Yes, you likely need to derate the ampacity of the 600 MCM conductors when running four sets of them in an overhead cable tray. Here's why and the relevant NEC sections

Why Derating is Necessary
Concentrated Heat

When you group multiple conductors carrying significant current in a confined space like a cable tray, the heat generated by each conductor can raise the ambient temperature within the tray. This elevated ambient temperature reduces the current-carrying capacity of the conductors.

NEC Table 310.15(C)(1)

This table provides adjustment factors for ampacity when there are more than three current-carrying conductors in a raceway, cable, or

cable tray. While your conductors are technically in sets of four, the aggregate number of conductors in the tray becomes the determining factor.

Relevant NEC Sections1. NEC 392.80 Ampacity of Cables and Conductors in Cable Trays. This section is the main section for determining the ampacity of conductors in cable trays. It sends you to other sections within Article 310.2. NEC 310.15(B)(3)(a)(ii) Adjustment Factors For cable trays, this section states that the derating factors of 310.15(C)(1) apply.
3. NEC 310.15(C)(1) Adjustment Factors

This table is the heart of the derating calculation. It provides adjustment factors based on the number of current-carrying conductors in the tray. You need to determine the total number of current-carrying conductors in the cable tray. In your case, with (4) sets of (4) 600 MCM conductors, you likely have 12 current-carrying conductors (assuming each set is a 3-phase circuit with a neutral that doesn't meet the criteria of not being considered current-carrying). Look up the appropriate adjustment factor from Table 310.15(C)(1) based on the number of conductors. For example, if you have 10-20 conductors, the adjustment factor is 50%.

How to Determine the Correct Ampacity1. Determine the base ampacity

Look up the ampacity of a 600 MCM conductor in NEC Table 310.16 based on its insulation type (e.g., THHN, XHHW) and the ambient temperature rating of the termination equipment.
2.

Count the current-carrying conductors

As mentioned above, you'll need to know how many conductors are considered current-carrying. This depends on the circuit configuration (single-phase, three-phase, presence of neutrals, etc.). Always consult a qualified electrician to determine the correct number.3.

Find the adjustment factor

Use NEC Table 310.15(C)(1) to find the adjustment factor corresponding to the number of current-carrying conductors.
4.

Calculate the derated ampacity

Multiply the base ampacity (from Table 310.16) by the adjustment factor (from Table 310.15(C)(1)).

Example

Let's assume: 600 MCM THHN conductor Table 310.16 base ampacity for THHN at 75°C terminals: 475 Amps (check the table for the specific conductor rating) 12 current-carrying conductors in the cable tray. Table 310.15(C)(1) adjustment factor for 10-20 conductors: 50% (0.50)Derated ampacity: 475 Amps 0.50 = 237.5 AmpsTherefore, in this example, each 600 MCM conductor would have a maximum allowable ampacity of 237.5 Amps in the cable tray.

Important Considerations Cable Tray Fill

You must also comply with the cable tray fill requirements outlined in NEC 392.22. This limits the amount of space cables can occupy in the tray and can further influence your conductor choices.

Ambient Temperature Correction

If the ambient temperature in the cable tray exceeds 30°C (86°F), you'll need to apply an additional correction factor from the bottom of Table 310.16.

Voltage Drop

Remember to consider voltage drop. Even if the conductors are adequately sized for ampacity after derating, voltage drop may necessitate using larger conductors.

Qualified Electrician

This is a complex calculation.

It is imperative to consult with a qualified and licensed electrician or electrical engineer to perform these calculations and ensure compliance with all applicable codes and standards. Incorrectly sized conductors can lead to overheating, fire hazards, and equipment failure.

In summary, yes, you almost certainly need to derate the 600 MCM conductors in your scenario. The relevant NEC sections are primarily 392.80, 310.15(B)(3)(a)(ii), and 310.15(C)(1). Always consult a qualified professional for these calculations. Flag for review