Cable Tray Fill Calculator
Check NEC 392.22 cable tray fill for multiconductor and single-conductor cables. Get fill %, pass/fail, and the minimum tray width for ladder, ventilated, and solid-bottom trays. Free, no sign-up.
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Explanation
Cable tray fill determines how many cables a tray can carry under NEC 392.22 for installations rated 2000 V or less. The code splits the rules two ways: by how the cables are built (multiconductor in 392.22(A) versus single-conductor in 392.22(B)) and by conductor size, because large cables must lie in a single layer while smaller cables may be stacked. This calculator applies the correct Table 392.22 limit, reports the required tray width, and checks the fill of a width you choose.
How the calculation works
Cables 4/0 AWG and larger (and single conductors 1/0–4/0 AWG or 1000 kcmil and larger) are limited by the single-layer rule: the sum of their outside diameters, Sd, must fit across the tray. Smaller cables are limited by cross-sectional area, A, against the maximum allowable fill area from Table 392.22(A) or 392.22(B)(1). The required inside width combines both contributions:
| Symbol | Meaning |
|---|---|
| W | Required inside tray width (in) |
| A | Sum of cross-sectional areas of cables under the area rule (in²) |
| Sd | Sum of outside diameters of single-layer cables (in) |
| R | Allowable area per inch of width (1.167 ladder, 0.910 solid bottom, 1.083 single-conductor) |
| D | Single-layer width factor (1.0 ladder, 0.9 solid bottom) |
Maximum allowable fill area
The area columns below are the maximum allowable fill area for cables under the area rule, in ladder or ventilated-trough trays. Solid-bottom trays reduce the multiconductor area allowance by about 22% and limit single-layer diameters to 90% of the tray width.
| Inside width (in) | Multiconductor <4/0 — Table 392.22(A) | Single-conductor 250–1000 kcmil — Table 392.22(B)(1) |
|---|---|---|
| 6 | 7.0 in² | 6.5 in² |
| 9 | 10.5 in² | 9.8 in² |
| 12 | 14.0 in² | 13.0 in² |
| 18 | 21.0 in² | 19.5 in² |
| 24 | 28.0 in² | 26.0 in² |
| 30 | 35.0 in² | 32.5 in² |
| 36 | 42.0 in² | 39.0 in² |
Choosing the right rule
The 4/0 AWG boundary is the key decision in 392.22(A). At 4/0 and larger, multiconductor cables are heavy enough that the code requires a single layer, so width — not area — is the limit. Below 4/0 the cables may be stacked and the area table applies. When a tray carries both, the large cables take a single-layer zone and the smaller cables fill the remaining width, which is exactly how the formula above combines Sd and A.
Single-conductor cables follow 392.22(B) instead and must run in ladder or ventilated trays only. To convert a conductor size to an outside diameter and cross-sectional area for these calculations, use the AWG wire size chart, which lists conductor area, diameter, and NEC 310.16 ampacity for copper and aluminum.
Notes and limitations
This tool checks the physical fill limits of NEC 392.22 only. It does not perform ampacity adjustment for the number of current-carrying conductors (NEC 392.80), and it does not cover cables rated over 2000 V, which are sized under 392.22(C)–(E). Always plan for future expansion, confirm conductor ampacity, and verify the final design against the current NEC and the authority having jurisdiction.