Let's break down your situation and the options

Understanding the Issues
MPPT Mismatch

MPPTs (Maximum Power Point Trackers) are designed to independently optimize the power extraction from each string. Connecting multiple strings with different characteristics (due to shading, panel degradation, or slight variations in manufacturing) to the same MPPT can result in suboptimal performance. The MPPT will track the average power point, which might not be ideal for any individual string.

Current Capacity

Your biggest constraint is the inverter's MPPT current limit of 24A. You have three strings at 8.68A each, totaling 26.04A. This exceeds the MPPT limit if combined into a single MPPT.

Negative Wire Size

The shared #8 AWG negative wire is a concern. It needs to be appropriately sized for the combined current of all three strings.

NEC Compliance

All wiring practices must adhere to the National Electrical Code (NEC) and any local amendments. Specifically, pay close attention to wire ampacity, overcurrent protection, and grounding requirements.

Analyzing Your OptionsOPTION 1: Splitting Positives and Negative Across MPPTs (Your Original Idea)

This is NOT recommended and likely violates safety and electrical code. Why it's problematic



MPPT Dependence

MPPTs operate as independent DC-DC converters. "Splitting" the common negative across multiple MPPTs can create unpredictable current flow and potentially damage the inverter. The inverter's internal circuits are not designed for this type of configuration.

Unbalanced Currents

Even if the strings were perfectly identical (highly unlikely), there would be no guarantee that the current would be distributed equally between the MPPTs. This could overload one MPPT while underutilizing the other.

Safety Concerns

It is impossible to ensure correct current flow and safe operation, and any such configurations would void the inverter's warranty and UL listing.

NEC Violation

It would likely violate NEC article 690, "Solar Photovoltaic (PV) Systems" specifically regarding wiring methods and equipment usage.

DC Ground Fault Detection

Your inverter relies on measuring the differential current of DC+ and DC- in order to detect a ground fault. When you "split" the common negative, this is no longer possible.

OPTION 2: Combining All Strings into a Single MPPTNOT RECOMMENDED due to exceeding current limitations. Why it's problematic



Current Overload

3 strings at 8.68A each, which is a total of 26.04 A. Since this is more than the max DC input, wiring these to a single MPPT could damage the inverter or cause it to operate inefficiently.

MPPT Underutilization

You're correct that you'd only be using 2 of the 3 MPPTs. While not inherently a problem (the inverter should be able to operate with unused MPPTs), it does mean you're not taking full advantage of the inverter's capabilities.

A BETTER SOLUTION

To make the most use of the existing wires you can do the following: Combine 2 strings in parallel This gives you an Imp of 17.36A (2 x 8.68 A) and a voltage equal to that of one string. Connect them to a single MPPT. As the amperage is lower than 24A you should be able to connect them to a single MPPT on the inverter. Connect the remaining string to a second MPPT. As the amperage is lower than 24A you should be able to connect them to a single MPPT on the inverter. Leave one MPPT unused This should be fine.

Important Considerations



Professional Installation

Solar PV installations are complex and potentially dangerous. I strongly recommend that you hire a qualified and licensed solar installer to perform the work. They will have the knowledge and experience to ensure that the system is installed safely and correctly, and that it meets all applicable codes and regulations.

Warranty

Incorrect wiring can void the inverter's warranty.

AHJ Approval

The final installation will need to be inspected and approved by your local Authority Having Jurisdiction (AHJ).

Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. The information provided above is for general informational purposes only and should not be considered a substitute for the advice of a qualified electrician or solar installer. Flag for review