Wire & Cable calculator
Cable Tray Fill Calculator
A 12 in ladder tray loaded to 4 in depth has 48 sq in of tray area; with 24 #12 THHN conductors at 0.0133 sq in each, the screen is about 0.319 sq in of cable area and 0.7% occupancy. This page is a preliminary cable-tray occupancy screen for early layout work. It adds cable planning area, compares that area against the tray area you entered, and shows a simple occupancy percentage and remaining-space screen. It is intentionally narrower than a full NEC 392 tray-fill review because actual code treatment changes with cable construction, tray type, voltage class, single-conductor versus multiconductor layouts, and one-layer width rules.
Updated June 21, 2026
A 12 in tray loaded 4 in deep has 48 sq in of area. With 24 x #12 THHN conductors, the page screens only about 0.319 sq in of cable area, or 0.7% occupancy.
Tray occupancy % = Total cable area ÷ tray area × 100
Enter tray width, planned loading depth, cable family, size, and cable count for a preliminary tray-occupancy screen
Calculator Inputs
Calculation Results
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Calculation history
Example Calculations
12 in ladder tray with 24 THHN conductors
Basic early occupancy screen for a lightly loaded open tray.
- Tray Type: Ladder tray
- Tray Width: 12
- Usable Depth: 4
- Voltage Class: Below 2,000 V
- Cable Type: THHN
- Cable Size: 12
- Number of Cables: 24
24 in solid-bottom tray with 12 MC cables
Conservative planning screen for a larger tray and multiconductor cable set.
- Tray Type: Solid-bottom tray
- Tray Width: 24
- Usable Depth: 4
- Voltage Class: Below 2,000 V
- Cable Type: MC 3 C
- Cable Size: 4
- Number of Cables: 12
How to Use
How to use the cable tray fill calculator
- Select the tray type and enter the tray width and the planned loading depth.
- Choose the voltage class. On this page, the higher-voltage option mainly triggers a warning so you do not mistake the result for a final medium-voltage layout check.
- Select a cable family, cable size, and number of cables.
- Review the tray area, total cable area, occupancy percentage, and the page's planning reference area.
- Use the result as an early space check only. Final tray design still needs the actual cable outside diameter or published area and the correct tray-fill method for the installation.
What the page returns
| Output | Meaning | How to use it |
|---|---|---|
| Tray area | Width multiplied by planned loading depth | Check whether the chosen tray geometry is even close to workable |
| Total cable area | Planning area per cable multiplied by cable count | Compare rough space usage between tray options |
| Occupancy percentage | Cable area divided by tray area | See how tightly packed the tray would be |
| Planning reference area | Simple screening benchmark used by this page | Start a conversation about tray size, not a final code sign-off |
Important scope notes
- This page does not return an NEC 392 compliance verdict.
- Single-conductor tray layouts often need separate width-based and installation-condition checks.
- Medium-voltage and over-2000V layouts frequently move to one-layer width rules instead of simple area percentages.
- Solid-bottom trays often need the most conservative final review because ventilation is more limited.
Example: a 12 in ladder tray loaded to a planned 4 in depth has 48 sq in of tray area. If it carries 24 conductors with a planning area of 0.0133 sq in each, total cable area is about 0.319 sq in and occupancy is only about 0.7%.
Second example: an 18 in wire-basket tray with a planned 3 in loading depth has 54.00 sq in of tray area before comparing the 36 #10 THHN conductors against the selected planning reference.
Use the Conduit Fill Calculator for raceway work, the Wire Size Calculator for conductor sizing, the Cable Ampacity Calculator for bundled cable ampacity review, and the NEC Raceway Fill Calculator when you need a different wire-routing screen.
After the occupancy screen, use the Cable Tray Fill Chart to document tray type, tray width, planned loading depth, cable family, cable count, spare capacity, and the separate ampacity review that still needs project-specific confirmation.
Common Applications
Early tray-width comparison before a detailed layout
Space screening for cable tray renovation or expansion work
Quick planning review for open tray versus solid-bottom tray concepts
Checking whether a tray concept is obviously loose or obviously crowded
Supporting scope discussions before manufacturer and code-detail verification
Frequently Asked Questions
Does this cable tray fill calculator prove NEC 392 compliance?
Why does the page treat single conductors conservatively?
Why is solid-bottom tray treated more cautiously?
Can I use this page for medium-voltage tray design?
What should I verify before final tray design?
What should I record after a cable tray fill result?
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