NEC 2017 Updates for PV Systems: Page 4 of 4
Inside this Article
ARC-FAULT PROTECTION WAIVER
In the 2014 revision cycle, CMP 4 implemented a subtle but significant change to 690.11 by removing the words “on buildings,” which meant that NEC 2014 essentially required all PV systems operating above 80 V to have dc arc-fault protection. Unfortunately, the product safety standard for dc arc-fault circuit protection, UL 1699B, at this time covers products working at currents up to 40 A only. It is difficult, if not impossible, to find arc-fault detectors for larger 200 A–400 A PV output circuits.
NEC 2017 addresses this problem by providing a dc arc-fault protection exemption for larger PV output circuits where these are not installed on buildings and the conductors are either underground or in metallic raceways or enclosures. The bad news is that this exception does not allow for the use of larger PV output circuits on buildings. The good news is that system designers are deploying 20 kW–50 kW 3-phase string inverters with integral string-level dc arc-fault detectors on most large rooftop PV installations.
NEW ARTICLE FOR LARGE-SCALE PV
With all the gnashing of teeth around rapid shutdown, many stakeholders may be unaware of a more significant revision that will be welcome news to anyone working on utility-scale PV systems. Specifically, NEC 2017 introduces Article 691, “Large-Scale PV Electric Supply Stations,” which provides a means of differentiation between requirements for decentralized building-mounted PV applications (Article 690) from those governing large-scale PV power stations that supply merchant power to the electricity grid (Article 691).
To clearly differentiate these two articles, the scope of Article 691 has very restrictive criteria. Per Section 691.1, “This article covers the installation of large-scale PV electric supply stations with a generating capacity of no less than 5,000 kW, and not under exclusive utility control.” Two informational notes follow. The first clarifies that facilities covered by Article 691 “have specific design and safety features unique to large-scale PV facilities and are operated for the sole purpose of providing electric supply to a system operated by a regulated utility for the transfer of electric energy.” The second informational note provides a reference to the National Electrical Safety Code (ANSI/IEEE C-2-2012); this code covers utility-controlled electric supply stations, which Section 90.2(B) designates as outside the scope of the NEC.
The rationale for developing Article 691 was that large-scale PV electric supply stations have more in common with power plants than with typical residential and commercial roof-mounted PV systems. Some of these PV power stations have capacities as large as 500 MW. Without any clear direction in the NEC on how to deal with these large facilities, some AHJs find themselves trying to enforce Article 690 requirements on these massive projects and questioning items such as grounding lugs and conductor sizing.
One of my power plant engineering colleagues works for a company that has designed and installed numerous wind and solar power stations. He has observed that inspectors rarely visit the company’s wind farms, whereas local AHJs inspect every single one of its PV power stations. While engineers have certainly made mistakes in large-scale solar facilities—just as they occasionally make mistakes in large coal and nuclear power plants—the idea that a local AHJ should enforce design and installation standards on these engineered merchant power plants makes no sense. Article 691 solves this problem for facilities that meet specific criteria.
In addition to providing these strict qualification guidelines, Article 691 details the design and construction documentation requirements for large-scale PV electric supply stations. For example, Section 691.6, “Engineered Design,” clarifies that upon request developers must provide to the AHJ design drawings and an independent engineering report that details compliance with Article 690, as well as any alternative methods that deviate from those described in Article 690 or other Code sections if applicable. To comply with 691.7, “Conformance of Construction to Engineered Design,” developers must document that PV plant construction complies with the engineered design and must provide an independent engineering verification report to the AHJ upon request.
Lastly, Article 691 provides brief direction on four areas where engineers are most likely to employ alternative design methods. These relevant sections include 691.8, “DC Voltage Calculations”; 691.9, “PV Equipment Disconnects”; 691.10, “Arc-Fault Protection”; and 691.11, “Grounding of Fences.”
ARTICLE 705 CHANGES
The three most important changes to Article 705 relate to center-fed panelboards, ac arc-fault protection for wire harnesses, and microgrids.
Center-fed panelboards. Connecting interactive PV systems to center-fed panelboards has become a persistent point of contention for residential installers, particularly those in western states where these panelboards are common. Many AHJs have required that installers remove and replace these panelboards to make a load-side connection, which not only is an unnecessary expense but also does nothing to improve system safety.
To eliminate this problem, CMP 4 added language to 705.12 that specifically addresses center-fed panelboards. Section 705.12(B)(2)(3)(d) now states: “A connection at either end, but not both ends, of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power source(s) output-circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar.”
Arc-fault protection for microinverter wire harnesses. During the 2014 revision cycle, CMP 4 added ac arc-fault protection requirements for utility-interactive inverters with exposed wire harnesses or output cables in 705.12(D)(6). There are two problems with this requirement. The first problem is that no listed ac AFCI protection equipment exists to meet this requirement. When required equipment is not available, language in Section 90.4 empowers AHJs to “permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code.” The second problem is that some AHJs have not deferred to 90.4 and have instead required that installers put microinverter ac cable systems in conduit. Since these wire harness and cable systems are not compatible with conduit, 705.6(D) has inadvertently disallowed microinverter systems in some jurisdictions.
To eliminate future opportunities for misinterpretation, CPM 4 removed 705.6(D) as part of the 2017 cycle of revisions. In addition, industry stakeholders have filed for a tentative interim amendment (TIA) that would officially remove this requirement from NEC 2014
Microgrid systems. A significant structural change is the addition of Part IV, “Microgrid Systems.” A new term in Section 705.2 defines a microgrid as a “premises’ wiring system that has generation, energy storage, and load(s), or any combination thereof, that includes the ability to disconnect from and parallel with the primary source.” An informational note clarifies that exclusions detailed in Section 90.2(B)(5) limit the application of Article 705 requirements to microgrid systems under the exclusive control of an electric utility. Since many microgrid systems include PV generation, the microgrid requirements will help installers and AHJs distinguish between requirements for these different electrical power systems.
Bill Brooks / Brooks Engineering / Vacaville, CA / brooksolar.com