Cable Tray Fill and Ampacity Worksheet
Compare cable area with an installation-specific allowable tray area, then apply an entered ampacity adjustment factor.
Tray and cable details
Enter the area allowed for this tray type, cable size, and installation method after checking the applicable table and rules.
Cable group 1
Use the manufacturer-listed outside diameter.
Enter the factor from the applicable ampacity table and installation conditions.
Estimated tray area utilization
About this worksheet
Enter cable counts, manufacturer-listed outside diameters, base ampacities, and verified adjustment factors for each cable group. Also enter the allowable cable area determined for the actual tray installation. The worksheet combines cable area across groups and reports adjusted ampacity for each group.
Method and assumptions
For each cable group, area equals cable count multiplied by the circular area calculated from its outside diameter. Total area is the sum across groups; fill percentage equals total area divided by the entered allowable area. Adjusted ampacity for each group equals its base ampacity multiplied by its entered adjustment factor.
Article 392 allows different fill and ampacity methods depending on tray construction, cable type, conductor size, and installation. This worksheet does not select those rules or determine the allowable area, base ampacity, or adjustment factor. Check each applicable condition and enter the resulting values for each cable group.
Interpret the result
A passing area comparison means only that the entered cable group fits within the allowable area you supplied. A passing adjusted ampacity means only that the factor you supplied produces the displayed ampacity. Verify those reference values against the actual installation before using them.
FAQs
Where do I get the allowable fill area?
Determine it from the applicable Article 392 table and rules for the tray type, cable type, size, and arrangement. This worksheet does not choose a universal percentage or table column.
Does the ampacity field calculate every tray ampacity rule?
No. For each cable group, enter the base cable ampacity and the factor from the applicable ampacity table and installation conditions. The result applies that factor and does not choose conductor temperature, ambient, grouping, or cable-listing rules for you.
Can this approve a cable tray installation?
No. Tray construction, cable listing, support, spacing, bonding, firestop, environmental conditions, and the adopted Article 392 rules require a separate review.
Sources
- NFPA 70, National Electrical Code, 2026 edition — Article 392 and its cable-tray fill and installation provisions; confirm the adopted edition and local amendments.
- NFPA Code-Making Panel 8 Second Draft Report, preliminary report — public code-development material for Article 392; it is not a substitute for the adopted final text.