Grounding Compendium for PV Systems

Why is PV system grounding so confusing and the subject so contentious? Perhaps because no concise yet detailed collection of information related to the topic has existed—until now, that is.

PV system grounding encompasses issues ranging from equipment grounding strategies, including bonding modules and grounding racking support structures, to system grounding considerations, including grounding electrode system options. Grounding PV systems correctly and effectively is difficult—and the topic is frequently contentious—because there is no one prescription for either the design process or the methods and materials. The difficulty of grounding PV systems also stems from the interactions of dissimilar metals used for racking structures, module frames and grounding devices. In addition, PV systems are frequently installed in harsh environments, creating situations in which traditional equipment and methods for bonding may not be adequate. Furthermore, PV systems often cover very large physical areas, interconnect to new or existing services, and include transformers of various voltages and configurations, all of which add complexity to designing and installing the grounding system.

Part of the reason why grounding PV systems correctly and effectively is difficult is that applicable UL standards—including UL 1703 (Flat-Plate PV Modules and Panels), UL 2703 (Rack Mounting Systems and Clamping Devices for Flat-Plate Photovoltaic Modules and Panels) and UL 467 (Grounding and Bonding Equipment)—contain requirements that are difficult to harmonize with one another and with the National Electrical Code. Most of the Code requirements related to grounding and bonding are found in Article 250, which describes methods and materials for grounding electrical systems. However, solar professionals also need to understand requirements found in Part V of Article 690 that relate specifically to PV system grounding, as these supplement or modify Article 250 requirements.

In this article, we take a practical approach to the NEC  requirements as applied to grounding PV systems: bonding modules, sizing and specifying equipment-grounding conductors, installing ac and dc grounding electrode conductors and systems, and so forth. In addition, we consider the implications of new methods for mounting modules, in particular racking solutions listed to UL 2703, and present best practices related to the design and installation of grounding systems.

Essential Terminology

While NEC Article 100 defines many of the following terms, here we explain them and put them in context.

Grounded, grounding. Since the NEC defines ground as “the earth,” these words can mean connected, or connecting, to the earth. More often they mean connected, or connecting, to a conductive device that is connected to the earth. The electric potential of the earth is assumed to be zero. 

Solidly grounded. This describes a direct connection to ground, one that does not include any additional impedance or resistance devices.

Bonded, bonding. Components and devices are considered to be bonded when they are connected in a manner that establishes electrical continuity and conductivity. A bonding conductor or bonding jumper is used to establish and maintain a bond.

System grounding. This refers to the practice of intentionally bonding one current-carrying conductor to ground. In ac systems, the grounded conductor is also known as the neutral conductor. DC systems can be negatively or positively grounded—based on the polarity of the grounded conductor—or ungrounded. The dc system grounding connection is accomplished through the main bonding jumper or, in the case of separately derived systems, via the system bonding jumper. System grounding on the dc side of a PV system generally occurs via a ground-fault protection circuit that is internal to a listed inverter, as shown in Figure 1.

Equipment grounding. This refers to the practice of bonding normally noncurrent-carrying metal equipment—like the module frames in Figure 1—to establish an electrically continuous path.

Grounding electrode. This is a conductive component through which an electrical system’s connection to ground is established. NEC Section 250.52(A) describes allowable grounding electrodes.

Grounding electrode system. This is what is formed when all grounding electrodes present at a building or structure are bonded, as required by NEC Section 250.50.

Grounding electrode conductor (GEC). The GEC connects the grounded conductor and/or the equipment grounding conductor to the grounding electrode system.

Continuous. When referring to wires used as GECs, this term indicates that the wire is installed as a single length of conductor without splices or joints, unless splicing is accomplished using irreversible crimp connectors or exothermic welds.

Sole connection. This term indicates that a GEC is connected to only one type of grounding electrode. This term is easily misunderstood. For example, a GEC can be connected to more than one grounding electrode—say, two ground rods—and still be considered to have a sole connection. Similarly, multiple GECs connected to the same grounding electrode are each considered to have a sole connection. When GECs have a sole connection, their maximum size is limited per NEC Sections 250.66 for an ac system and 250.166 for a dc system.

Equipment-grounding conductor (EGC). EGCs are used to connect all exposed, normally noncurrent-carrying metal equipment to the grounding-electrode system and to the grounded conductor if present.

Ground fault. This is an unintentional connection between an ungrounded current-carrying conductor and a normally noncurrent-carrying grounding conductor or grounded metal equipment—such as an enclosure, raceway, module frame or rack component—characterized by undesired current flow on the EGC.

Effective ground-fault current path. This refers to an intentionally constructed, electrically conductive low-impedance path with adequate capacity to conduct ground-fault current. 

Voltage to ground. In a grounded electrical system, this term refers to the nominal or measured difference in electrical potential between a given conductor and the grounded conductor or ground, as measured in volts. Note that for ungrounded systems, this refers to the difference in electrical potential between a given conductor and any other conductor in the circuit.

Listed. This term refers to equipment and materials included in a list published by an evaluation organization that is acceptable to an AHJ—such as a nationally recognized testing laboratory (NRTL)—and that performs periodic production inspections. The product listing must either state that designated standards are met or indicate suitability for a specific purpose. Depending on the evaluating organization, equipment may need labeling, per the definition in NEC Article 100, to be considered listed.

Identified. This term applies to equipment that is recognized as suitable for a specific purpose, environment and application, or as meeting a particular Code requirement. For example, NRTL-listed and -labeled equipment is considered identified.

Bonding and Equipment Grounding 

NEC Sections 250.4(A) and (B) require the bonding of electrically conductive materials and equipment to establish an effective ground-fault current path. Along with Section 250.110, these sections require the bonding of normally noncurrent-carrying metal components that are likely to become energized in the event of a fault. While many of these components—such as metal enclosures and conduit—are common in other types of electrical installations, PV module frames and racking present unique challenges, as do the harsh environmental conditions that PV arrays are subject to.

The NEC makes a distinction between bonding and grounding, although the words are often used interchangeably. Note that bonding two pieces of metal does not necessarily accomplish grounding, as the latter requires an effective path to ground in addition to the bond between the metal pieces. Article 250, Part V, details bonding methods, materials and requirements referenced by other Sections of Article 250, including Part VII, “Methods of Equipment Grounding.” Article 690, Part V, supplements the requirements for equipment grounding and bonding found in Article 250.

