Okay, let's break down the EGC sizing and OCPD selection for your PV combiner box output. You've got most of the calculations correct, but there are a few nuances to consider to ensure compliance and safety.

Recap of your Calculations



Module Isc

13.95A

Bifacial Gain

8.4%

Effective Isc

13.95A 1.084 = 15.12A

690.8(A)(1)(1) Multiplier

15.12A 1.25 = 18.90A

690.8(B)(1) Multiplier

18.90A 1.25 = 23.63A

Total Calculated Current

12 strings 23.63A = 283.56A

Key Considerations and NEC References


OCPD Selection (NEC 690.9)

The OCPD protects the conductors from overcurrent. It must be rated to protect the ampacity of the conductors. The OCPD rating must not be less than 125% of the calculated circuit current, per NEC 690.9(B) & 690.8(B)(1) Therefore, your calculated current of 283.56A 1.25 = 354.45A would be the next standard OCPD of 400A.
2.

EGC Sizing (NEC 250.122)

The Equipment Grounding Conductor (EGC) size is based on the rating of the overcurrent protective device protecting the circuit. You will need to choose a size that corresponds with the OCPD that will be protecting the circuit.

Important

You CANNOT simply round down to the next available OCPD size just to save on conductor cost.3.

Combiner Box Rating

The fact that your combiner box is rated for 400A is important, but it doesn't directly dictate your OCPD size. The rating means it can handle up to 400A, but the OCPD size protects the conductors.4.

Conductor Ampacity (NEC 310.15 and Chapter 3 Tables)

You must select conductors that have an ampacity equal to or greater than both: The calculated circuit current (283.56A in your case) The OCPD size (what you select in the previous steps)

Step-by-Step Decision Process


Calculate the Required Conductor Ampacity

As previously mentioned, your calculated circuit current is 283.56A, and the OCPD must be rated to at least 125% of this value. 283.56A 1.25 = 354.45A2.

Choose OCPD

The next standard OCPD size equal to or greater than 354.45A is 400A. You cannot use 300A.3.

Determine the Required EGC Size (Based on OCPD)

Using NEC Table 250.122 (or the metric version if applicable), find the minimum EGC size for a 400A OCPD.

For Aluminum

Table 250.122 typically requires a

1 AWG Aluminum EGC for a 400A OCPD.4. Confirm Conductor Ampacity

Verify that the conductors you choose (for the current-carrying conductors, not the EGC) have an ampacity of at least: 283.56A (the calculated current) Your OCPD size 1.25 (The NEC requires the conductor ampacity to be no less than 125% of the OCPD used to protect the conductors)

In summary



OCPD

400A

Minimum EGC (Aluminum)

1 AWG

Verify that the Current-Carrying conductors' ampacity is equal to or greater than the calculated current and the OCPD1.25.Important Considerations



Temperature Correction

PV installations often experience high ambient temperatures. Apply temperature correction factors to your conductor ampacity ratings as required by NEC 310.15.

Raceway Fill

Ensure that your conductors and EGC fit properly within the selected conduit or raceway, adhering to raceway fill requirements in Chapter 3 of the NEC.

Voltage Drop

While not directly related to safety, voltage drop is crucial for PV system performance. Calculate the voltage drop for your chosen conductor size to ensure it's within acceptable limits.

Local Codes

Always check with your local electrical inspector or authority having jurisdiction (AHJ) for any specific requirements or interpretations of the NEC in your area.

Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional electrical engineering advice. Consult with a qualified electrical engineer and follow all applicable codes and regulations for your specific installation. Flag for review