You've hit a common point of confusion. Let's break it down and address the code requirements

Understanding 230.79

NEC 230.79 deals with the Rating of Service Disconnecting Means. It's primarily concerned with the overall service disconnect for the entire building, not a disconnect dedicated solely to a PV system's output. While it's understandable why someone might look at it, it's usually not directly applicable to a load-side or line-side PV tap disconnect. The key is that your PV is connecting to the service after the main service disconnect, making those rules less directly applicable.Why 230.79(D) Doesn't Apply (Probably)230.79(D) "All Others" is the catch-all, and the 60A minimum is intended for general residential services. This section usually applies to the main service disconnect. Since your PV tap is on the load/line side of the existing service, it is generally assumed that the existing residential service already meets the 60A minimum requirements of 230.79.

Relevant NEC Articles for PV Disconnects

The controlling sections for PV disconnects on a line-side tap are found in NEC Article 690 (Solar Photovoltaic (PV) Systems). Specifically: 690.8(A)

Calculation of Circuit Currents - This section dictates how you calculate the maximum current the PV system will produce. This is where your 26.25A figure comes from (inverter output x 1.25).

690.9

Overcurrent Protection - This governs the overcurrent protection (fuses or breakers) required for the PV system.

690.13

All Conductors - This section requires that a PV system that connects to a line side tap must be installed with a disconnect means that isolates ALL conductors.

705.12(A)

Supply Side Source Connections. The tap conductor sizing and overcurrent protection must be compliant with 705.12(A).

Your Situation - Is the 30A Disconnect Acceptable?

Here's how to assess whether your 30A disconnect is code-compliant:1. Conductor Ampacity

The conductors between the PV system and the tap on the service conductors must have an ampacity at least equal to 125% of the inverter's continuous output current (your 26.25A figure), according to 690.8(A)(1).
2.

Overcurrent Protection

The fuses/breakers protecting the PV system's conductors must be sized according to 690.9. The 30A fuses you're using are likely acceptable if the conductors are rated to handle 30A.
3.

Tap Conductor Rules (705.12(A))

Because you are doing a supply side source connection, the requirements of 705.12(A) must be met.

In summary, a 30A AC disco is likely acceptable if

The conductors between the PV inverter and the service tap are rated for at least 30A. The 30A fuses provide the required overcurrent protection for those conductors (based on their ampacity). The supply side source connection meets the requirements of 705.12(A).

Why the Engineering Firm Might Have Concerns (Possible Explanations) They were misinterpreting 230.79

It's possible they were focused on the overall service rating and applying it to the PV disconnect inappropriately.

They were considering future expansion

They might have been looking ahead and anticipating a larger PV system in the future, and suggesting a 60A disconnect to accommodate that.

Local Amendments

Some jurisdictions have local amendments to the NEC that might impose stricter requirements.

Oversized Service

It is also possible that the engineering firm thought the service was undersized for the home's loads, and the additional PV connection exacerbated the issue.

Recommendations


Clarify with the Engineering Firm

Ask them specifically* why they believe a 60A disconnect is required, citing the relevant sections of the NEC that support their position. Get a clear explanation that goes beyond simply referencing 230.79(D).
2.

Verify Conductor Ampacity

Double-check that the conductors used for the PV system's AC output are rated for at least 30A (or whatever the fuse/breaker size is).3.

Confirm 705.12(A) Compliance: Make sure that the tap conductor size and overcurrent protection complies with 705.12(A).By thoroughly reviewing the relevant NEC sections (especially 690 and 705) and communicating with the engineering firm, you can determine the correct disconnect size for your PV system. Remember to consult with your local Authority Having Jurisdiction (AHJ) for any local requirements. Flag for review