Okay, let's break down how to size the Equipment Grounding Conductor (EGC) between the combiner and the AC disconnect in your scenario, referencing the National Electrical Code (NEC). Keep in mind that I am an AI and not a qualified electrician. This information should be used for educational purposes only and not as a substitute for advice from a qualified professional.

Understanding the Context Combiner Box

This is where multiple PV source circuits (from solar panels) are combined in parallel.

AC Disconnect

This is a safety switch that allows you to disconnect the AC output of the inverter from the grid.

EGC (Equipment Grounding Conductor)

This conductor provides a low-impedance path for fault current to return to the source, tripping the overcurrent protective device (circuit breaker or fuse) and preventing dangerous voltage buildup on equipment enclosures.

NEC Sections to Consult

The primary sections of the NEC you'll need are: NEC 250.122

This is the general section for sizing EGCs for AC systems. It is the core of what you need.

NEC 690.45

This section may have specific requirements for grounding PV systems, so make sure to verify if there are any modifications or additions for PV systems.

Steps to Size the EGC

Here's a step-by-step guide, assuming your AC disconnect is fed by a standard AC circuit:1. Determine the Overcurrent Protective Device (OCPD) Size

Find the rating (in Amperes) of the circuit breaker or fuse protecting the circuit that feeds the AC disconnect. This is crucial. This information should be available on the inverter's nameplate or the panel schedule. Let's assume, for example, that the OCPD is rated at

60 Amperes.2. Use NEC Table 250.122

Go to NEC Table 250.122. This table provides the minimum size of the EGC based on the rating of the overcurrent protective device. In our example (60A OCPD), Table 250.122 will likely indicate a minimum EGC size of

#10 AWG Copper. (Double-check the table to confirm based on the latest NEC edition.)3. Adjust for Increased Size of Ungrounded Conductors (If Applicable)

NEC 250.122(B) states that if the ungrounded conductors (the "hot" wires) are increased in size for voltage drop, the EGC must be increased proportionally.

Example

If you originally needed #6 AWG conductors for the ungrounded conductors but upsized them to #4 AWG to reduce voltage drop, you would need to increase the EGC size proportionally. This involves calculating the ratio of the increased conductor's circular mil area to the original conductor's circular mil area, and applying that ratio to the required EGC size. This rarely occurs and is more for long wire runs.4.

Consider Conductor Material (Copper vs. Aluminum)

Table 250.122 has separate columns for copper and aluminum EGCs. Make sure you're using the correct column based on the material of your EGC. Aluminum EGCs will generally need to be a larger AWG size than copper for the same OCPD rating.5.

Fuses vs. Circuit Breakers

Table 250.122 applies to both fuses and circuit breakers.
6.

Minimum Size

The EGC must be no smaller than #14 AWG copper or #12 AWG aluminum, as stated in NEC 250.120(C).

Important Considerations and Clarifications Continuous Loads

If the load is considered a "continuous load" (operates for 3 hours or more), you must size the circuit and the conductors (including the EGC) to 125% of the continuous load current.

Conduit Type

The type of conduit you're using (e.g., metal, PVC) can affect grounding requirements. Metal conduit can sometimes serve as the EGC, but this is subject to specific rules in NEC 250.118. If using non-metallic conduit, you must install a separate EGC.

PV-Specific Rules

Always double-check NEC 690.45 and any other relevant PV-specific sections in the NEC to ensure compliance. There might be additional or modified requirements for EGC sizing in PV systems.

Local Codes and AHJ

Your local electrical codes or the Authority Having Jurisdiction (AHJ) may have additional requirements or interpretations. Always consult with your local AHJ.

EGC to Grounding Electrode Conductor

When connecting the ac EGC to the grounding electrode conductor at the service equipment, you are required to use a bonding jumper. This is per 250.24(C). This jumper is required to meet the size requirements of table 250.102(C)(1). This may increase your wire size beyond table 250.
122.

Example Summarized

Let's say: OCPD rating: 60 Amperes Conductor material: Copper No up-sizing of ungrounded conductorsThen, based on NEC Table 250.122 (check the actual table in the current NEC edition!), you would likely need a #10 AWG Copper EGC.

Disclaimer: This information is for educational purposes only and does not constitute professional electrical advice. You must* consult with a qualified and licensed electrician to properly design and install your electrical system according to the NEC and local codes. Incorrect sizing or installation of the EGC can create a serious safety hazard. Flag for review