Section 690.6 of the National Electrical Code covers AC modules—those that output AC current to tie directly to the grid. These modules are a fairly new technological development. Although this section does not cover all of the Code-related rules, there are some key points that designers and installers need to be aware of.
As with many sections within the Code, it is worthwhile to read the definitions. Section 690.2 defines an AC module as “a complete, environmentally protected unit consisting of solar cells, optics, inverter and other components, exclusive of tracker, designed to generate AC power when exposed to light.”
There are some products that look a lot like an AC module, but they are actually a microinverter connected to a PV module. This only can lead to confusion and misapplication of the Code. A true AC module is one with the inverter integrated into the module design at the factory—not added after the fact. Some industry people even contend that a true AC module will not have any exposed DC wiring; that the only wiring external to the module is the inverter’s output conductors (since there is nothing in the definition that prohibits exposed DC conductors, this is not an absolute requirement). But the presence of exposed DC conductors opens up parts of these NEC sections to interpretation. There are listed AC modules that have exposed DC wires. Since these are listed as an AC module by a nationally recognized testing laboratory (NRTL), my opinion is that the DC wiring can and should be considered internal to the listed product. In contrast, some AC modules integrate the inverter within the module’s junction box, keeping all DC conductors concealed.
Section 690.6 (A) states that, since any DC circuits are considered internal wiring, the requirements for PV source-circuit conductors do not apply to AC modules.
DC conductor sizing and maximum voltage calculations are not required by Section 690.6(A), since they are provided by the AC module manufacturer and are covered in the module’s listing. According to the NEC’s second subsection in this section, the output from an AC module is considered an inverter output circuit and as such must follow those rules for circuit sizing and current calculations in 690.8. This results in calculations and conductor sizing that is the same as for any other inverter output circuit used in grid-tied PV systems.
Read the 690.8 subsection titles carefully, as the requirements set within vary based on the circuit you are considering. For example, 690.8(A)(3) applies to AC module circuits, but the other three current definitions within 690.8(A) do not. The maximum inverter output circuit current is defined as the continuous output current rating. To properly size the conductors and overcurrent protection, refer to 690.8(B)(1) and (2). This process is similar to the conductor and sizing calculations used for string inverters, as the Code does not differentiate between inverter technologies. You should also follow the manufacturer’s listing instructions for this portion of the installation, which may provide important details like the maximum number of AC modules per branch circuit and maximum ampacity of the overcurrent devices.
Section 690.6(E) provides allowances for overcurrent protection per 240.5(B)(2). This section deals with the ampacity and allowable overcurrent protection for fixture wiring, the factory-supplied conductors connected to the AC module. This section deals specifically with the fixture wire but does not specifically address the size of the branch-circuit conductor. Designers and installers do not have control over the conductors attached to the modules but are responsible for the branch circuits from the point of interconnection to the PV installation. When sizing conductors and overcurrent protection for these circuits, use the established methods outlined in 690.8(A) and (B).