Alternating Current PV Modules in the NEC

If you attended Solar Power International 2011 in Dallas, you know that ac modules are among the latest and greatest PV product innovations. What you may not realize is that the NEC has long had clear guidelines for the deployment of ac PV modules.

The National Electrical Code 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 sunlight.” This definition is found in Section 690.2 and was added to the 1999 edition of the NEC. Section 690.6, which applies specifically to ac modules, also first appeared in 1999. In the four Code revisions since 1999, the content related to ac PV modules is substantially unchanged. This is remarkable given the rate at which new PV technologies are being developed. While some of these technological innovations outpace the development of associated codes and standards, NEC Article 690 and UL 1741 both anticipated the successful commercialization of ac PV modules by more than a decade.

This article provides an overview of NEC 2011 as it applies to ac modules and provides guidelines for meeting NEC requirements for an ac PV system. While the interpretations and recommendations made in this article are consistent with the intent of the NEC, they are provided for informational purposes only and should not be considered enforceable. Ultimately, the AHJ is responsible for enforcing Code requirements or approving equipment, and may even have its own requirements.

PRODUCT EVALUATION

UL Standards Technical Panels made up of industry stakeholders generally develop product safety standards for North America. The subject matter experts on these panels develop and revise standards using a process of consensus. A Nationally Recognized Testing Laboratory (NRTL), such as CSA, ETL, TUV or UL, then uses these published standards to evaluate the safety of manufactured equipment.

While UL 1741 is currently used for evaluating and listing ac modules, this standard focuses on inverters, charge controllers and other BOS components such as combiner boxes. Therefore, the photovoltaic device used in an ac module must still comply with UL 1703, which covers traditional flat-plate PV modules. In effect, an ac module needs to meet the same safety and performance standards and operate in the same environments as both traditional utility-interactive inverters and flat-plate PV modules. Since UL 1741 is not allencompassing, it is left to the NTRL’s discretion to decide which aspects of the product’s construction—beyond those outlined in UL 1741 and UL 1703—qualify it to become a listed ac module.

One question that often arises is whether an ac module can have exposed dc conductors. John Wiles, program manager for the Southwest Technology Development Institute at New Mexico State University, is a longtime contributor to the development of codes and standards related to PV systems and components. He notes that “neither the NEC nor UL 1741 is specific enough” to answer this question. As a result, listed products are coming to market that have had this question answered differently by various manufacturers and testing laboratories. Wiles concludes, “We will not know if future changes to UL 1741 or NEC 2014 will clarify this situation until those changes—if any—are adopted.”

Note that manufacturers provide written guidelines for the benefit of system designers, installers and the AHJ. The NRTL evaluates these installation instructions, which are packaged with the product, as part of the listing process. This document contains important requirements specific to the device, and failure to follow these guidelines may void the product listing.

The NEC recognizes that manufacturers and testing laboratories may have specific product installation requirements that differ from general requirements found elsewhere in the Code. The Informational Note accompanying Section 110.3(A) clarifies: “Special conditions of use or other limitations and other pertinent information may be marked on the equipment, included in the product instructions, or included in the appropriate listing or labeling information.” When this is the case, follow the instructions provided with the equipment. Section 110.3(B) clearly states, “Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.”

AC MODULES VS. MICROINVERTERS

While an ac PV system may be functionally equivalent to a PV system made up of dc modules and microinverters, these are very different systems from the point of view of the NEC. An ac module is specifically defined in the NEC, whereas a microinverter is not. Similarly, UL 1741 includes content specific to ac modules (Sections 80–86). As a result, an NRTL can test, list and label a product as an ac module.

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