The National Electrical Code
The National Electrical Code (NEC) published by the National Fire Protection Association (NFPA) is arguably the most comprehensive electrical installation code that is available from any source. However, as a published document, the safety achieved from using this Code is only as good as the people applying it. And those people, including the equipment installers and the electrical inspectors are only human and must use their knowledge and experience to implement those requirements and verify that the installation is correct.
The authority each inspector has varies greatly from jurisdiction to jurisdiction. Personal confidence and local procedures and policies frequently dictate AHJ actions. Some inspectors are required to follow the exact letter of the code as best they can interpret it and not deviate from what the actual language of the code says. In other regions, inspectors have more latitude and may apply their own interpretation of the code as well as look at other editions of the code to determine what best fits the situation at hand. Of course, local codes may supplement or modify the requirements of the National Electrical Code. And those local codes are also subject to interpretation.
Can Inspectors Get Too Much Information?
I am sometimes criticized for providing the readers of these “Perspectives on PV” articles in the IAEI News, with too much information, behind the scenes details, and engineering information. It has been noted that too much information may be confusing and muddy the water when it comes to inspecting PV systems, thus possibly delaying the inspection process and possibly increasing the cost of the PV installation (Photo 1).
I would reply to such critics as follows. We need to have the inspection community and also the installers as educated as possible and to be as knowledgeable on PV installations and the Code requirements for those installations. In that way, as these systems get increasingly complex, the safety of the public can be assured. Restricting the flow of information is no way to achieve that goal. If the information presented is too complex or confusing, let me know and I will elaborate in future articles or in personal communications.
The Edition of the Code In Force Varies
While some states adopt the current edition of the NEC when it is first published and becomes effective on 1 January of the code year, other jurisdictions sometimes delay the adoption of the code for various reasons until later in the year. However, many jurisdictions are not on the current code, and they are enforcing codes that may be one or two or more editions older than the current code. The use of these older codes may create some problems for the inspectors.
As each edition of the code changes, the Underwriters Laboratory (UL) Standards that apply to the equipment addressed in the Code are updated to reflect the most recent requirements in those new codes. As the UL Standards change, equipment manufacturers must build equipment meeting the most recent and current standards. And, since this equipment that is made to the newer standards and the latest requirements in the code can be different in form, fit, function, and application to the code requirements being enforced, it may be difficult to inspect this newer equipment or even permit it to be installed (photo 2).
For example, the 2011 and 2014 NEC require that there be DC PV arc-fault circuit interrupters (DCPVAFCI) in the DC PV circuits. UL Standards are being changed to allow these devices to be installed in the PV inverters or as separate devices. However, earlier codes do not require these devices; and to some extent when they are installed with the newer inverters it may pose inspection challenges, particularly when there is an external device required in the PV source or output circuits. Using an older edition of the Code poses issues on how the device should be installed or inspected.
Code Changes Have Impact
From the 2005 to 2104 editions of the NEC, there have been a number of changes in the code requirements found in sections 690.64(B) and 705.12 (D) dealing with making the utility connections for utility interactive inverters. They have been addressed in some detail in articles in this “Perspectives on PV” series over the years.
The Utility Interconnection Has Been Clarified
Briefly, the calculations for conductor sizes differ between the 2005, 2008, 2011, and 2014 editions of the NEC. These differing calculations result in different conductor size requirements and the specific allowances for different size conductors between residential (dwelling) and commercial PV installations.
As each edition of the code is developed, an attempt is made to make the specific requirements clearer, and to provide greater safety. Significant amounts of engineering analysis on existing electrical systems, new PV equipment capabilities, and how that equipment can best be used result in code requirements that ease installation requirements, reduce costs and increase safety.
However, a jurisdiction still enforcing an early edition of the code may not be able to take advantage of these increased safety capabilities unless the inspector has the authority to use requirements in the later editions of the code. Detailed knowledge of the changes in the Code, and why they were implemented gives the inspector the information needed to make a decision on using a later edition of the code for a specific requirement and specific installation.
