Cable Tray Fill Chart
How much cable a tray may legally carry: the allowable fill areas of NEC Table 392.22(A) for multiconductor power cables by tray width, the tighter solid-bottom column, and the single-conductor rules of 392.22(B) — plus the Sd formulas that govern whenever 4/0 and larger cables share the tray. Sum your cable cross-sections, find your width, and check you're under the number.
Allowable fill area by tray width
| Tray width (in) | Ladder / vented, multi <4/0 (in²) | Solid bottom, multi <4/0 (in²) | Single cond. 250–900 kcmil (in²) |
|---|---|---|---|
| 6 | 7.0 | 5.5 | 6.5 |
| 9 | 10.5 | — | — |
| 12 | 14.0 | 11.0 | 13.0 |
| 18 | 21.0 | 16.5 | 19.5 |
| 24 | 28.0 | 22.0 | 26.0 |
| 30 | 35.0 | 27.5 | 32.5 |
| 36 | 42.0 | 33.0 | 39.0 |
The Sd formulas — mixing in big cables
Sd is the sum of the outside diameters, in inches, of every 4/0-or-larger cable in the tray (1000 kcmil and larger, for single conductors). Those cables claim a single-layer strip, and the printed formulas shrink the area budget for everything else: ladder / ventilated trough: Column 1 − 1.2·Sd · solid bottom: Column 3 − 1·Sd · single conductors mixed with 1000 kcmil+: area − 1.1·Sd. When the large cables are alone, skip the area math entirely: single layer, summed diameters no greater than the tray width. The practical sizing shortcut is to compute the width each group needs separately — Sd inches for the big cables, area ÷ (area-per-inch-of-width) for the small — and add the widths.
What this table doesn't cover
Control and signal cables get their own, more generous rule: trays with a usable depth of 6 inches or less may fill 50% of their cross-section (40% in solid-bottom pans) — area-based, no table lookup. Ventilated channel trays carry their own small fill table in 392.22. And fill is only the geometry half of tray design: conductor ampacity in tray follows 392.80 (which can invoke the bundling adjustment factors when cables stack), and if the tray itself is your equipment grounding conductor, its metal cross-section must clear the cable tray grounding chart. As always, verify against the code edition your jurisdiction enforces.
Common questions
How full can a cable tray be?
For multiconductor power cables smaller than 4/0 in a ladder or ventilated-trough tray, the summed cable cross-sections may occupy the Table 392.22(A) Column 1 area — 21 in² in an 18-inch tray, which works out to about 39% of a 3-inch loading depth. Solid-bottom trays are tighter (16.5 in² at 18 inches), and cables 4/0 and larger don’t use area math at all: they lie in a single layer whose summed diameters fit the tray width.
Why do 4/0 and larger cables have to be in a single layer?
Heat. Big cables stacked on each other can’t shed heat into the airflow the ampacity tables assume, so the code keeps them one layer deep with their combined diameters (Sd) no wider than the tray. When they share a tray with smaller cables, the printed formulas carve their strip out of the area budget: Column 1 minus 1.2·Sd in ladder tray, Column 3 minus 1.0·Sd in solid-bottom.
Can I run single-conductor cables in cable tray?
Yes, but under 392.22(B) rules: 1/0 AWG is the minimum size, ventilated trays only (never solid bottom), and the fill method depends on size — 1000 kcmil and larger lie in a single layer with summed diameters within the tray width; 250 through 900 kcmil use the area column on this page; 1/0 through 4/0 also go single-layer, diameter-summed. Many specs additionally require bundling each circuit’s phases together to limit unbalance.
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