What are common mistakes in conduit fill calculations?

The classic conduit fill mistakes: forgetting that equipment grounds count toward fill, borrowing EMT capacity numbers for PVC whose inner diameter is smaller, and treating flat cable like round wire when Note 9 says to use its widest dimension.

The most expensive one is separate from fill entirely: packing a legal 40 percent pipe with current-carrying conductors and skipping the ampacity derating that starts above three. A pipe can pass fill and still fail thermally.

Runner-ups: using interior conduit dimensions from the wrong schedule, ignoring conductor insulation type since THW is fatter than THHN, and forgetting the 60 percent nipple allowance only applies at 24 inches or less.

Each error traces to using one table where two applied.

The conduit fill calculator applies the right table for the raceway, insulation, and count automatically.