You've hit on a point of confusion that many in the industry have raised regarding the 2020 NEC change to 690.6(A). Let's break it down

The Issue:
The Goal of 690.6(A)

Originally, 690.6(A) was designed to offer an exception for

AC modules themselves. An AC module has a DC/AC inverter integrated into the module. This reduces the DC voltage to a low level within the module before the circuit leaves the module. Thus, this exemption allows for lower voltage DC circuits within the module to avoid certain restrictive requirements. The Expansion to AC Module Systems

The 2020 NEC expanded this exemption to "AC module systems," defined as systems comprised of AC Modules. As you correctly point out, there aren't DC circuits between AC modules in a typical AC module system. The DC is contained within each module and converted to AC there. So, why expand the exception to the entire system?

Possible Interpretations & What It Might Allow Now

While it's true that there are no DC circuits between AC modules in a typical system, the expanded wording may be intended to cover scenarios where the system includes other DC components or aspects. Here are some possibilities:1.

DC Control Circuits Within the System

While the main power circuit between modules is AC, some AC module systems might have low-voltage DC control circuits for monitoring, communication, or shutdown. This change might be intended to exempt these low-voltage DC control circuits from certain stricter requirements.2.

DC Components within the Inverter

The AC module inverter itself is inherently a DC circuit with its own set of components that can be viewed as a DC circuit in the AC module system.3.

DC optimization of modules

Some AC modules can have some type of optimization within the module that can be viewed as a DC circuit in the AC module system

Practical Implications (What Changed?)

The practical difference that this change makes in the field may be subtle. Here are the things that may have changed It could allow manufacturers to design their systems with greater flexibility, provided they maintain safety. The exemption might streamline inspection and approval processes, as inspectors might be less likely to require certain labeling, wiring methods, or other requirements related to DC circuits if the system is demonstrably composed entirely of AC modules and has no other concerning DC circuits.

In conclusion: The change to 690.6(A) in the 2020 NEC to include "AC module systems" is indeed a source of confusion. However, by considering the possibilities, we can see why the wording was changed. It is intended to cover certain scenarios where low voltage DC circuits can be included with AC module systems. Flag for review