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.”
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.
Isolated inverters. Most utility-provided ac circuits have a grounded current-carrying conductor, as do traditional dc PV source and output circuits. Therefore, a mechanism is needed to prevent the injection of normal operating current from one system into the other through the equipment-grounding paths, which are bonded to the grounded conductors.
The most common method of separating the grounded ac system from the grounded dc system in PV applications is through the use of frequency-based or transformer-based inverter topologies. These inverter designs include either high- or low-frequency transformers that electrically isolate the ac and dc systems. (For an overview of inverter operation and topologies, see “How Inverters Work,” April/May 2009, SolarPro magazine.)
In UL 1741, the standard for inverters and other equipment intended for use with distributed energy resources, an isolation transformer is defined as “a transformer having its primary winding electrically isolated from its secondary winding and constructed so that there is no electrical connection—under normal and overload conditions—between the secondary and primary windings, between the primary windings and the core, or between separate adjacent secondary windings, where such connection results in a risk of fire or electric shock.”
Isolation transformers provide what is known as galvanic isolation between electrical systems, meaning that there is no physical or electrical connection between the input and output power. Because transformers use magnetic induction to transfer power from one circuit to another, there is no need for a metallic conduction path between the two systems. A dielectric barrier—such as an air gap, electrical insulation or both— is used to provide adequate isolation between the primary and secondary sides of the transformer, based on the highest expected voltages in the two systems. The presence of isolation transformers in the inverter prevents any stray or fault currents from traveling between the grounded ac system and the grounded PV system.
Historically, all residential and many commercial utilityinteractive inverters include an isolation transformer within the inverter enclosure. To simplify transportation and installation, inverters used for large commercial applications may have an external isolation-transformer cabinet. Inverters that include an isolation-transformer as part of the listed product are specifically designed and tested for use with grounded PV systems and are often referred to as isolated inverters or transformer-isolated inverters.
Non-isolated inverters. When we switch to an ungrounded PV system, we eliminate the need for electrical isolation. Since neither the positive nor the negative PV system circuit conductor is intentionally connected to ground, there is no parallel current-carrying path between the dc and the ac electrical systems. This means that inverters used with ungrounded PV systems do not require an isolation transformer. In grid-direct applications, utility-interactive transformer-isolated inverters cannot be used with ungrounded PV arrays, because the GFDI is internally bonded to one of the PV input-circuit conductors. Since these inverters ground the array by default, they are incompatible with an ungrounded PV array. A different inverter topology is needed.
In 2005, when Section 690.35 was added to the Code, UL 1741 did not address safety concerns related to inverters intended for use with ungrounded PV systems. Therefore, a draft approach was developed based on IEC 62109, the equivalent International Electrotechnical Commission standard. It was not until April 2010 that UL formally published a Certification Requirement Decision (CRD) containing content specifically related to inverters designed for use with ungrounded PV systems. The expectation is that the definitions and safety requirements in this CRD will be incorporated into UL 1741 after the Standards Technical Panel has reviewed and commented on them.
Inverters listed to UL 1741 for use with ungrounded PV systems do not include an isolation transformer. While as a result these are commonly referred to as transformerless inverters, it is worth noting that this particular term does not appear in either the UL standard for utilityinteractive inverters or the NEC. Instead, UL 1741 defines an “inverter that does not provide galvanic isolation between its input and output circuits” as a non-isolated inverter. The NEC includes neither term, referring simply to ungrounded PV systems and inverters “listed for the purpose.” The problem with this inconsistent nomenclature is that it can be confusing.
Bill Brooks, principal at Brooks Engineering, is an active participant in the UL 1741 Standards Technical Panel as well as in the code-making panel that has purview over NEC Article 690. According to Brooks, “The NEC is going to have to adopt a more accurate concept of isolated and non-isolated.” He continues: “The most common ungrounded PV systems installed in the US are non-isolated systems. While 690.35 is very clear on the requirements for ungrounded PV systems, contractors and AHJs do not necessarily understand that non-isolated means ungrounded and that Section 690.35 of the NEC applies. To make matters worse, not all literature provided by the manufacturers of non-isolated inverters makes it clear that 690.35 must be followed.”
Eliminating the Isolation Transformer
Inverter topology determines whether an application calls for a grounded or an ungrounded PV system. In the US, where grounded systems are common, most inverters incorporate an isolation transformer. Figure 1 shows a representative single-phase grid-tied inverter used in the US.
While the isolation transformer represents a significant part of the overall volume and cost of the inverter, it also performs the following three important functions.
Output filtering. The transformer, being a reactive component, helps filter the inverter’s pulse-width modulation signal. The transformer is generally not the only reactive component in the filter design. An inductor (identified in Figure 1) and a capacitor (the large sky-blue component in the top right corner near the control board of the inverter) provide additional filtering so that a pure sine wave is generated at the inverter’s ac output.