Center Fed Panelboards
Center fed panelboards are another area that has received attention in various editions of the code. Previous articles in this series on center fed panelboards and the possibility of overloading them with the PV connection have received both positive and negative comments. Some engineers and inspectors say that there has never been a problem with connecting PV systems to center fed panelboards and they should be allowed. Other inspectors say that they have felt existing panelboards that are already warm to the touch and were reluctant to add PV to them whether it was center fed or not, due to the potential for adding too much heat to the panelboard. My feeling is that Murphy’s Law applies here, and if it can happen, it will happen—it being an overloaded center fed panelboard due to an added PV connection. Before the adoption of the 2014 NEC, center fed panelboards were not mentioned in articles 690 or 705. There appeared to be no way to legally place a PV connection on a center fed panelboard and meet the “opposite end from the utility end” requirement. However, the 2014 NEC addresses the PV connection to center fed panelboards and allows them where engineering supervision is involved (705.12(D)(3)(d). See photo 3.
So the AHJ, with some understanding of the requirements in the 2014 NEC, could allow an alternate methods-and-materials variance when dealing with center-fed panelboards, even though the code adopted by that that jurisdiction is earlier than the 2014 NEC and does not mention a connection allowance.
Conductor Sizes and Ampacities
The ampacity requirements and calculations for conductors used in PV source and PV output circuits were expanded and clarified in the 2011 NEC. See “Perspectives on PV” in the March-April 2015 IAEI News. Essentially, the revised code established requirements on how to deal with the conditions of use requirements. It explained that the 125% continuous current factor was not to be applied sequentially with the conditions of use factor, but that they were to be applied separately and the largest resulting conductor size used for the circuit. Inspectors enforcing earlier editions of the code could review these calculations, note that they are clarifications of the intent of the code and apply the language from the 2011 and later additions of the Code to sizing these conductors.
Significant discussions related to the grounding of electrical systems are widespread, and the grounding of PV systems is no exception. The NEC, the NEC Handbook, the Soares Book on Grounding, and numerous other documents have thoroughly discussed and debated the grounding of electrical power systems. The Internet abounds with documents, websites, and blogs that discuss the grounding of PV systems.
One prominent website maintains that the grounding requirements found in the NEC for PV systems and other electrical systems are unsafe and might even be dangerous. Arguments presented, to some extent, are based on maintaining electrical grounding requirements for machines while preventing or minimizing the potential for damage to those machines from nearby lightning strikes. When considering these suggestions for modified grounding, the AHJ should keep in mind that grounding systems that may be effective for protecting machinery may not be safe or effective for protecting humans from nearby lightning strikes. And, if the code is in error or needs to be changed in this area, there have been numerous opportunities to change it, which so far have not occurred.
Another area relating to grounding requirements is the appropriate grounding for carport structures holding PV arrays. While the articles 250 and 690 grounding requirements for such systems are clear-cut, it should be noted that the metal posts placed in the ground to support the carport roof may indeed serve as not only auxiliary grounding electrodes, but also the primary grounding electrodes if they are sufficiently deep in the ground [690.47(D)]. See photo 4. Of course, appropriate electrical paths including bonding jumpers and grounding conductors or grounding electrode conductors may be needed to ensure compliance with Code requirements.
PVC Conduits on Roofs
When it comes to rooftop installations where PV modules are usually mounted, the issue and questions of conduit types come up. Because of its ease of use, some installers would like to use PVC conduit (Article 352, Rigid Polyvinyl Chloride Conduit: Type PVC). There are several issues associated with using PVC conduit in this application. The first is the temperature limitation shown in section 352.12(D) where the maximum ambient temperature is 50° C (122°F).
While ambient temperatures in the vicinity may be 50° C or lower, the temperatures on a flat roof with parapets minimizing cooling breezes may well exceed 50° C. And if additional heating is provided by the conduits being in sunlight, the conduit temperature may very likely exceed 50° C by a significant margin. Coupled with these potentially high temperatures are the significant amounts of expansion that PVC conduit experiences under changes in temperature. This expansion and contraction of the conduit poses significant requirements on securing the conduit and supporting it, so it meets code requirements and does not sag excessively when hot.