Note that regardless of system voltage, or whether the system is grounded or ungrounded, equipment grounding is required on all PV systems. Equipment grounding is indispensable because it provides a path for ground-fault current, which is what enables the operation of the overcurrent protection and ground-fault protection devices that protect people and property from harm. The EGC has to continue from the PV array to the other equipment in the system, and must run with the circuit conductors once they leave the vicinity of the PV array.

Grounding and Bonding Connections

To bond two pieces of metal in a Code-compliant manner, you must make a connection that effectively makes two pieces of metal into one, at least in terms of electrical continuity. Per Section 250.8(A), permitted methods for making bonding and grounding connections include listed pressure connectors, terminal bars, exothermic welds, machine screw–type fasteners engaging not less than two threads or secured with a nut, thread-forming machine screws engaging not less than two threads, connections that are part of a listed assembly and other listed means. Devices that depend on solder alone cannot be used for grounding or bonding equipment.

Steve Szczecinski is a product applications engineering manager at ERICO, a company that pioneered exothermic welded connections for electric railway applications. Szczecinski notes: “Exothermic welding is a superior connection system because it results in maximum contact area with no voids or gaps, which means no spaces for moisture to penetrate. Exothermic connections do not loosen over time due to thermal cycling, and high-current tests show that exothermic welds can carry more current than the conductors themselves.”

Appropriate lugs and hardware must be used to bond enclosures and equipment to the equipment-grounding system to ensure continuity and sufficient fault-current capacity. Every single point of bonding is critical: The failure of any one point could render the equipment-grounding system, as well as overcurrent protection or ground-fault protection devices, ineffective.

Bonding enclosures and conduit. NEC Section 250.12 requires the removal of nonconductive materials such as paint from the surfaces used to form the electrically conductive bond. Further, in keeping with Section 110.3(B), installers must follow manufacturer instructions, which may mean using a specific grounding busbar or lug for bonding, or selecting a specific location on the device. Typically, enclosure-grounding points include a threaded boss or a flared hole to provide the minimum two-thread depth required for bonding with thread-forming or machine screws.

Section 250.97 requires electrical continuity for metal raceways and cables with metal sheaths that contain any conductor other than service conductors for circuits that are over 250 V to ground. Many PV systems have dc voltages of more than 250 V to ground—or between the positive and negative poles in ungrounded systems—and are subject to this requirement. It is especially relevant to PV systems on buildings where the dc circuits need to be in metallic raceway or Type MC (metal clad) cable until the first accessible disconnect. On large ground-mounted systems, metallic raceways provide protection for cables or are used to transition from underground PVC to enclosures and equipment above grade.

The connectors used with metal raceways typically provide electrical continuity, in which case installers need to address only connections to enclosures and equipment. Most standard fittings do not provide continuity when metal raceways enter an enclosure via concentric or eccentric knockouts—unless the hole is opened up to its full size. Therefore, most inspectors expect to see bonding bushings installed wherever metal raceway is used on the dc side of a PV system.

Exceptions to Section 250.97 apply in the following circumstances: where there are no reducing washers or concentric or eccentric knockouts, or “where a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection.” In these instances, the following alternatives to bonding bushings are allowed: threadless connectors and couplings for metal-sheathed cables, two locknuts (one inside and one outside) for rigid or intermediate metal conduit, one locknut inside the box where fittings are used with shoulders that seat firmly against the enclosure, or a listed fitting. Some electrical enclosures with eccentric or concentric knockouts are listed as providing a reliable bonding connection. This suitability must be identified and generally means that there is only one eccentric knockout ring.

Note that Section 250.86 does not require a metal elbow in an underground nonmetallic conduit to be bonded. Since friction may damage a PVC elbow during a large conductor pull, metal elbows are sometimes used for their superior mechanical characteristics. As long as the metallic elbow is under at least 18 inches of cover or encased in at least 2 inches of concrete, it is considered to be “isolated from possible contact” and does not need to be bonded to the equipment-grounding system.

Where expansion fittings are used with metallic raceways, Section 250.98 requires that the fittings and telescoping raceway sections be electrically continuous. This can be accomplished via either an internal, built-in flexible bonding jumper across the expansion gap, or an external field-installed identified bonding jumper that is of sufficient length and flexibility and is appropriate for the environment.

Bonding modules and racks. UL 467 covers general devices for grounding. A wide variety of manufacturers produce familiar types of electrical grounding lugs and connectors—ground rods, ground rod clamps, split bolts, grounding bushings and so forth—and certify them to UL 467. This standard is also used to list some PV-specific devices. For example, BURNDY’s stainless steel Wiley WEEB and ILSCO’s tin-plated aluminum SGB-4 are both listed to UL 467. When installed correctly, and in a suitable environment, listed products can be used to bond any piece of metallic equipment in a PV system to an EGC.

UL 1703, the equipment standard for flat-plate PV modules, requires manufacturers either to provide certified module frame grounding means with their product or to specify acceptable means in detail in the installation manual. Until recently, the most common method for bonding modules was to connect a direct-burial–rated lug to a marked grounding hole on the module frame, using a stainless steel thread-forming screw. Typically, a copper EGC is connected to each module’s grounding lug and to lugs connected to metallic support structures, and continued on to the rest of the equipment-grounding system.

While this is certainly an acceptable method, concerns include the following:

  • Potentially unclear manufacturer instructions
  • Possible galvanic corrosion due to dissimilar materials
  • Inappropriate use of indoor-rated devices
  • Connections loosening over time due to expansion and contraction, even when torqued correctly at installation
  • Incompatibility between modules and the devices listed for grounding them
  • The expense of the materials and labor to install lugs and connect an EGC to each module

UL 2703. The UL 2703 Standard was developed in part to address perceived shortcomings associated with relying solely on UL 1703 as a governing standard for grounding PV modules. For example, many innovative grounding solutions for PV applications do not originate with module manufacturers, but rather with companies that provide structural mounting solutions.

