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.
Example
You enter
- Interior width (in) 4
- Interior height (in) 4
- Total conductor area (in², Ch.9 Table 5) 2.5
- Current-carrying conductors 18
You get
- Interior area (in²) 16
- Allowed fill (20%, in²) 3.2
- Used fill 15.625
Details, formula, and sources
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.
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.
NEC is free to read at nfpa.org/freeaccess. The 30-conductor limit excludes signal and certain conductors per the article exceptions; exceeding 30 current-carrying conductors triggers ampacity adjustment, not prohibition.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: width_in, height_in, conductor_area_in2, ccc_count, interior_in2, allowed_in2, used_pct
- Fill limit conductor cross-section no more than 20% of the wireway interior cross-sectionNEC 376.22(A) / 366.22(A)
- Conductor count more than 30 current-carrying conductors requires the 310.15(C)(1) adjustment factorsNEC 376.22(B)