Your calculation is accurate based on the 2023 NEC and the information you've provided. Let's break it down and address the potential concerns: NEC Section 392.22(A)(1)(b)

This section indeed states that for cables #4/0 and smaller, the sum of the cross-sectional area of all cables shall not exceed the maximum allowable cable fill area in Column 1 of Table 392.22(A).

Table 392.22(A)

For a 24-inch wide cable tray, Column 1 specifies a maximum allowable cable fill area of 28 square inches.

12/2 MC Cable Dimensions

You correctly use the diameter of .475 inches for 12/2 MC. This is a critical step as the exact dimensions vary by manufacturer. Always consult the manufacturer's data sheets for the specific cable you are using.

Area Calculation

You accurately calculated the area of a single 12/2 MC cable: (.475 / 2)^2 PI = 0.17725 in^2. It seems like you calculated area as (0.475)^2=0.2256 sq. in. It will slightly increase the allowable amount.

Cable Capacity Calculation

28 square inches / 0.17725 square inches = 158. However using the 0.2256 results in 125. So yes your calculation and numbers are accurate.

Important Considerations and Potential Issues


Manufacturer's Data Sheets

ALWAYS use the specific diameter provided by the MC cable manufacturer's data sheet. Diameters can vary slightly between manufacturers. Using accurate dimensions is critical for compliance.2.

Cable Bundling

Although you are not required to derate based on 392.80(A)(1)(a) due to the number of current carrying conductors, densely packed cables can still create localized heat buildup. Consider the overall heat dissipation of the tray system. While the NEC allows this fill, it doesn't guarantee optimal performance or long-term cable life in all environments. This is more of a practical engineering consideration than a code violation, but it's crucial.
3.

Accessibility

While the NEC allows for a high fill rate, remember the customer's requirement for easier retrofitting and expansion. A completely full tray makes adding or removing cables difficult, defeating the purpose of the tray system.4.

Physical Arrangement

Consider how the cables will be arranged in the tray. While not explicitly required, uniform distribution will help with heat dissipation and make future modifications easier. "Cramming" the cables can lead to issues.5.

Cable Tray Load Capacity

Ensure that the cable tray itself is rated to handle the total weight of all the cables you plan to install, plus any future additions.6.

Other Cables

Remember to account for all cables in the tray, including control cables, data cables, etc. If those cables exceed 2.15 inches then it requires a different calculation that may impact capacity.
7.

AHJ Interpretation

The Authority Having Jurisdiction (AHJ) has the final say on code interpretation. It's always a good idea to discuss your design with the AHJ to ensure they agree with your interpretation and calculations.

Recommendations



Derate Strategically

Even though not required by code for CCC count, consider voluntarily derating (reducing the allowable fill) to improve airflow and facilitate future modifications. For example, aim for 60-70% fill rather than 100%.

Larger Tray

If budget allows, consider using a slightly larger cable tray (e.g., 30-inch or 36-inch). This provides more room for expansion and improves heat dissipation.

Document Everything

Keep meticulous records of all cable types, dimensions, fill calculations, and tray load calculations. This documentation will be invaluable during installation and for future maintenance.

Consider Cable Spacing

Implement methods to maintain spacing between cables within the tray to promote airflow and prevent overheating, even if derating is not strictly required.

In conclusion, your calculations are correct based on the information you provided and the 2023 NEC. However, it's important to consider the practical implications of a fully loaded cable tray and the customer's needs for future expansion. Prioritize ease of access, airflow, and maintainability when designing the cable tray system. Flag for review