How does NEC 392.22 limit cable tray fill?

For multiconductor cables 4/0 AWG and larger, the sum of cable diameters must not exceed the tray width. For smaller cables, the total cross-sectional area must not exceed the allowable area from Table 392.22(A), which depends on tray type and width.

Why are 4/0-and-larger cables treated differently?

Large cables are stiff and lie in a single layer, so the code limits them by the sum of their diameters against tray width rather than by area. Smaller cables can bundle, so an area-fill limit governs them instead.

How are mixed cable sizes handled?

When both large and small cables share a tray, the large-cable diameters occupy part of the width and the remaining width is available for the area-method cables. The tool applies this combination so both rules are satisfied together.

Why does a solid-bottom tray allow less fill?

Solid-bottom trays restrict airflow around the cables, reducing heat dissipation, so NEC Table 392.22(A) gives them a smaller allowable fill area than ladder or ventilated-trough trays of the same width.

Does cable tray fill affect ampacity?

Yes. Fill and the number of conductors interact with the ampacity derating rules in NEC 392.80. Staying within the fill limit is the first step, but you must also confirm the cables carry their required current after any applicable derating.

Cable Tray Fill Calculator (NEC 392)

NEC Article 392 governs how many cables you can run in a tray. Overfill a tray and the cables overheat and become hard to maintain; this calculator applies the 392.22 fill rules for ladder, ventilated-trough, solid-bottom and channel trays so you can verify a layout before you install it.

How it works

NEC 392.22 splits cables by size. Multiconductor cables 4/0 AWG and larger are limited by the sum of their diameters against the tray width:

Σ(diameters) ≤ tray width

Smaller multiconductor cables are limited by total cross-sectional area against the allowable from Table 392.22(A), which varies by tray type and width. When both sizes share a tray, the large cables occupy part of the width and the remaining width sets the allowable area for the smaller cables. Single-conductor cables from 250 to 900 kcmil use the Table 392.22(B) area column, and 1000 kcmil and larger use the diameter-sum rule.

Worked example

A 12 inch ladder tray (allowable area 21 sq in) carrying six 0.75 inch and three 1.2 inch multiconductor cables (all under 4/0), plus two 1.8 inch 4/0 cables:

Σ large diameters = 2 × 1.8 = 3.6 in  ≤ 12 in  → ok
width left for area method = 12 − 3.6 = 8.4 in
allowable area for 8.4 in ≈ 21 × (8.4/12) = 14.7 sq in
small-cable area = 6 × π(0.375)² + 3 × π(0.6)² ≈ 2.65 + 3.39 = 6.04 sq in
6.04 ≤ 14.7 → PASS

Tips and notes

Use the outside diameter of each cable, not the conductor size, when entering values. Solid-bottom trays carry a lower allowable area than ladder or ventilated trays because they trap heat. Passing the fill check is necessary but not sufficient: confirm the cables still meet their required ampacity after the derating rules in NEC 392.80, since fill and conductor count both affect heat dissipation.