UL 2703 allows module mounting and clamping components to be evaluated and listed as a grounding means. Many mounting system manufacturers are already certifying products to the draft UL 2703 standard. UL enlisted the involvement of subject matter experts from the solar and electrical industries to review public comments and modify the draft, and it is now seeking ANSI accreditation for the standard.

The advantage of using a racking system that is listed for bonding module frames per UL 2703 is that it reduces both materials and labor. The act of installing a module and torquing the mounting hardware to the specified value bonds the module to the mounting system. Special teeth or nubs that bite through the anodized finish on module frames and make a solid low-resistance connection to the rails are often enough to accomplish the bonding connection; in other cases, the bond is established with specialized connection hardware. However it is done, an electrical connection is made simultaneously with the mechanical connection, and the net result is an electrically continuous unit.

Note that UL 2703 product listings are often system level, meaning that the listings apply to specific configurations of modules, racks, clamps, grounding hardware and so forth. With a system-level listing, specific product configurations only are listed. Even a minor change requires additional testing and a change to the system listing, or the system is not listed. The entity that initiates the process with the NRTL—which is usually the racking manufacturer—is responsible for choosing the products to include in the test. Since it is not practical to evaluate every imaginable configuration, listed system options are limited to the manufacturer’s product configurations.

In a report published by the Solar America Board for Codes and Standards (Solar ABCs’ Interim Report, March 2011), “Grounding Photovoltaic Modules: The Lay of the Land,” author Greg Ball expects that UL 2703 can serve a more general purpose than system-level listings: “UL is planning to list grounding components independent of the racking certification through this standard. There is also the intent to establish subsystem-level testing of bonding—for example, tests using multiple modules and components connected together, rather than single connections—and impedance requirements for metal apparatus containing multiple strings of modules. It is expected that the standard will quantify ampacity and cross-sectional area requirements, similar in principle to the approach taken for the cable tray systems used instead of conduit to support electrical and communication cables.”

Product selection and installation. System-level listings under UL 2703 can put a lot of responsibility on PV system designers, installers and inspectors, who must verify that the configuration used is in fact listed. John VanWinkle, an engineer in training with the mounting system manufacturer Schletter, cautions: “It is important to review module manufacturer installation instructions to determine what is acceptable. Each module has different requirements, and each manufacturer will specify requirements for equipment that is used to secure and bond the module.”

This is one of the reasons that some manufacturers list PV-specific grounding components to UL 467. Jeremy Turner, the engineering manager for the racking manufacturer Daetwyler Clean Energy, points out: “There are really two major components to UL 2703. On the one hand, there are grounding and bonding considerations; on the other, there are structural considerations, which include mechanical strength and material suitability.”

Turner explains: “Different NRTLs have different interpretations about how to list products in light of these separate requirements. For example, Intertek has decided to list grounding and bonding components separately from structural components, which makes sense given all the different types and applications for racking structures. Since our stainless steel grounding hardware is listed to UL 467 as connectors, we can offer integrated grounding and bonding solutions with all Daetwyler racking products. This hardware provides multiple redundant ground paths and is not only designed to break through the anodized module coatings, but is also vibration resistant so that this bond is maintained for the life of the product. We use serrated hardware throughout the rack structures to maintain the system bond and ensure safety.”

Note that when the equipment-grounding system includes separate rack or cable tray sections, they must be bonded or identified as EGCs. Among other things, this means that installers need to include bonding means around expansion joints, unless the joint itself is identified as a bonding jumper or device.

David Del Vecchio is a senior engineer with Strata Solar, a project developer and EPC. Del Vecchio encourages installers to exceed minimum equipment-grounding requirements to ensure system safety over time. He cautions: “Your ground-fault protection is only as effective as your return ground path. Let’s assume you are using a UL 2703–listed racking system, and the installation requirements call for a single EGC connection. What happens if this single connection fails over time, perhaps because it was under- or over-torqued or otherwise improperly installed? Now you have a section of the array that is effectively ungrounded.”

Del Vecchio concludes: “If a ground fault occurs at a location without an effective return ground path, there is no ground-fault detection. The inverter will continue to run and the consequences can be disastrous. Therefore, when grounding rooftop racks, I always recommend multiple EGC attachments beyond the minimum requirements, because redundant grounding points in a bonded section of an array ensures a return path for ground-fault current and GFP detection.”

Equipment Grounding Conductors

Per NEC Section 250.118, the Code allows 14 types of EGCs, which are either run with or used to enclose the circuit conductors. The EGC must have one or more of the following characteristics:

  • A copper, aluminum or copper-clad aluminum conductor [250.118(1)]
  • Metallic conduits such as RMC, IMC or EMT [250.118(2), (3), (4)]
  • Flexible metal conduits or tubing such as FMC, LFMC or FMT [250.118(5), (6), (7)], provided that numerous conditions are met; for example, a separate EGC is often required with flexible metal conduit and tubing
  • The armor of Type AC cable [250.118(8)]
  • The copper sheath of a mineral-insulated metal-sheathed cable [250.118(9)]
  • Type MC cable, provided that it contains an EGC or some combination of the metallic sheath and the internal EGC is identified as an EGC [250.118(10)]
  • Cable trays or cablebus framework as permitted in 392.10 and 392.60 or 370.2 [250.118(11), (12)]
  • Metal raceways, auxiliary gutters or surface metal raceways listed as electrically continuous or listed for grounding [250.118(13), (14)]

Eric Sadler is the engineering sales manager for Snake Tray, a company that manufactures cable tray wire and cable management solutions. Sadler notes: “We can say that a cable tray is a ‘UL Classified’ EGC if it has a cross-sectional area of continuous bonded steel equal to or greater than 0.2 square inches. In our case, this varies by product line. On one hand, our 802 Series Mega Snake, which can span distances up to 20 feet, is UL Classified as an EGC based on its cross-sectional diameter. On the other, our 407 Series Solar Snake Tray is listed to UL 2239, which is a structural standard that has nothing to do with grounding.”