Voltage step-up. The maximum voltage that an inverter can output is about 10% less than the maximum voltage that can be produced on the dc side of the system. On the one hand, the typical maximum dc voltage in the US is 600 Vdc, and the actual operating voltage can be as low as 330 Vdc. On the other, the output voltage of the inverter must match the grid’s maximum voltage. For a 240 Vac installation, the peak grid voltage can be as high as 373 Vac. For a 480 V 3-phase ac system, the maximum peak grid voltage can be as much as 747 Vac. An isolation transformer makes it possible to step up the dc input voltage to match the grid voltage.
Decoupling ac from dc. With grounded PV systems, the dc system ground needs to be isolated from the ac system ground so that they are not coupled through the source circuits.
Unfortunately, an isolation transformer also decreases the inverter efficiency by 1%–2% and lowers the overall system efficiency as a result. If the isolation transformer is eliminated, then inverter and system efficiency can be improved. However, the functions performed by the isolation transformer need to be addressed before it can be eliminated.
Benefits of Non-Isolated Inverters
As we will show, ungrounded PV systems have additional BOS requirements compared to conventional grounded PV systems. Ungrounded PV systems also require special inverters specifically designed and listed for use with ungrounded arrays. So why would anyone choose to go this route?
It turns out that many of the potential benefits of deploying ungrounded PV systems are specifically associated with the use of non-isolated inverters. The advantages most commonly attributed to non-isolated inverters include higher efficiency, improved economics and increased ground-fault sensitivity.
Higher efficiency. Advanced Energy has sold its bipolar transformerless inverters into commercial and utility-scale PV applications since August 2007. According to Tucker Ruberti, the company’s director of segment marketing, “The most obvious benefit of a transformerless architecture is higher inverter efficiency.” As an example, the weighted CEC efficiency of Advanced Energy’s transformerless Solaron 250 kW inverter is 97.5%, which is 1% higher than that of the company’s transformer-isolated PVP250kW inverter.
While modern transformers are exceptionally efficient, losses that can never be entirely eliminated occur in the core and in the windings. These losses are dissipated as waste heat, which is one of the reasons that 60-Hz transformer-based inverters often have relatively large heat sinks.
Since large transformers are generally more efficient than smaller ones, it is not uncommon to see a higher efficiency differential between isolated and non-isolated inverters at smaller inverter capacities. For example, SMA America’s 8 kW transformerless inverter (SB 8000TL-US) has a CEC efficiency of 98%, which is a full 2% higher than the efficiency of its 8 kW transformer-isolated inverter (SB 8000US). Generally speaking, non-isolated inverters are 1%–2% more efficient than equivalent isolated inverters.
Output filtering can easily be accomplished with the addition of properly designed filter elements, such as an additional inductor, capacitor or both. Voltage step-up can be addressed with higher dc utilization voltages. For single-phase inverters, operating voltages in the 330–600 Vdc range are generally adequate for utility interconnection. However, interconnecting a single-stage, non-isolated inverter to a 480 Vac 3-phase system requires higher utilization voltages, such as 1,000 Vdc. Alternately, a boost stage can be added to a 600 Vdc non-isolated inverter to allow for 480 Vac interconnection or to allow for lower minimum input voltages and a wider MPPT window. Decoupling is the critical issue when moving from grounded to ungrounded systems.
Figures 2a and 2b show an inverter schematic with and without an isolation transformer. In both diagrams, the dc system is grounded. With the isolation transformer (Figure 2a), there is no direct path between the dc and ac grounds. However, once the transformer is replaced with line inductors (Figure 2b), there is a direct short through the coupled grounds.
To avoid the coupled-ground problem, either the ac or the dc ground must be removed. It is not possible to remove the ac system ground for premises wiring of less than 1,000 V because the NEC does not permit this. However, the removal of the dc system ground is addressed in NEC Section 690.35, “Ungrounded Photovoltaic Power Systems.” —Adapted with permission from SolarWorld Engineering Bulletin 1002–2010 (see Resources)
Improved economics. Since non-isolated inverters are more efficient, they have the potential to increase a PV system’s specific yield and improve a customer’s return on investment as a result. In theory, non-isolated inverters should also cost less to purchase, ship and install than isolated inverters. As Wiles explains in Home Power magazine, “The transformer is usually heavy, costly and bulky—decreasing efficiency and increasing the inverter’s size and shipping costs.”
Eliminating the isolation transformer in a utility-interactive inverter may also enable additional savings. Verena Arps is the director of technical sales at REFUsol, an inverter manufacturer with a line of transformerless 3-phase string inverters ranging in capacity from 16 kW to 24 kW. Arps points out that REFUsol’s transformerless inverter topology does not require active cooling: “Because the inverters are more efficient, the internal heating losses are decreased, which allows for the elimination of active cooling components.”
Even when active cooling components are included, non-isolated inverters generally have a lower parts count than their isolated counterparts. A reasonable claim can be made that they have less embodied energy than transformer-isolated inverters since they are smaller and lighter. These same attributes could make them easier to install.
