A 12-inch ladder tray loaded with smaller power cables only, checked against the area-based fill rule for cables under 4/0 AWG.
| Tray type | Ladder tray |
| Category | Power/mixed cables |
| Tray dimensions | 12 in wide x 4 in deep |
| Cables (all smaller than 4/0 AWG) | 6 cables, 0.6 in diameter each |
With no 4/0 AWG-or-larger cables present, NEC §392.22(A)(1)(b) applies: total cable cross-sectional area must not exceed a percentage of the tray's cross-sectional area, with usable depth capped at 3 inches for this category.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Cable tray fill area | ≤ 14.004 in² | 1.696 in² | ✓ PASS |
Key insight: The 3-inch depth cap in the area formula means a deeper tray doesn't keep buying more allowed fill area indefinitely — beyond 3 inches of usable depth for this cable category, NEC caps the calculation at 3 inches regardless of the tray's actual physical depth, so oversizing tray depth has diminishing returns for fill capacity specifically (though it may still help with cable bend radius or future additions).
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Open Cable Tray Fill calculator →Single-conductor cables use a sum-of-diameters rule against tray width instead of an area-against-depth rule, and are not permitted in solid-bottom trays at all under §392.22(B) — the area-fill approach assumes multiconductor cables stacking somewhat randomly, which doesn't represent how single-conductor cables (often run in trefoil or flat formation for magnetic field cancellation) actually sit in a tray.
Once any cable reaches 4/0 AWG or larger, NEC switches to a hybrid width-based rule combining a sum-of-diameters term for the large cables with an area term for the smaller ones — reflecting that large cables are assumed to lie in a single layer across the tray width rather than stacking, which the pure area-fill rule for small cables doesn't capture.