However, it is possible to install the Solar Snake Tray according to Code, Sadler explains: “To comply with the NEC, the 407 Series product needs to be bonded to the rest of the grounding system using listed split bolts, one half of which makes a compression connection to the tray spline while the other half connects to a copper grounding wire. Because it is a continuous metallic pathway, it bonds easily from unit to unit and to the rest of the system. When it is mechanically fastened, you get a bond. To support a ground-fault condition, every fourth stick is additionally bonded to a grounding conductor, which is typically 6 AWG copper.”

NEC Article 690 discusses other devices and methods of installing EGCs for bonding PV module frames and racking components. Section 690.43(C) addresses the use of a structural racking system as an EGC. Metal mounting systems are allowed for grounding purposes provided that either they are identified as EGCs or the individual racking sections are bonded together and to the grounding system using identified bonding jumpers. Note that identified is used here rather than listed; this provides AHJs with more leeway when making interpretations.

Sections 690.43(D) and (E) allow the use of devices that are listed and identified as appropriate for bonding module frames to racking parts and adjacent modules. This presents an alternative to mechanically attaching a grounding lug to a marked grounding point on each module frame. The Wiley WEEB from BURNDY is the classic example of a listed module-to-mount (and module-to–adjacent module) bonding solution.

Sizing EGCs. EGCs for ac and dc circuits in PV systems are sized according to NEC Table 250.122. This table specifies a minimum allowable conductor size based on the rating of the overcurrent protection device (OCPD) protecting the circuit conductors, and the EGC material (copper, aluminum or copper-clad aluminum). For example:

  • A 14 AWG copper EGC (or 12 AWG aluminum) is sufficient for a circuit protected by a 15 amp OCPD, which is typical of a fuse rating used in a PV source-circuit combiner box. 
  • A 6 AWG copper EGC (or 4 AWG aluminum) is sufficient for a circuit protected by a 200 amp OCPD, which is typical of a fuse rating used in PV output circuits connected to the input combiner of a 500 kW inverter.

While the absolute minimum EGC size allowed is 14 AWG, Section 250.120(C) requires that EGCs smaller than 6 AWG be “protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of building or structures and where not subject to physical damage.” For this reason, many PV system installers use 6 AWG copper EGCs to bond modules and racking, on roof-mounted systems in particular, and then reduce the EGC to the minimum size allowable per Table 250.122 where it transitions into conduit.

Multiple circuits. What about the common scenario where multiple PV source circuits run in the same raceway, cable tray or enclosure? Chuck Ladd, a solar system designer and PE with PowerSecure, explains: “The basis for sizing EGCs per 250.122 is to protect against a single fault, which is why 250.122(C) allows us to size the EGC for multiple circuits based on the largest single overcurrent device and not the sum of all the overcurrent devices.”

As an example, imagine that you need to run six pairs of PV source-circuit conductors from a rooftop array down to an inverter-integrated input combiner that is fused at 15 A per circuit. If all of the source-circuit conductors are run in a single conduit, then a single 14 AWG copper EGC meets the minimum Code requirement. However, if the source-circuit conductors are split between two separate conduits, then each conduit needs a 14 AWG copper EGC.

Parallel conductors. Current-carrying conductors are frequently wired in parallel on the ac output of large inverters, or on the dc side of the system between subarray combiners and the inverter dc input bus. NEC Section 250.122(F) requires installation of a full-sized EGC in each of the parallel raceways or cables, sized based on Table 250.122. This can create difficulties if you want to use cables in parallel. A cable typically consists of multiple insulated current-carrying conductors, plus a smaller bare EGC. Since this EGC is sized according to the ampacity of the circuit conductors, it is likely undersized if cables are used in parallel.

Proportional size increases. According to Section 250.122(B), if ungrounded conductors are increased in size, then you must increase the size of the EGC proportionally. As described in the NEC 2011 Handbook, this requirement applies when conductors are upsized “to compensate for voltage drop or for any other reason related to proper circuit operation,” but does not apply when conductors are upsized “for the purposes of ampacity adjustment, correction, or both.” While 250.122(B) applies to the ac side of a PV system, the following language in 690.45(A) exempts PV source and output circuits: “Increases in equipment-grounding conductor size to address voltage drop considerations shall not be required.”

Section 690.45 provides additional EGC size requirements for the less-common cases of circuits without OCPDs, or systems without ground-fault protection.

Learning from failure

An equipment-grounding system fails when it no longer maintains electrical continuity between all components. To prevent failures, carefully consider whether the methods and materials are suitable for the intended use and for the installation environment. Galvanic corrosion, which occurs when dissimilar metals like aluminum and copper are in direct contact, has been a common cause of equipment-grounding system failures in PV arrays. (See “Galvanic Corrosion Considerations for PV Arrays,” SolarPro magazine, June/July 2011.)

According to the Solar ABCs’ interim grounding report, the following materials are generally galvanically compatible: nickel, tin, zinc, zinc-aluminum alloys, 5000 or 6000 series aluminum alloys (alloyed with magnesium and silicon), commercially pure aluminum, and stainless steel containing a minimum of 16% chromium. Note that copper is not on this list. PV system designers and installers must provide reliable means of galvanic isolation—such as stainless steel washers or components with suitable platings or coatings—when using copper conductors to bond dissimilar metals.

Appropriate grounding solutions are inherently location- and application-specific. While tin-plated copper grounding lugs are a de facto industry standard for bonding aluminum module frames to copper EGCs, they may corrode over time if installed in a salty, humid or otherwise harsh environment. While we often see corrosion above ground, it is also a concern below ground. (See “Corrosion Impacts on Steel Piles,” SolarPro magazine, December/January 2012.) ERICO’s Szczecinski points out, “All connections below grade must be UL rated for direct burial as required by Code.” He continues: “Underground corrosion can be caused by moisture and exposure to oxygen, chlorides, sulfides, certain soil bacteria or stray currents, especially dc. Long-term studies show that stainless steel grounding electrodes provide the longest service life, but they are used only in the most corrosive soil conditions due to cost. Copper grounding conductors provide the optimal combination of service life and cost. If theft deterrence and cost are important concerns, copper-bonded steel conductors, which have a copper plating over a steel core, are an alternative to solid copper. While galvanized steel is also an option, it has the shortest service life; studies show an expected service life of 15 years, compared with 30 years or more for copper systems.”

