Okay, let's break down NEC 690.9 in the 2020 edition as it relates to your question about combiner boxes, inverter DC inputs, and overcurrent protection. The language is dense, and it's a common area of confusion in PV system design.

The Key Principles from 690.9 You Need to Understand Overcurrent Protection is Generally Required

690.9 generally mandates overcurrent protection for PV source circuits and PV output circuits to protect conductors from overcurrent events (like short circuits and ground faults).

Sizing the Overcurrent Protection

The overcurrent protection device (OCPD) is sized to protect the conductors. The conductor's ampacity must be greater than or equal to 125% of the calculated maximum current (usually Isc x 1.25 x additional factors if temperature/conduit fill derating applies). The OCPD is often sized no larger than that conductor ampacity.

Purpose of String Fuses

The fuses in the combiner box (on each input string) are primarily there to protect the modules and the module wiring from reverse currents if one module develops a fault. They are not necessarily sufficient as the sole overcurrent protection for the output circuit conductors between the combiner box and the inverter.

Fault Current Considerations

The available fault current at the DC input of the inverter can be very high. Without proper overcurrent protection on the output of the combiner box (or integrated into the inverter), a fault downstream could damage conductors and equipment.

Scenario: Combiner Box with String Fuses Only

In your scenario, you have a combiner box with fusing on the input strings (i.e., each PV string coming into the box is fused). The critical question is: Do you need overcurrent protection between the combiner box and the inverter DC input in addition to the string fuses?

The answer is generally yes under the NEC. Here's why, with reference to 690.9 (2020 NEC), and how to think about it:1. 690.9(A) General. This section is a broad statement that PV source circuits and PV output circuits shall have overcurrent protection.2. 690.9(B) Overcurrent Device Required. (This is the crux of your question.) This section states that a fuse or circuit breaker shall be used. The string fuses in the combiner box do provide overcurrent protection for each PV Source Circuit, however, there is no overcurrent device to protect the PV Output circuit, the circuit between the combiner box and the inverter.3. 690.8(A) Calculation of Maximum Circuit Current. To size conductors and OCPD, you need to determine the maxiumum current in the output circuit. This will generally be determined by adding up all of the short circuit currents of the combined strings after accounting for any derating factors.4. 690.8(B) Conductor Ampacity. The output circuit ampacity shall be at least 125 percent of the maximum circuit current.5. 690.9(B)(2) Circuit Breaker or Fuses. Here, it stipulates that the OCPD must be readily accessible and rated to interrupt the available fault current.6. Available Fault Current

The available fault current calculation is critical. You'll need to account for the short-circuit current from all the parallel strings, the temperature coefficient of Isc, and other factors (e.g., cable impedance). The output OCPD of the combiner box (or within the inverter) must be rated to interrupt this fault current. String fuses alone are not designed to do this on the output circuit.

Therefore, even with string fuses, you will generally need overcurrent protection on the output of the combiner box (or integrated into the inverter) to protect the conductors between the combiner box and the inverter.When Might You Not Need Separate Overcurrent Protection on the Output?

There are a few, specific circumstances where you might not need a separate OCPD on the output of the combiner box: Inverter with Integrated Overcurrent Protection

If the inverter explicitly has overcurrent protection at its DC input terminals that meets the requirements of 690.9 and is properly sized for the output circuit, you might be able to rely on that. The inverter documentation must clearly state this and provide the necessary interrupting rating and other specifications.

Listing and Labeling

If the entire system (combiner box and inverter) is listed and labeled as a complete assembly where the manufacturer has demonstrated that the system meets the overcurrent protection requirements, you can follow the manufacturer's instructions. This is relatively rare.

Important Considerations and Best Practices



Consult the Manufacturer's Instructions

The manufacturer's instructions for the combiner box and the inverter are paramount. Follow their recommendations regarding overcurrent protection and conductor sizing.

AHJ Approval

Always get approval from your Authority Having Jurisdiction (AHJ). They have the final say on whether your design meets code requirements.

Documentation

Provide clear and comprehensive documentation to the AHJ, including: Calculations of maximum circuit currents and available fault currents. Datasheets for all components (modules, combiner box, inverter, OCPDs, conductors). Wiring diagrams showing all overcurrent protection.

In Summary:

While string fuses in a combiner box are essential for module protection, they typically don't satisfy the NEC's requirement for overcurrent protection of the output circuit* between the combiner box and the inverter. You'll generally need a separate OCPD on the output of the combiner box (or relying on appropriately sized and rated protection within the inverter) to protect against faults in that circuit. Always follow manufacturer's instructions and get AHJ approval. Flag for review