltage fluctuations. For DC, the Power supply could be your voltage regulator or maybe it is your battery charger.

Most AC equipment have stated 10% tolerance minimum. Same for most DC electronics, however from past experience, electronics seem more sensitive to voltage fluctuations. Furthermore electronics seem to have the consequence of burning out, whether it be from high current on voltages below tolerance or voltages above tolerance. Whereas AC equipment will suffer minimal damage for short duration voltage fluctuations exceeding their stated tolerances.

So the design of 5% plus fluctuation in the Source should be below 10%.

Anyway, I suggest to target 2.5% and accept up to 5%. As others mentioned, spend the money and design/do it right the first time....it is typically easily justified. It is almost the same as ordering cable extra-long.