System Grounding

With the exception of some 3-phase delta systems, the vast majority of ac electrical systems in the US are solidly grounded. So it is not surprising that the vast majority of PV systems installed to date in the US are grounded on the dc side. In a grounded PV system, either the positive or the negative current-carrying conductor is bonded to ground through the inverter’s ground-fault–protection device. While the NEC has allowed ungrounded PV systems since the 2005 cycle of revisions, the unavailability of compatible equipment slowed the adoption of these systems. In recent years, however, ungrounded PV systems, which use non-isolated (transformerless) inverters, have gained significant traction in the US market. (See “Ungrounded PV Power Systems in the NEC,SolarPro magazine, August/September 2012.)

On many larger PV installations, system grounding is necessary at multiple points, via either a main bonding jumper or a system-bonding jumper. This is not to say that any source—whether utility service, inverter or transformer—should have more than a single point where the system grounding connection is made; rather, on large sites you may encounter multiple transformers, each creating a separately derived system; multiple inverters, each requiring GECs or taps; and even multiple service entrances, each requiring a system-grounding connection.

Where transformers are installed as part of a PV system, they generally create separately derived systems, in which there is no electrical connection between the circuit conductors on the primary side of the transformer and those on the secondary side, other than connections through EGCs, metal enclosures or raceways. Exceptions are transformers installed as part of a service or autotransformers with a single winding, which do not result in separately derived systems.

As an example, consider a step-down transformer installed to provide power to a data acquisition system that requires a lower voltage than the inverter ac output connection. You likely need to consider this transformer as a separately derived ac system, in which case it must follow the extensive requirements found in NEC Section 250.30. Note that where the transformer, the source of the separately derived system, is installed outdoors (as is typical in PV systems), Section 250.30(C) requires that a grounding-electrode connection be made at the transformer “to one or more grounding electrodes in compliance with 250.50.”

DC system ground

PV systems without a direct connection—other than equipment grounding—between grounded conductors on the dc side of the system and grounded conductors on the ac side must have dc system grounding. This means that a dc grounded conductor must be connected to a dc grounding system that is bonded to the ac grounding system. These general requirements apply to all grounded PV systems, but not to ungrounded PV systems or systems using ac modules.

Grounded systems. Frequency- or transformer-based inverters operate using a grounded PV array, and the dc and ac sides of the system are galvanically isolated. Therefore, dc system grounding is required, and you must bond the dc and ac grounding systems. Grid-direct inverters for grounded systems have a marked terminal for connecting the dc GEC.

NEC Section 690.42 requires that the dc system-grounding connection be made at a single point in the dc circuit. In grounded utility-interactive PV systems, this system-grounding connection is made via a listed inverter’s internal ground-fault–protection device. It is therefore critical for installers to ensure that the only circuit grounding connection point made is the grounded conductor-to-ground bond inside the inverter. If an inspector, plan checker or electrician expects to see otherwise, refer them to the Section 690.42 Exception: “This bond, where internal to the ground-fault equipment, shall not be duplicated with an external connection.” Any additional system grounding connection creates an unwanted parallel path for ground-fault currents.

Ungrounded systems. Transformerless inverters are not isolated and operate using an ungrounded PV array. There is no dc system grounding at all. Since there is no dc grounded conductor, there is no grounded conductor-to-ground bond. In the SolarPro magazine article “Ungrounded PV Power Systems in the NEC (August/September 2012), John Wiles points out that if a non-isolated inverter adheres strictly to the UL 1741 standard, it will not even have a marked GEC terminal.

While the NEC has never required a dc GEC for transformerless inverters, language in Section 690.47(B) left room for misinterpretation by not explicitly addressing this issue. Therefore, the code-making panel responsible for Article 690 added the following paragraph to Section 690.47 during the 2014 revision cycle: “An ac equipment-grounding system shall be permitted to be used for equipment grounding of inverters and other equipment, and the ground-fault detection reference for ungrounded PV systems.” (Emphasis added.)

Systems using ac modules. AC modules are not required to have a dc system ground. This is partly established in NEC Section 690.6(A), which states, “The requirements of Article 690 pertaining to photovoltaic source circuits shall not apply to ac modules.” Further, this language was added to Section 690.47(C) in the 2011 NEC: “This section does not apply to ac PV modules.” Note that these sections apply specifically to listed ac modules with no field-serviceable dc wiring, which is a category of equipment that is distinct from microinverters. (See “Alternating Current PV Modules in the NEC,SolarPro magazine, June/July 2012.)

Grounding Electrodes

The effectiveness of a connection to ground varies based on the type, depth and quantity of grounding electrode(s), as well as soil conditions and seasonal variability. NEC Section 250.52 details what can be used as a grounding electrode and under what conditions. Per Section 250.52(A), at least one of the following is required for each grounding system:

[1] Metal underground water pipe is suitable for use as a grounding electrode if it is in contact with the ground for 10 feet or more. Note that per Section 250.68(C)(1), if a grounding electrode connection is made to a water pipe inside a building, it must occur within the first 5 feet from the pipe’s point of entrance into the building.

[2] Building steel is suitable provided at least one structural member is in contact with the ground for at least 10 feet, or the structural steel column is bolted to a concrete-encased electrode.

[3] Concrete-encased electrodes are suitable assuming that each electrode is at least 20 feet long and is made of either at least ½-inch–diameter steel or 4 AWG bare copper. A concrete-encased electrode is commonly referred to as a Ufer ground. Note that a vapor barrier, insulation or gravel backfill between the concrete and the earth dramatically reduces the electrode’s effectiveness (and that of building steel). 

[4] Ground rings are suitable provided that each ring is at least 20 feet long and made of 2 AWG copper. The ring needs to encircle the structure at a depth of 30 inches below the surface. Ground rings are commonly installed around large inverter pads and serve as grounding electrodes for multiple inverters.

[5] Ground rods are suitable provided that each rod is at least 8 feet long; is not less than ⅝ inch in diameter; and is stainless steel, or copper- or zinc-coated steel. Ideally, a ground rod extends into the permanent moisture level of the soil. If a ground rod cannot be driven perpendicular to the ground or at a 45° angle or less, then it can be buried, provided that the depth is at least 30 inches. Ground rods are common at residential buildings as part of the existing electrical system and as additional electrodes associated with PV systems; they are also frequently installed in ground-mounted PV array fields.

