Let's break down 690.31(C)(2) and how it applies to your PV cable tray design, specifically addressing the stacking issue.

Understanding 690.31(C)(2)



(1) Single Layer

This is the overarching requirement. All single conductors must be installed in a single layer. This is designed to maximize heat dissipation. No conductor should be completely buried under others, except as permitted in (2).

(2) Pair Exception (Stacking)

This section provides a limited exception to the single-layer rule. It says that pairs of conductors (positive and negative conductors of the same PV source circuit or PV output circuit) can be stacked directly on top of each other. This is allowed because these conductors carry closely related currents and therefore tend to generate similar amounts of heat and the current cancels out a lot of the magnetic fields.

(3) Cable Tray Fill

This provides the overall fill constraint for the cable tray. The sum of the diameters of all conductors in the cable tray (whether single or stacked pairs) cannot exceed the internal width of the cable tray. This ensures adequate space for heat dissipation and avoids excessive compression of the conductors.

Applying it to Your Scenario


Wire Diameter and Tray Width

You've correctly calculated that with a 24-inch cable tray and a #8 PV wire diameter of 0.331 inches, you could technically fit 24 / 0.331 = approximately 72.5 single conductors if you weren't stacking, according to (3).
2.

The Stacking Dilemma

Here's where the confusion arises. You're thinking that because you're stacking pairs (positive and negative from the same string), you're only using half the tray width for each string. This is the correct interpretation.3.

The Catch

The key is that while you're allowed to stack pairs, you're still limited by the total diameter fill constraint of (3). You can't just ignore the fact that you're stacking and assume you can cram double the number of strings into the tray. The stacked pair, while occupying less width in the tray, still has a combined diameter that contributes to the total fill.

The Correct Calculation

Since you're stacking pairs, consider each pair (positive and negative of a string) as a single "unit" for the purpose of calculating cable tray fill. The diameter of this "unit" is still 0.331 inches (since you are only accounting for the width).

Strings in the Tray

With a 24-inch tray width, you can still only fit approximately 72 strings. (24" / 0.331" per pair = 72.5 pairs)

Why This is Important

The single-layer requirement (with the limited stacking exception) is primarily about heat dissipation. PV cables can generate significant heat, especially when carrying high currents. Overfilling the cable tray, even if you can physically cram more conductors in, can lead to: Overheating of the conductors Reduced conductor lifespan Potential fire hazard Derating of the wire's ampacity

Practical Considerations for Your 1 MW Project



Cable Tray Size and Quantity

If 72 strings per 24" tray isn't enough, you have a few options:

Wider Cable Trays

Use wider cable trays to accommodate more strings per tray.

Multiple Cable Trays

Run multiple cable trays in parallel. This is often a more cost-effective solution than using extremely wide trays, and it provides better redundancy.

Ampacity and Wire Size

Double-check that your #8 PV wire is adequately sized for the current it will be carrying, considering derating factors for ambient temperature, number of conductors in the tray, and any other applicable factors (NEC 310.15). You might need larger gauge wire, which will further reduce the number of strings you can fit in a tray.

Cable Tray Spacing

Ensure proper spacing between cable trays to allow for adequate ventilation.

Engineering Review

For a 1 MW project, it's highly recommended to have a qualified electrical engineer review your cable tray design to ensure compliance with all applicable codes and standards and to optimize the design for performance and safety.

AHJ (Authority Having Jurisdiction)

Consult with your local AHJ early in the design process to get their input and approval on your cable tray system. They may have specific requirements or interpretations of the code that you need to follow.

In Summary:

You can* stack positive and negative conductors from the same string in your cable tray, but you're still limited by the overall fill requirement (sum of conductor diameters) of 690.31(C)(2)(3). Focus on a design that prioritizes heat dissipation and compliance with all applicable codes. Use a qualified electrical engineer to review your design, especially for a project of this scale. Good luck! Flag for review