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 widthSmaller 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 → PASSTips 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.
Tray type and fill limit comparison
NEC Table 392.22(A) sets the maximum allowable fill by tray type and width. Ladder and ventilated-trough trays allow the most fill because air can circulate around cables on all sides. Solid-bottom trays restrict convective airflow under the cables, so the NEC assigns them a lower allowable area for the same tray width.
| Tray type | Fill method | Typical allowable area at 12 in width |
|---|---|---|
| Ladder | Area (small cables) / diameter (large) | ~21 in² |
| Ventilated trough | Area / diameter | ~21 in² |
| Solid bottom | Area / diameter | ~16 in² |
| Channel | Special limits (smaller size) | Per Table 392.22(A) |
The exact values come from the NEC table — use these figures for direction only; always verify against the current edition of NFPA 70.
Interaction with NEC 392.80 ampacity derating
Passing the fill limit does not mean your cables meet their required ampacity. NEC 392.80 requires an ampacity correction when the number of current-carrying conductors in the tray exceeds a threshold. The derate factor is applied to the ampacity the cable would otherwise have from Table 310.15(B)(16) or its manufacturer’s table.
The key rule: if a multiconductor cable tray is not ventilated and contains more than three current-carrying conductors, adjustment factors apply. Engineers often find that a tray can pass the fill check while a subset of the cables in it need to be upsized to compensate for the ampacity derate.
Segregation and separation requirements
NEC 392.6 addresses how different circuit types should be separated or segregated within a tray system. For example:
- Power conductors (600 V or less) and Class 1 control conductors may share a tray if certain conditions are met.
- Class 2 and Class 3 circuits generally must be separated from power conductors.
- Medium-voltage cables (over 600 V) have their own segregation requirements and typically cannot share a ladder tray with low-voltage power cables without a solid barrier.
The fill calculator addresses total cable area, not segregation. Always review Article 392 and the circuit-type rules in Articles 725 and 800 for mixed-system trays.