You are correct in your understanding. Here's a breakdown and confirmation of your approach, along with some key considerations

Your Approach is Correct:
You start with the base ampacity from NEC Table 310.16. You recognize that auxiliary gutters, while not specifically defined as raceways, function similarly and affect heat dissipation. You intend to apply derating (adjustment) factors based on the number of current-carrying conductors and other relevant conditions.

Explanation and Considerations


Starting Point - Table 310.16

NEC Table 310.16 is the starting point for determining conductor ampacity. It's the baseline. You look at the conductor size, insulation type, and temperature rating to find the initial ampacity.2.

Auxiliary Gutters are Not Raceways (Directly)

You're right, auxiliary gutters are defined separately in Article 366, and are not explicitly listed as raceways. This is a crucial point. You can't assume that the ampacity adjustments for specific raceway types apply directly.3.

Applying Adjustment Factors (Derating)

This is the most important part. Since auxiliary gutters are not raceways, you use the general derating rules for conductors in free air. NEC 310.15(C) is key.

Number of Conductors

The most common adjustment is due to the number of current-carrying conductors. This is essential. Section 310.15(C)(1) outlines the adjustment factors based on the number of conductors in a raceway or cable. Although an auxiliary gutter is not a raceway, the intent of preventing overheating applies. Use the same derating factors as you would for conductors in a raceway.

Ambient Temperature

If the ambient temperature exceeds 30°C (86°F), you must apply an ambient temperature correction factor. This is found at the bottom of Table 310.16.

Adjustment Factors

It's not related to your specific question of auxiliary gutters, but it is important to always be aware of the conductor termination temperature rating. If the wire termination is rated for a lower temperature than the wire's insulation (e.g., wire is rated 90C, termination is rated 75C), you must use the lower ampacity value.4.

Article 366 - Auxiliary Gutters

Article 366 provides specific rules about auxiliary gutters, including:

Fill Restrictions (366.22)

The sum of the cross-sectional areas of all conductors at any point in the gutter cannot exceed 20% of the interior cross-sectional area of the gutter. This is a crucial safety requirement to prevent overcrowding and overheating.

Conductor Bending Space (366.58)

Sufficient bending space must be provided at terminals.

Enclosure Size

This relates back to the fill requirements. Make sure you select a gutter large enough to accommodate all the conductors and splices you need to make.5.

Key Differences from Raceways

Because auxiliary gutters are NOT raceways, you typically can't use raceway fill calculations. Article 366 dictates different fill requirements for gutters. Also, raceway conductor derating tables (other than the generic "number of conductors in a raceway or cable") aren't applicable.

In Summary


Start with Table 310.16.2. Apply Number of Conductors Adjustment (310.15(C)(1)). Use the same derating factors as if they were in a raceway.3. Apply Ambient Temperature Adjustment (if applicable).4. Adhere to fill requirements in Article 366.22 (20% fill rule).5. Ensure proper bending space (366.58).6. Consider voltage drop, if relevant.Important Considerations for Best Practice



Consult with a Qualified Electrician/Engineer

Especially for complex installations, it's always best to consult with a qualified professional to ensure compliance with the NEC and local codes.

Documentation

Keep clear documentation of your calculations, including the conductor sizes, insulation types, ambient temperature, number of conductors, and any applied derating factors.

Safety First: Overestimating conductor size is always preferable to underestimating. Err on the side of caution.By following these steps, you can accurately determine the ampacity of conductors installed in auxiliary gutters while adhering to the NEC and ensuring a safe and reliable electrical system. Flag for review