Conduit fill is a simple geometric ratio with a code limit attached: sum each cable’s cross-sectional area as π(OD/2)² times its quantity, divide by the raceway’s internal area, and compare. NEC Chapter 9 caps conduit at 40% fill when it carries more than two conductors; multi-conductor cables in cable tray get a more generous 50% under NEC Article 392. Selecting a raceway means finding the smallest standard conduit diameter — or tray width and depth — whose internal area keeps the total cable area under the applicable limit.
Fill is only half of what crowding a raceway costs. Bundled conductors cannot shed heat, so the ampacity of every cable in the group drops: the NEC grouping factor of Table 310.15(C)(1) steps down from 1.0 for one to three current-carrying conductors to 0.80 for four to six and lower beyond, and the ambient-temperature factor of Table 310.15(B)(1) multiplies on top. Six conductors in a raceway at 40 °C combine to 0.80 × 0.88 = 0.704, turning a 100 A cable into a 70.4 A one — a raceway can pass its fill check and still leave its conductors short on capacity.