Practical Application of NEC 2017

Every 3 years the solar industry receives an updated set of instructions for designing and installing PV systems—the National Electrical Code. Although adoption dates for the new Code vary by jurisdiction, many states will be operating under NEC 2017 by the end of this year.

In this article, we discuss the practical implications of NEC 2017 for PV system design and installation; we also provide specific information about the intent of the new requirements, compliance strategies, cost implications and perspectives from industry experts. This article references callout tags, shown in Figure 1, that provide system designers with a quick guide to 2017 Code changes.

Note that large-scale PV systems are not a main focus of this article. One of the changes in NEC 2017 is the introduction of Article 691, “Large-Scale Photovoltaic (PV) Electric Power Production Facility,” which applies to non–utility-controlled solar farms with a capacity greater than or equal to 5 MWac. The focus of this article is those residential, commercial and industrial PV systems that must comply with Article 690.

Callout A: Functional Grounded PV Systems

NEC 2017 introduces a definition for a functional grounded PV system, which is one that “has an electrical reference to ground that is not solidly grounded.” Unsurprisingly, this category includes PV systems that the industry previously referred to as ungrounded, which includes the majority of contemporary string inverters and module-level power electronic (MLPE) devices. Surprisingly, the functional grounded concept also applies to PV systems previously referred to as grounded, which includes legacy systems and large-scale systems that use transformer-isolated inverters.

An informational note in 690.2 elaborates: “A functional grounded PV system is often connected to ground through a fuse, circuit breaker, resistance device, non-isolated grounded ac circuit, or electronic means that is part of a listed ground-fault–protection system. Conductors in these systems that are normally at ground potential may have voltage to ground during fault conditions.” Any technician who has had to troubleshoot a ground fault in a so-called “grounded” PV system knows that “if a ground fault is indicated, normally grounded conductors may be ungrounded and energized.” That is because these systems make the connection to ground via a fuse, which does not meet the solidly grounded definition in Article 100.

The implications of the functional grounded PV system concept ripple throughout NEC 2017, affecting requirements related to disconnecting means, overcurrent protection, wiring methods and conductor identification. While the practical implementation of these changes is not difficult, especially in residential applications or systems with MLPE, it may take some time for installers and inspectors to align their practices and expectations with the new requirements.

Jason Fisher is Solar City’s principal compliance engineer as well as a member of Code-making panel (CMP) 4, which is responsible for Articles 690 and 705. Fisher notes: “In the short term, the new PV system grounding configurations in 690.41 will likely cause some confusion for the installation and enforcement communities. While the new grounding configurations are more comprehensive and accurate than those in previous Code editions, they do have implications beyond grounding practices related to conductor color coding, overcurrent protection and disconnecting means. Since these new installation methods are most appropriate for our current limited electrical systems, I am confident that everyone will become accustomed to these changes over a short period of time. Once you understand the definition of a functional grounded PV system, you realize that we almost never install solidly grounded or ungrounded PV arrays.”

According to Bill Brooks, principal of Brooks Engineering and another member of CMP 4, the expanded system grounding configurations in 690.41 will actually simplify system design, installation and inspection. Brooks points out that CMP 4 was able to eliminate Section 690.35, “Ungrounded PV Systems,” in its entirety as part of the 2017 cycle of revisions. The definition of a functional grounded PV system includes not only PV systems deployed with transformerless (TL) or non-isolated inverters, but also PV systems deployed with transformer-isolated inverters. Brooks concludes: “We now have a single wiring method that works for all types of PV systems and inverters. This single wiring method will help contractors and AHJs, saving everyone time and money. It is also safer.”

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