NEC 2017 Updates for PV Systems: Page 2 of 4
Inside this Article
Table 1 (see Inside This Article) provides a high-level overview of the vast number of changes to and extensive reorganization of Articles 690 and 705 implemented during the 2017 revision cycle. Since I cannot address all of these revisions in detail, here I focus on the most important changes related to PV system design and deployment. Some of these changes address long-standing pain points for installers and inspectors. In jurisdictions where PV system stakeholders and AHJs have an open dialogue, it may be possible to defer to the most recent revisions to NFPA 70 in certain circumstances.
Article 100 of the NEC defines solidly grounded as “connected to ground without inserting any resistor or impedance device.” Solidly grounded ac electrical systems are the most common way to supply power to loads in the US. When PV systems were new to the NEC, it was important that we design them in a similar fashion, with a solidly grounded system conductor, as doing so increased AHJ acceptance.
As the number of fielded PV systems grew, industry stakeholders realized the importance of dc ground-fault protection. The NEC first codified requirements for dc ground-fault protection in the 1990s; subsequent revision cycles extended these requirements to cover virtually all PV systems. Early dc ground-fault protection systems used an overcurrent-protection device in the grounded conductor-to-ground bond. The implementation of this simple design effectively replaced solidly grounded PV systems with not so solidly grounded PV systems. However, everyone continued to refer to these as grounded PV systems, out of fear that AHJs would otherwise cry foul.
NEC 2017 frees us from this confusion by introducing a new definition under 690.2. It defines a functional grounded PV system as one “that has an electrical reference to ground that is not solidly grounded.” This definition adopts terminology commonly used in Europe to describe how PV systems are referenced to ground in practice. An informational note further clarifies: “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.” In other words, virtually all of the PV systems installed over the last two decades are functional grounded rather than solidly grounded systems.
Design implications. This simple change in our understanding of PV system grounding has profound design implications. Not only does it impact where you place disconnects and overcurrent protection in PV circuits, but it also allows for a unified approach to these parameters. As long as we treated one subset of PV systems as solidly grounded and another subset as ungrounded, for example, you needed to have two sets of design standards.
By acknowledging that all modern PV systems are not solidly grounded (ungrounded or functional grounded), CMP 4 was able to eliminate 690.35, “Ungrounded PV Systems,” in its entirety. We then defined a single set of design standards, shown schematically in Figure 3, that apply to the dc side of a functional grounded PV system:
- Overcurrent protection is required in only one leg of a PV circuit [690.9(C)]
- Disconnecting means are required in both legs of a PV circuit [690.15]
- USE-2 or PV Wire is allowed as single-conductor cable in a PV array [690.31(C)]
These unified design standards solve a number of problems for installers and inspectors. As long as we treated some PV systems as solidly grounded, for example, opening the “grounded” conductor created the appearance of a Code violation in the minds of many AHJs and inspectors due to requirements in Article 240. However, if a ground fault occurs in a fuse-grounded PV system and the “grounded” conductor is bolted rather than switched, the only safe way for a field technician to service the system is to work at night. The CMP addressed this issue in NEC 2014 by creating an exception to 690.17(D) that allowed a disconnect switch for opening an accessible grounded conductor. Only qualified persons could access the switch, which was dedicated to PV array maintenance only. In NEC 2017, the revised language in 690.15 eliminates all of this confusion. It not only improves safety in the field but also eliminates the need for at least three previously required warning signs, including 690.5(C), 690.7(E) and 690.35(F).
The replacement of legacy grounded inverters with new and improved transformerless inverters is an important pain point that NEC 2017 addresses. Though legacy grounded inverters are susceptible to blind spots in ground-fault detection, earlier Code cycles held these systems to less-restrictive wiring method requirements than “ungrounded” systems with transformerless inverters, even though the latter offer improved ground-fault protection. Both USE-2 and PV Wire were Code-compliant single-conductor wiring methods for grounded inverters, whereas only PV Wire was compliant with transformerless inverters. The unified design standards in NEC 2017 eliminate this distinction and allow installers to replace grounded inverters with transformerless inverters without having to upgrade single-conductor wiring. To bring a legacy system into compliance with NEC 2017, installers need only rewire or replace the dc disconnects.