An alternating-current (AC) photovoltaic (PV) module is defined in Section 690.2 of the 2005 National Electrical Code (NEC) 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 sunlight.” These modules combine a standard PV module and a separate stand-alone AC/DC inverter to produce direct current (DC) that must be transformed into AC for use by standard appliances.
Electrical contracting firms need to be aware of this type of PV module because a PV installation based on this technology may fit the customer’s needs better than a traditional system.
An AC PV module looks the same as a standard PV module. The difference is found on the back. Standard PV modules have a junction box for connecting the positive and negative module terminals to the DC PV source circuit. The AC PV module has a dedicated inverter, sized for the module, with ungrounded, grounded and equipment-grounding output terminals for connection to a dedicated 120-volt branch circuit.
An AC PV module is an interactive PV system that connects directly to the building distribution system and operates in parallel with the utility service. It can deliver power to the utility’s distribution system through the building service.
As a result, the AC PV module inverter must meet all of the requirements of IEEE Standard 929-2000 entitled IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems. This standard addresses the interconnection of PV system generating 10-kilowatts (kW) or less to the utility grid, but its requirements can be applied to PV systems of any size. The standard simplifies the PV system interconnection by providing a uniform standard that can provide a safe working environment for linemen, safeguards utility equipment and protects utility customers.
NEC Section 110.2 states, “conductors and equipment required or permitted by this Code shall be acceptable only if approved.” The term “approved” is defined in NEC Article 100 as “acceptable to the authority having jurisdiction.” Most Code-enforcement agencies require that electrical equipment be tested in accordance with UL standards by UL or another qualified testing laboratory and be listed or labeled.
Therefore, the AC PV module inverter needs to be tested in accordance with UL 1741, titled, Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources (Revision 11/7/2005), based on the requirements of IEEE Standard 929.
Advantages of AC PV modules
Because the inverter is an integral part of the unit, and an array of AC PV modules can be installed in accordance with NEC Section 690.6. This section allows the AC output of the PV array to be connected to the building distribution system through a dedicated branch circuit, with the branch-circuit breaker serving as the disconnecting means if it meets the requirements of NEC Section 690.17.
This simplifies PV system installation and can significantly reduce its initial cost. In addition, each AC PV module is a stand-alone unit and additional units can be added in parallel, making the system scalable based on the customer’s power needs and financial capability.
Operationally, AC PV modules offer a number of advantages over standard PV modules. With each module operating independently from others making up the array, the failure of any modules will not affect the operation of the overall system. With traditional PV installations, the failure of one PV module in the string impacts the operation of the entire string. Similarly, shading is not a problem with AC PV modules because each module is operating independently and not affected by other parallel modules.
Since the NEC defines an AC PV module as a complete system, it must be manufactured, tested and installed as an integrated system. Small inverters for one or two PV panels have been produced since the 1990s, but do not meet the NEC definition of an AC PV module. These stand-alone inverters could not be installed in accordance with NEC Section 690.6, which specifically addresses AC PV module installation requirements and provides the technical and cost advantages over a standard PV module installation as discussed in this article.
These older inverters were also not tested and listed in accordance with the latest requirements of UL 1741 and may not qualify for utility and state tax incentives, which may be important to your customer. Currently, manufacturers are working on inverters that meet current UL 1741 requirements and will soon be available for manufacturing AC PV modules that meet the NEC. EC
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.