SettingsSettings for this calculationUS
The cable tray's usable internal width.
The clear width INSIDE the side rails, not the catalogue width, which is measured over them and is larger by two rail thicknesses. On a ladder tray it is the clear span of the rungs. Ordering to the catalogue width and filling to it is how a tray specified with spare capacity arrives without any.
The cable tray's usable internal depth.
Measured inside, from the tray floor to the top of the side rail. Treat the capacity this produces as a ceiling rather than a target: fill rules generally limit loading to less than the full rail depth, and cables piled to the rail lose heat to each other rather than to the room. Depth is also the dimension that decides whether a tray can be re-entered later without unloading it.
The sum of the cross-sectional areas of all cables in the tray.
This is the sum of each individual cable's overall cross-sectional area, not just its conductor area.
Cable tray fill percentage
29.7 %
Maximum allowable fill percentage varies by tray type (ladder, solid-bottom, ventilated) and cable type (single vs. multiconductor) per NEC 392.22 — verify your specific tray/cable combination's allowable maximum against the code rather than a single universal limit.
- Tray usable cross-sectional area
- 47 in²
They open the calculator with your figures already in it
Cable Tray Fill Percentage Calculator: 29.68 % — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Cable tray fill percentage = total cable cross-sectional area ÷ tray usable cross-sectional area × 100, per NEC Article 392 cable tray fill requirements (specific maximum fill percentages vary by tray type and cable type per NEC 392.22)
Inputs used
- Tray Usable Width
- 11.75 in
- Tray Usable Depth
- 4 in
- Total Cable Cross-Sectional Area
- 13.95 in²
Intermediate steps
- Tray usable cross-sectional area
- 47 in²
Confidence note: Maximum allowable fill percentage varies by tray type (ladder, solid-bottom, ventilated) and cable type (single vs. multiconductor) per NEC 392.22 — verify your specific tray/cable combination's allowable maximum against the code rather than a single universal limit.
What this calculation does not cover
- An area percentage is not always the rule that governs. For the larger multiconductor cables, and for the larger single conductors, NEC 392.22 stops being an area test and becomes a sum-of-diameters test with the cables laid in a single layer — a tray can sit well inside any percentage and still be non-compliant because the cables were stacked.
- Fill has nothing to say about heat. Cables bundled in a tray lose the free-air rating they were picked on and derate against the number of current-carrying conductors and how tightly they lie, so a legally filled tray can still be running conductors past their allowable ampacity — maintained spacing and a single layer are the cure, and neither shows up in a percentage.
- Cross-section is not weight. Armored and medium-voltage cable fills a tray slowly and loads it quickly, so a run sitting at a comfortable fill percentage can be over the tray's rated load class long before it is anywhere near its fill limit.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Cable tray fill percentage = total cable cross-sectional area ÷ tray usable cross-sectional area × 100, per NEC Article 392 cable tray fill requirements (specific maximum fill percentages vary by tray type and cable type per NEC 392.22)
Which documents these citations point at
- National Electrical Code (NFPA 70) — Article 392, 392.22 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
Cite this page
Your workspace
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Protection this work requires
- Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.
Tools and safety for this job
To first-fix electrical. Generic types, no brands, no prices.
- Assume every cable is live until proved dead at the point you will touch, with a two-pole tester you have just proved on a known source.
First-fix electrical: This site does not publish a tool list for electrical installation work. In every market it serves, fixed wiring is either reserved to a registered electrician or notifiable to a building authority, and the failure mode is a fire or an electrocution months later rather than a visibly bad job on the day. The calculator gives you the quantities to discuss and to buy against. The installation is a job for a qualified electrician, and the certificate they issue is the point of them.