Ungrounded PV Power Systems in the NEC
Inside this Article
No, the sky is not falling—the ground just got lifted. However, while Code allows for the elimination of the grounded dc system and circuit conductors, all of the associated non–current-carrying conductive materials in an ungrounded PV system are still connected to earth.
Unlike in Europe, where ungrounded electrical systems are the norm, grounded electrical service architectures predominate in North America. Utilities supply a grounded ac service to all dwellings in the US. Therefore, the electrical wiring in US residences includes a normally current- carrying conductor that is intentionally connected to ground. This circuit conductor meets both the grounded conductor and the neutral conductor definitions found in Article 100 of the National Electrical Code. The electrical wiring in most US commercial buildings also includes a grounded conductor.
The NEC does allow for the use of ungrounded electrical systems—and specifically ungrounded PV systems—provided that certain requirements are met. In this article, we detail the Code requirements for designing and installing ungrounded PV systems while reviewing some fundamental concepts and definitions relevant to the task. We also describe the unique equipment and installation practices necessary to properly deploy these systems. It is particularly important that system integrators, plan checkers and inspectors understand what non-isolated utility-interactive inverters are, as well as how and when they are used. We discuss them in detail, including their unique ground-fault protection features.
While we believe that our interpretations and recommendations are consistent with the intent of the Code, the AHJ always has the final say regarding the acceptability of specific equipment and installation practices. Since it is not uncommon for AHJs to have requirements that exceed those found in the NEC, system installers need to do their due diligence to understand any requirements unique to their locality. The concepts, definitions and distinctions that we outline in this article should make it easier for designers and installers to communicate effectively with plan checkers and inspectors regarding ungrounded PV systems and non-isolated inverters, which are both relatively new and uncommon in North America, and may be misunderstood as a result.
Ungrounded Electrical Systems
John Wiles, program manager for the Southwest Technology Development Institute, has written extensively for solar professionals and electrical inspectors about ungrounded PV systems. In a Home Power magazine article on this topic (see Resources), Wiles explains: “When we discuss grounded versus ungrounded electrical systems, we are addressing whether one of the current-carrying circuit conductors, like the ac neutral conductor, is grounded or not.” In other words, an ungrounded system is simply an electrical circuit in which none of the current-carrying conductors is bonded to ground. It is not unusual to hear ungrounded circuits referred to as floating circuits—even though this term is not defined in the NEC or product standards—since no solid ground connection is made at the source or anywhere within the circuit.
Wiles points out, “Except for ungrounded 3-phase deltaconnected transmission and distribution systems, most of our electrical systems in the US have a grounded circuit conductor.” Control systems are another example of an ac electrical system that is commonly ungrounded in the US. Section 250.22 of the Code also details a variety of dedicated circuits that are required to be ungrounded.
Lifting the PV system ground. Since Article 690 was first included in the 1984 NEC, Section 690.41 has required a grounded conductor in dc PV power circuits. However, in the subsequent Code cycle, this section was modified to clarify that this general requirement applies only to two-wire PV arrays with a system voltage over 50 Vdc. Accordingly, system grounding is not required in many small stand-alone PV systems, such as those that perform remote lighting, monitoring or water pumping.
Note, however, that the term system voltage in Section 690.41 is generally interpreted as the maximum PV system voltage as described in Section 690.7(A), which must be corrected for the lowest expected ambient temperature. Therefore, the system voltage in all PV systems with a nominal voltage of 48 Vdc—and even some systems with a nominal voltage of 24 Vdc—will exceed 50 Vdc. That means the low-voltage system grounding exception found in this section is not broadly applicable. Further, while PV systems with a system voltage of 50 Vdc or less are not required to have a grounded conductor per se, Section 690.41 still requires them to use “other methods that accomplish equivalent system protection and that utilize equipment listed and identified for the use.”