How much voltage drop is acceptable?

Acceptable voltage drop is 3 percent on a branch circuit and 5 percent for the feeder and branch combined. That comes from the NEC's informational notes to 210.19 and 215.2.

Strictly speaking it is a recommendation, not an enforceable rule, except in a few special cases like fire pumps and sensitive electronics circuits where the code makes it binding.

In practice 5 percent is where problems start. Motors run hot, LED drivers flicker, and long 120V runs feel it worst because every volt is a bigger share of the total.

On a 12V system the same percentages bite ten times harder, so low-voltage designers usually hold drop under 3 percent.

Check any run in seconds with the AC voltage drop calculator, which cites the formula it used next to the result.