Ungrounded PV Power Systems in the NEC: Page 2 of 12

It was not until Section 690.35 was added to the 2005 NEC that it became possible to deploy Code-compliant ungrounded PV systems with higher utilization voltages— at least in theory. In practice, it would be a few more years before the UL-listed inverters and source-circuit conductors required for the installation of ungrounded PV systems in a cost-effective manner became commonly available.

Power-One was one of the first companies to introduce inverters in North America that were specifically designed for use with ungrounded PV arrays. Roy Allen, a technical sales specialist engineer with the company, explains, “Prior to 2008, there were no economically feasible methods for accommodating a floating array, one with no electrical reference to ground.”

What changed in 2008 is that the term Photovoltaic (PV) Wire was added to Section 690.35(D) of the NEC. PV Wire is a type of single-conductor cable specifically designed for use in ungrounded PV source circuits. It was not until listed PV modules became available from the factory with PV Wire cable assemblies, and system integrators could purchase PV Wire in bulk through conventional distribution channels, that ungrounded PV arrays could be installed in the same plug-and-play fashion as conventional grounded PV arrays.

Establishing Common Ground

According to NEC Section 690.4(E), “qualified persons” must wire PV systems. Article 100 defines a qualified person as “one who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved.” Ideally, the same holds true when it comes to the design of a PV system. However, even knowledgeable and trained persons may have difficulty discussing issues related to grounding.

While it is not difficult to ground electrical systems per se— to make the necessary connections physically and mechanically— many of the terms we use to describe these activities sound similar, even though they have distinct technical meanings. The fact that terms with different meanings sound similar is a potential pitfall when discussing ungrounded PV system designs and installations with plan checkers and inspectors. System designers and installers can lose credibility with this critical audience if they are not careful to say what they mean and mean what they say. Even veteran electricians, PV system designers and installers may want to refamiliarize themselves with the following definitions adapted from Article 100 of the NEC before reading further in this article or trying to describe an ungrounded PV power system to an AHJ.

Equipment-grounding conductor: the conductive path(s) installed to connect normally non–current-carrying metal parts of equipment together and to the system-grounded conductor, the grounding-electrode conductor, or both

Ground: the earth

Grounded (grounding): connected (connecting) to ground or to a conductive body that extends the ground connection

Grounded conductor: a system or circuit conductor that is intentionally grounded

Grounding electrode: a conducting object through which a direct connection to earth is established

Grounding electrode conductor (GEC): a conductor used to connect the system-grounded conductor or the equipment to a grounding electrode or a point in the grounding-electrode system

Ungrounded: not connected to ground or to a conductive body that extends the ground connection

Isolated vs. Non-Isolated Inverters

The most important Code requirement related to ungrounded PV systems is found in Section 690.35(G), which reads: “The inverter or charge controllers used in systems with ungrounded photovoltaic source and output circuits shall be listed for the purpose.” Since most PV systems in North America are grounded, most listed inverters available to this market are designed for use with grounded PV systems only. To deploy ungrounded PV systems, inverters that are specifically designed and tested for use with ungrounded arrays are needed. The main difference between an inverter designed for use with grounded PV systems and one designed for ungrounded PV systems is whether the inverter topology includes an isolation transformer. Inverters used in grounded PV systems include an isolation transformer, whereas inverters used in ungrounded PV systems do not.

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