[6] Other listed grounding electrodes are allowed, including electrolytic ground rods, also called enhanced or active ground rods. These refillable tubes contain metallic salts that leach into the soil, providing a low-resistance path to ground that improves over time.

[7] Plate electrodes are suitable, provided the surface area exposed to ground is not less than 2 square feet and the thickness of the plate is at least 0.06 inch for iron or steel or not less than ¼ inch for nonferrous metals. Per Section 250.53(H), plate electrodes must be buried at least 30 inches below grade.

[8] Unlisted local metal underground systems or structures such as tanks, piping systems or well casings are allowed, provided they are not bonded to a metal water pipe. Coatings like paint or enamel can impair the connection to ground and render the object ineffective as a grounding electrode.

When any of these allowable grounding electrodes are present at a building or structure, NEC Section 250.50 requires that they all be connected with bonding jumpers to form a grounding-electrode system. However, it provides an exception for concrete-encased electrodes: Where these cannot be accessed without disturbing the concrete, they do not need to be bonded to the grounding-electrode system. Generally speaking, options 1–3 and 8 will already be in place; options 4–7 may already be in place or may be installed as part of a PV system. Note that aluminum and underground gas piping systems are not permitted for use as grounding electrodes, according to Section 250.52(B).

Supplemental electrodes. Per Section 250.53(A)(2), systems with a primary electrode that is a rod, pipe or plate require a supplemental electrode, unless the primary electrode has a resistance to ground of 25 ohms or less. A valid assessment of ground resistance requires a fall-of-potential test, which can be accomplished with meters like ERICO’s ERITECH EST401 Ground Resistance Tester or Fluke’s 1623 GEO Earth Ground Tester. A three-point fall-of-potential test measures the resistance to ground of ground rods or grids; a four-point fall-of-potential test measures soil resistivity and can locate areas of low soil resistance for a better connection to ground.

For smaller PV systems, it may be easier to install a supplemental electrode than to conduct a valid fall-of-potential test. This Code requirement to have a resistance to ground of 25 ohms or less does not apply to systems with ground rings or concrete-encased electrodes. Any supplemental electrode installed to lower the resistance to ground must be positioned at least 6 feet from the primary electrode, and a supplemental bonding electrode connection must be installed. Note that 25 ohms is a minimum safety provision, which may not be adequate for all applications. The IEEE 142-2007 Standard, which covers grounding practices for commercial and industrial power systems, recommends that the resistance to ground be kept to 5 ohms or less, which can be accomplished by increasing the grounding electrode surface area.

In the 2008 NEC, Section 690.47(D) requires “additional electrodes for array grounding” at all ground- and pole-mounted PV arrays; the electrode must be directly connected to the array frame or structure using a GEC sized per Section 250.166. While this requirement was deleted during the 2011 cycle of revisions, this should not be interpreted as a signal that PV arrays do not require additional electrodes. Section 250.32 requires that a grounding electrode be installed at structures with feeders. As defined in Article 690, a PV array includes a support structure and foundation, and thus requires an electrode. Furthermore, the code-making panel responsible for Article 690 approved language for the 2014 NEC that explicitly requires additional electrodes for all ground- and pole-mounted systems, in accordance with Section 250.52.

For a large ground-mounted system, the question becomes: What portions of the system are considered separate structures or arrays and thus require grounding electrodes? Many system engineers choose to install an approved electrode, such as a ground rod, for each row of a ground-mounted PV array. While the piles or piers that support the racking structure provide an electrically conductive connection to ground, they typically do not meet the Code requirements for use as grounding electrodes. A critical difference between Section 690.47(D) (as it appears in NEC 2008 and NEC 2014) and Section 250.32 is that the former requires a structure-specific GEC sized according to Section 250.166, whereas the latter requires only the connection of an EGC sized in accordance with Section 250.122 to equipment, structures, frames and the grounding electrode.

Ladd explains PowerSecure’s conservative design approach for ground-mounted arrays: “In addition to the Code-required equipment grounding, we attach each section of racking together with a bare solid copper conductor or similar, and attach each row of racking to a buried bare 1/0 copper wire. There is no Code requirement for this additional grounding. We do this to ensure that there is no difference in potential between any portion of the array and also to provide a flow path for lightning energy.”

Grounding Electrode Conductors

The general GEC-sizing requirements for ac systems differ somewhat from those for dc systems. While allowable GEC material and installation methods are generally the same for both the ac and the dc side of the inverter, NEC Article 690 provides some additional options that are unique to PV systems.

AC GEC sizing. GECs for ac systems are sized per NEC Table 250.66, based on the size of the largest ungrounded service entrance conductor or equivalent area of parallel conductors. For example, one 300-kcmil conductor requires a 2 AWG copper GEC (1/0 AWG aluminum); two parallel sets of 300-kcmil conductors, equivalent to 600-kcmils, require a 1/0 AWG copper GEC (3/0 AWG aluminum).

The minimum size allowable is 8 AWG copper (6 AWG aluminum). Meanwhile, the maximum GEC size required for sole connections according to Section 250.66 is also limited based on the type of electrode that the GEC connects to. For ground rods or connections to pipes or plate electrodes, the maximum size required is 6 AWG copper (4 AWG aluminum); for concrete-encased electrodes (Ufers), the maximum size required is 4 AWG copper; and for ground rings, the GEC does not need to be larger than the size of the conductor used for the electrode, which is at least 2 AWG copper per Section 250.52(A)(5).

DC GEC sizing. GECs for dc systems are sized according to NEC Section 250.166, which has maximum size limits similar to those in Section 250.66, and the same absolute minimum size of 8 AWG copper (6 AWG aluminum). Sections 250.166(A) and (B) specify that the dc GEC shall be at least equal in size to the neutral conductor. For systems without a neutral, the GEC shall be at least equal in size to the largest conductor in the system, except as allowed under Section 250.166(C) through (E).