Another issue that is less well known is that the Sunlight-Resistant marking on the PVC conduit indicates that it has passed a 720-hour accelerated UV test to meet the standard. The 720 hours of accelerated UV testing is equivalent to about 2 ½ years of outdoor exposure in the sunny Southwest. There is some question about the longevity of PVC conduit and whether it meets its original specifications under these high-temperature conditions and after years or decades of UV exposure that will be found in a PV installation. Other PVC electrical components such as PVC jacketed UF cable and nonmetallic liquidtight flexible conduit also marked Sunlight-Resistant have been known to fail in less than six years in hot sunny environments. As before, the AHJ armed with this information can make decisions on whether PVC conduit should be allowed in any particular installation.
Electrical Connectors on PV Modules
Each PV module comes with an attached set of cables (one positive and one negative), and those cables have connectors on the ends for connecting to other modules or to the PV source circuits. There are numerous manufacturers of connectors based both here in the US and overseas. Historically, there have been one or two prominent connector manufacturers whose connectors have been used on the majority of modules. However, that is changing, and modules from various manufacturers have many different connectors on them. While connector manufacturers may make statements that their connectors are compatible electrically, and mechanically with the connectors from other manufacturers, and those connectors appear to mate correctly, the suitability of mating connectors from two different manufacturers has not been standardized nor is compliant with any particular UL standard.
UL Standard 6703 deals with the connectors that are attached to PV modules and other connectors used in PV systems, both ac and dc. The standard is very specific in stating that if connectors from two different manufacturers are to be mated together, they must be evaluated, tested, and certified as being electrically and mechanically compatible. While that certification is possible, it is not practical.
The tests, evaluations, and certifications certainly could be accomplished at a certain point in time. However, the two manufacturers are making their connectors in isolation; they are using their unique mechanical assembly methods; they are using unique materials (both plastic insulators and metallic electrical contact components) and that information is proprietary. Product revisions are common. In many cases, the two connector manufacturers are using two different Nationally Recognized Testing Laboratories (NRTL) for certification to the UL 6703 standard. Confidentiality agreements between the manufacturers and the NRTLs prevent the dissemination of the material specifications. Follow-up testing by the NRTL is, at most, conducted every three to six months to determine what changes have been made in those specifications and materials for a particular connector. Without nearly continuous testing and certification of newly produced products from each manufacturer, it is not possible to verify that the electrical and mechanical compatibility between the two manufacturers continues to exist over time. See photo 5.
The AHJ must be aware of this issue and should consider refusing to approve any PV system that relies on electrical connections using mated connectors from two different manufacturers. A visual inspection and hand operation of the connector may indicate a satisfactory electrical and mechanical interface, but this is not a substitute for laboratory testing. Under the extremes of the environment associated with rooftop PV systems, those contacts may not be reliable for the decades-long life of a PV system.
There is a solution to this issue although not inexpensive. Many PV systems integrators, when faced with differing connectors, are using a short length of cable with different connectors on either end creating an adapter cable. At some expense and a potential reduction in the reliability of the overall electrical system, connector compatibility is assured over the life of the system.
Knowledge Is Power and Leads to Safer PV Systems
PV installers and electrical inspectors should attempt to maintain awareness of the continuing changes in product specifications, the standards that drive those changes, and the Codes that require the standards to be updated. In this way, the proper installation of that equipment can be assured even though the Code may not specifically address a particular piece of equipment in a particular application—a situation becoming more common every day.
AHJs need the knowledge and the authority to adapt the code in the ever-challenging career of a person actively charged with insuring the safety of the public.
For More Information
The author has retired from the Southwest Technology Development Institute at New Mexico State University, but is devoting about 25% of his time to PV activities in order to keep involved in writing these “Perspectives on PV’ articles in the IAEI News and to stay active in the NEC and UL Standards development process. Seven to eight-hour presentations are still available on PV and the Code, and they cover 2008–2014 NEC requirements. He can be reached at e-mail: email@example.com, phone: 575-646-6105
The Southwest Technology Development Institute web site maintains a PV Systems Inspector/Installer Checklist and all copies of the previous “Perspectives on PV” articles for easy downloading. A color copy of the latest version (1.93) of the 150-page, Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices, written by the author, may be downloaded from this web site: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html.