Wireway / Auxiliary Gutter 20% Fill (NEC 376.22)
Sizes a metal wireway (and, by 366.22, an auxiliary gutter) against the NEC 376.22 limits: conductor cross-section no more than 20% of the interior, and the 30 current-carrying conductor threshold for ampacity adjustment (4x4 in carrying 2.5 in^2, 18 CCC -> 15.6% fill, within). Over 30 conductors triggers a 310.15(C)(1) adjustment, not a prohibition; signal conductors and the article exceptions are excluded from the count. The AHJ governs.
Formula and source
interior = width x height; allowed = 0.20 x interior; used% = conductor_area / interior x 100; over 30 current-carrying conductors triggers the 310.15(C)(1) ampacity adjustment.
Metal wireway conductor fill and number of conductors, NEC 2023 376.22; the same 20% rule applies to auxiliary gutters (366.22), by name.
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