In residential and small commercial grid-direct PV systems, sizing the GEC so that it is equal to the largest dc conductor is unlikely to present issues for system designers or installers, even if the current-carrying conductors are upsized due to voltage drop considerations. In larger grid-direct PV systems, however, sizing the GEC equal to the largest dc conductor may not even be possible, based on the size and configuration of the GEC terminal or lug inside the inverter. Since grid-direct PV systems are inherently current limited on the dc side, the AHJ may allow sizing of the dc GEC according to NEC Table 250.66, in which case the maximum required GEC does not exceed 3/0 copper (250-kcmil aluminum) for a service entrance conductor or a parallel set of service entrance conductors.

Sizing the GEC according to Table 250.66 can prove problematic when systems include batteries—which are capable of delivering a tremendous amount of current in a fault situation—since battery cables are often sized at 2/0 AWG or larger, even on systems with an inverter rated at 4 kVA to 6 kVA.

Note that while NEC Table 250.66 requires a maximum GEC size of 3/0 copper, Section 250.28(D)(1) states that when service entrance conductor(s) are larger than 1,100-kcmil copper (1,750-kcmil aluminum), main and system bonding jumpers must be sized such that they have at least 12.5% of cross-sectional area of the largest phase conductor. This can result in bonding jumpers larger than 3/0 AWG copper (250-kcmil aluminum).

According to explanatory text in the NEC Handbook accompanying Section 250.28(D)(1), “the main and system bonding jumpers are placed directly in the supply-side ground-fault current return path. Therefore…it is necessary to maintain a proportional relationship between the ungrounded conductor and the main or system bonding jumper.” Thus, on large PV systems, main bonding jumpers may be larger than the GEC. While dc system bonding jumpers may also be required to be larger than the GEC, this component may be built into the inverter (via the ground-fault protection circuit). Per Section 250.168, the dc bonding jumper must not be smaller than the dc GEC.

GEC material and installation. NEC Section 250.62 details allowable materials for GECs: copper, copper-clad aluminum or aluminum, any of which can be solid, stranded, insulated, covered or bare. Section 250.64 describes approved GEC installation methods.

Per Section 250.64(A), aluminum and copper-clad aluminum GECs cannot be used in masonry, soil or corrosive conditions, and they cannot be terminated within 18 inches of the ground in outdoor locations. Section 250.64 includes requirements for securing GECs and protecting them from physical damage. Note that 8 AWG GECs must be protected in metal conduit or tubing (RMC, IMC, or EMT) or PVC conduit. Often system designers and installers opt to use a larger GEC to avoid this requirement. GECs larger than 8 AWG need to be protected only if they are exposed to physical damage, as interpreted by the AHJ. For example, an unprotected 6 AWG GEC can be run on a building surface if it is not subject to physical damage.

NEC Section 250.64(C)(1) through (4) requires GECs to be electrically continuous, so you must splice wire GECs using irreversible crimp connections that are listed as grounding and bonding equipment, or by exothermic welding. Follow installation instructions precisely: With a crimp connection, use properly sized crimps and install them with the specified compression tool and die; with exothermic welding, make sure the form is the correct size and shape. In both cases, ensure that the conductor is clean and free of damage both before and after splicing it.

While sections of busbars can be connected to form a GEC, this practice is more typical of commercial and industrial applications. This allowance does not extend to the types of busbars seen in combiner boxes or residential-sized service equipment. Instead, it applies to busbars with a minimum cross-sectional area of ¼ inch thick by 2 inches wide, as described in 250.64(D)(1). Busbars like this are commonly used to connect GEC taps to a common GEC—for example, in a building with multiple transformers.

When a GEC is installed in a ferrous metal raceway, the raceway must be continuous, per NEC Section 250.64(E). Typically this is accomplished by bonding the raceway to the enclosure in which the GEC is terminated, and then using a bonding bushing or other fitting where the GEC emerges and connects to the grounding electrode. That bonding jumper needs to be at least as large as the GEC. As the NEC Handbook explains, “These bonding connections are necessary so that the ferrous raceway does not create an inductive choke on the grounding-electrode conductor.” Another way to eliminate the choke effect is to use PVC conduit, as allowed in Section 250.64(B).

DC GEC taps. NEC Section 690.47(B) permits a single common dc GEC to serve multiple inverters. The GEC is connected to each inverter via GEC taps, which must be sized in accordance with Section 250.166. The tap conductors must be made electrically continuous, using the methods outlined in Section 250.64.

The rules regarding dc GECs and GEC taps leave more room for interpretation than those for ac GECs and GEC taps in NEC Section 250.64(D)(1). The common GEC for a utility- or service-supplied system is sized based on the service entrance conductor. DC GEC taps to individual service disconnecting means are sized based on the conductors supplying each separate service. Circuits on the dc side of a PV system are often aggregated differently from those on the ac side.

As an example, imagine an 82 kW PV system that is utility interconnected using six 12 kW inverters. Not only are the dc conductors never aggregated from one inverter to the next, but also they may never be aggregated before they reach the inverters. If the inverter model features an integrated source-circuit combiner, then the largest dc conductor in the system could conceivably be 10 AWG copper. In this case, the dc GEC taps to each inverter need to be at least 8 AWG, per NEC Section 250.166. Technically, you do not need to size the common dc GEC larger than the largest tap (8 AWG). However, if an AHJ interprets the PV power circuits as parallel service entrance conductors per Section 250.66, then the size of the common dc GEC must be based on the sum of the cross-sectional areas of all the dc ungrounded conductors.

Systems with ac and dc grounding requirements. Section 690.47(C) was completely rewritten for NEC 2011. This section applies to transformer-isolated PV systems, which do not have a direct connection between the dc grounded conductor and the ac grounded conductor. Therefore, you must bond the dc grounding system to the ac grounding system. Section 690.47(C) specifies three methods for providing that bond.

Separate and bonded. Per Section 690.47(C)(1), a separate dc grounding-electrode system can be installed, provided it is bonded to the ac grounding-electrode system, as shown in Option 1 in Figure 2. The ac and dc GECs must be sized based on their respective NEC sections, and the grounding electrodes must be bonded using a conductor no smaller than the larger of the ac or dc GECs. Note that the dc GEC and/or the bonding conductor between the grounding electrodes cannot replace the required EGCs.

