Cable Tray Fill Calculator and Bundle Calculator

Cable tray fill formula

Tray fill is total cable cross-sectional area over usable tray area. One round cable of outer diameter d has an area of pi x (d/2)², so n cables occupy n x pi x (d/2)². The tray's usable area is inside width x loading depth, where loading depth is the depth you intend to fill rather than the full side height.

Fill (%) = 100 x N x pi x (d / 2)² / (W x H), where N is cable count, d is cable outer diameter, W is tray inside width and H is usable loading depth. Use the same length unit throughout. For several cable diameters, calculate each group's area and add those areas before dividing by W x H.

The dropdown diameters are illustrative inputs, not specifications for every cable in a category. If your datasheet gives a different diameter, use it in the formula. Count round cable jackets, not conductors or fibres inside them. A duplex assembly with two separate legs cannot be treated as one round cable without checking its actual shape. Use inside dimensions rather than the tray's outside width, and reserve space taken by dividers or other obstructions.

Bundle outer diameter uses small-bundle ratios for one through seven equal round cables: one cable has its own diameter, and two side-by-side cables span twice that. Above seven, D = d x the square root of 4N/3 is a lattice approximation. The second result adds an illustrative 12 percent clearance allowance. Neither figure includes connector bodies, crossovers or a guaranteed installed shape. The bundle diameter guide explains the geometry and its limits.

Two-layer estimate is 2 x floor(W / d). In a 300 mm tray with 7.4 mm cables, 40 fit side by side, giving 80 in two straight layers. This is a layout scenario, not an NEC capacity rating or a promise that every lead will remain removable. Crossovers, straps and drop-outs still need space. Do not substitute the bundle's enclosing-circle area for the sum of individual cable areas when calculating tray fill.

NEC 392.22 fill limits by tray and cable type

This reference separates common cases; the calculator above only computes geometry. Eaton's manual explains the 2014 NEC provisions, while Legrand's Cablofil selector lists corresponding cable and tray cases. Confirm the adopted code edition, cable listing and manufacturer's tray classification before applying a row.

Wire basket is not assigned a rule solely by its appearance. Legrand lists control, signal or data under the open-bottom case in its selector; use the documentation for the basket you specify. Telecom runway also needs its own manufacturer guidance. The NEC limits and tray sizing guide explains classification and access planning in more detail.

Worked examples for basket, ladder and solid tray

These examples use round cables of an assumed 7.4 mm outside diameter. Each cable occupies pi x 3.7² = 43.0084 mm². The dimensions are illustrative usable areas, not product specifications.

Basket: reserve space beyond the percentage

The default basket example gives 100 x 4,128.8 / 30,000 = 13.8 percent. Its bundle estimate is 83.7 mm, or about 94 mm with the illustrative allowance. Yet 96 cables exceed the separate two-layer estimate of 80. Decide how the cables will be grouped and reached at drop-outs before treating unused cross-sectional area as available capacity.

Ladder: compare with the correct cable case

For the control/signal-only case described in 392.22(A)(2), 50 percent of 15,000 mm² is 7,500 mm². The example's 6,193.2 mm² is below that area limit. This comparison does not check cable eligibility, rung support, load rating, ampacity or access. Adding power cables changes the case and can invalidate that percentage comparison.

Solid bottom: the same geometry can exceed the limit

For the control/signal-only case in 392.22(A)(4), 40 percent of the same tray area is 6,000 mm². The example exceeds it by 193.2 mm². Geometrically, floor(6,000 / 43.0084) gives 139 cables at that limit. The contrast with ladder illustrates why selecting tray type is necessary before interpreting a fill percentage.

What the fill assessment means

The output bands use 25 percent as an illustrative initial target and 40 percent as a comparison point. They are planning labels, not code pass/fail results. To estimate a count at a chosen fraction f, use floor(f x W x H / (pi x (d / 2)²)). A larger tray depth can lower the displayed percentage while leaving the width for accessible layers unchanged. Check the applicable credited depth and avoid assuming that taller rails always permit more cable.

What this does not tell you

• Bend radius at drop-outs. The turn out of the tray and down into a rack is usually what runs out of room first. See bend radius and rack layout.
• Heat from remote power. Current in a bundle heats its centre, and allowable current per conductor falls as bundle size grows. A tray of Power over Ethernet runs is not the same installation as an identical tray of unpowered links.
• Load and span. Tray is rated for a working load at a given support spacing. Multiply cable mass per length by cable count and check the manufacturer's span/load table. Pathway type changes both the rating and how the cable is supported.
• The real cable. Outer diameter is squared in the fill calculation, so take it from the datasheet of the cable being bought. See cable bundle diameter.

Sources and assumptions

The worked numbers are derived from the stated dimensions and circle-area formula. The 12 percent allowance and two-layer target are explicit planning assumptions. Installation references are:

• Eaton Cable Tray Manual, 2014 NEC edition, pages MAN-23 and MAN-24 for the selected fill cases.
• Legrand Cablofil cable fill reference, for cable-type and tray-type distinctions.
• NFPA 70 code editions and access, for checking the applicable requirements.
• NVIDIA cable management guidance, for strain relief, access and airflow considerations.

All guides