Separate and bonded grounding-electrode systems are used on residential and commercial retrofits, as well as large utility-scale systems. A dc grounding electrode, such as a new ground rod or ring, is installed as part of the PV system and connected via a dc GEC to the marked point in the inverter. The dc grounding electrode is then bonded to the premise’s existing ac grounding-electrode system, such as a ground rod or Ufer, or, in large-scale systems, to the newly installed ac grounding electrode on the secondary side of a medium-voltage transformer that is between the inverter and the utility grid.

Common grounding electrode. NEC Section 690.47(C)(2) allows for a common grounding electrode (or grounding-electrode system) to serve both the ac and dc systems, as shown in Option 2 in Figure 2. The dc GEC cannot replace the required ac EGC. In the event that the ac grounding electrode is not accessible, the dc GEC can connect directly to the ac GEC per Section 250.64(C)(1), which allows for irreversibly crimped or exothermically welded connections.

This method is widely employed for a variety of PV system types, from residential to utility-scale systems. For example, a ground ring around an inverter and transformer pad can serve as both the ac and the dc grounding electrode. The allowance for connecting to the ac GEC is helpful when the existing grounding electrode is a Ufer. Be aware, however, that if an existing ac grounding electrode is inaccessible, then there may be no way to verify that the resistance to ground is less than or equal to 25 Ω, as required in Section 250.53(A)(2). In such cases the AHJ may require a supplemental grounding electrode; if so, this must be installed at least 6 feet away from the existing electrode and bonded to it.

Combined grounding conductor. A combined grounding conductor can serve as both the dc GEC and the ac EGC, as shown in Option 3, Figure 2. While Section 250.121 specifies that an EGC cannot be used as a GEC, Section 690.47(C)(3) amends this general Code requirement and provides an allowance exclusively for PV systems. On the face of it, installing one conductor instead of two seems like the simplest and least expensive method. In practice, the requirements associated with installing a combined grounding conductor may complicate things.

Since the combined grounding conductor serves two functions—dc GEC and ac EGC—it must be sized according to the function that requires the largest conductor: The size of the ac EGC is determined based on NEC Table 250.122, and the size of the dc GEC is determined based on Section 250.166. Further, the combined conductor must either run unspliced or irreversibly spliced from the inverter to the grounding busbar in the associated ac equipment, which refers to the ac equipment that the ac GEC connects to. As a result, you must crimp pigtails to the combined grounding conductor whenever you need to connect an EGC to any electrical equipment—including disconnect switches, production meters, switchgear and so forth—located between the inverter and the final termination point at the ac GEC. Lastly, all other GEC installation requirements still apply, such as bonding ferrous raceways.

While the option to use a combined grounding conductor is applicable to all PV system types, it is most common on systems that use microinverters with an internal dc system bonding jumper. While the microinverter trunk cable may include an EGC, the grounding conductor connected to the microinverter chassis runs from the PV array to the associated ac equipment, serving as both the dc GEC and the ac EGC.

Ken Gardner, the principal engineer at Gardner Engineering Alternative Energy Services, explains his design approach for Enphase microinverter systems: “We like to use the SnapNrack mounting system because then we can use WEEBs to bond the solar modules and microinverters. We then run 6 AWG solid copper wire between WEEB bonding lugs on the rails to a SolaDeck junction box, at which point we transition to 8 AWG stranded wire using an irreversible splice. The 8 AWG ground is required because this is the minimum size conductor allowed for use as a dc GEC for the microinverters per Section 250.166. The 8 AWG ground also serves as the EGC and is therefore bonded to the rooftop junction box. We install the 8 AWG ground with the current-carrying conductors—typically 8 or 10 AWG for residential systems—in metal flex, usually run to the main service panel where we land the ground on the main grounding lug per Section 690.47(C)(3). In theory, we could use Type MC cable, but the grounding conductor inside is usually a size smaller than the main current-carrying conductors and is too small to serve as a GEC.”

Bonding grounding electrodes. When one or more type of electrode is present at a single structure, or additional or supplemental electrodes are installed (whether for the same or separate systems, services or transformers), they must be bonded to the grounding-electrode system. Additional electrodes installed in an array field do not need to be bonded to the grounding-electrode system except via their connection to the equipment-grounding system.

NEC Section 250.53(C) covers bonding jumpers in grounding-electrode systems. It refers to rules in Sections 250.64 and 250.66 regarding the sizing, installation and protection of bonding jumpers, and to Section 250.70, concerning approved methods for making grounding and bonding connections to electrodes.

Per Section 250.70, installers may use listed means—including lugs, pressure connectors or clamps—and exothermic welds for making connections to electrodes, provided that the product is suitable for the intended use and for the environment. For example, clamps used to connect a GEC to a ground rod must be rated for direct burial. They must also be rated for use with the materials that they connect to, both the GEC and the electrode. In addition, they must be rated for the diameter of the GEC and ground rod. If a clamp or fitting is used to connect multiple conductors to an electrode, then the device must be listed for use with multiple conductors.

Per Section 250.64(F)(1) through (3), you can bond electrodes to form a grounding-electrode system using one of three allowable methods:

  • Option 1: Run a GEC to any one of the grounding electrodes and use bonding jumpers to connect the GEC to all of the additional electrodes.
  • Option 2: Run individual GECs to each electrode. (Keep in mind, however, that a bonding jumper is required to connect the ac and dc electrodes in PV systems with transformer-isolated inverters.)
  • Option 3: Connect bonding jumpers between each electrode and a busbar of at least ¼ inch by 2 inches, using listed connections or exothermic welds. If you do not use the busbar as the GEC, then you must connect a GEC that is at least as large as the largest bonding jumper to the busbar.

In practice, installers often need to accommodate a combination of electrodes in the field, and the order in which GEC or bonding connections are made to these electrodes matters. If you are daisy-chaining two or more types of electrodes using bonding jumpers, then the GEC or bonding jumper must be as large as the largest required by any electrode in the chain, until that electrode is connected.

CONTACT

Rebekah Hren / o2 Energies / Winston-Salem, NC / o2energies.com

Brian Mehalic / Solar Energy International / Winston-Salem, NC / solarenergy.org

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