The extent to which the raw material reductions associated with non-isolated inverters translates to up-front cost savings still remains to be seen. In today’s market, a non-isolated inverter may cost about the same as an equivalent isolated inverter from the same manufacturer. However, non-isolated inverters have yet to achieve manufacturing efficiencies of scale. They are still a specialty or niche product compared to isolated inverters. Most industry experts agree that transitioning to non-isolated inverters will eventually drive inverter costs down in North America.
SolarEdge has developed a unique utility-interactive PV system that consists of module-level dc-to-dc power optimizers coupled with proprietary non-isolated inverters. According to John Berdner, the company’s general manager for North America: “Non-isolated inverters offer the best chances for future cost reductions since they do not include the large transformers found in low-frequency transformer-isolated inverter designs and have far fewer components than highfrequency transformer-isolated designs.”
Increased ground-fault sensitivity. When people refer to the safety benefits associated with ungrounded PV systems, they are almost certainly referring to the fact that non-isolated inverters are more sensitive to ground faults than isolated inverters. In a SolarPro magazine article (February/March 2011) identifying the limitations of GFDI systems used in listed isolated inverters, Brooks points out, “The only way to get ground-fault detection below 1 amp as part of the GFP scheme for large PV systems is to unground or resistively ground the array circuit, just as they do in Europe and Japan.” He continues: “Contemporary European inverters, for example, can detect changes in ground current as low as 300 mA, which is an order of magnitude lower than our solidly grounded systems.”
While the differential is less pronounced in residential applications, non-isolated string inverters are still three times as sensitive to ground faults as isolated string inverters. At present, transformer-isolated string inverters up to 15 kW in capacity typically use a 1 A GFDI fuse to provide ground-fault protection. UL 1741 allows higher-capacity inverters to use GFDI fuses with higher ratings. For example, inverters rated more than 250 kW in capacity are allowed to use a 5 A fuse. Since a fuse located between the grounded current-carrying conductor and the ground bond most commonly provides this protection, the time required to open this fuse is determined by the physical response time of the fuse itself, which varies depending on temperature and the amount of current flowing across it during the fault event.
The electronic GFP strategy employed by non-isolated inverters used on ungrounded PV arrays allows for much lower and more consistent current and trip-time settings. Non-isolated inverters 30 kW and below sold on the market today are tested to the current UL CRD requirements of 300 mA maximum fault current and 0.3 second maximum trip time. Additionally, there is a “sudden ground-fault current change and response time” requirement that causes the operation of this protection circuit at levels as low as 30 mA and as quickly as 0.04 seconds. Besides reducing the potential shock hazard in a PV array, this means that ground faults are identified and the fault current is stopped more quickly, before it turns into an arcing fault capable of starting a fire.
Furthermore, since non-isolated inverters test for ground-fault currents at the start of each day— before the inverter goes online—they can detect potential ground-fault conditions before a fault occurs. For example, compromised conductor insulation may first manifest as a high-resistance fault and only later as a low-resistance fault. The groundfault protection scheme used in transformer-isolated string inverters may respond to the low-resistance fault only, whereas the scheme used in non-isolated string inverters is more likely to identify the highresistance fault condition.
SMA America was the first manufacturer to certify non-isolated inverters to UL 1741, using UL as its Nationally Recognized Testing Laboratory (NRTL). Greg Smith, a technical training specialist with the SMA Solar Academy, notes, “Plan checkers and inspectors may mistakenly think that a non-isolated inverter is unsafe because it doesn’t have the isolation transformer in it.” The reality is just the opposite, Smith explains: “Because non-isolated inverters check for PV isolation resistance before connecting to the grid and producing power, current is never flowing in a potentially unsafe array with ground faults.”
Ground-Fault Protection in Non-Isolated Inverters
Since AHJs occasionally question the safety of ungrounded PV systems, it is helpful to understand how the ground-fault protection system works in a non-isolated inverter. UL developed the increased ground-fault protection requirements for non-isolated inverters in concert with the PV inverter industry. These requirements address the unique conditions that ground faults can present in an ungrounded PV system. The process is under way to formally add these requirements for the testing and listing of non-isolated inverters to the published UL 1741 standard.
The ground-fault protection system used in non-isolated inverters includes a regular test of PV array insulation resistance. This test is performed by an isolation monitor interrupter (IMI), which UL defines as “a device that monitors the insulation resistance of a PV array circuit to ground and prevents energization of the inverter ac output circuit or disconnects an energized output circuit when the PV array input resistance drops below a predetermined level.” The IMI performs the PV array insulation resistance test in the early morning hours, when the PV source-circuit voltage is high but there is not enough current for the inverter to begin operating.
The IMI measures any current leakage between all the conductors in the PV circuit to ground and identifies levels of leakage current above set values. This technique is very similar to the insulation tests that electricians perform on unenergized electrical conductors using portable megohm meters. If the tested source-circuit conductor insulation resistance is below a minimum level, the inverter will not interconnect with the utility. If the source-circuit conductor passes the test, the inverter will initiate its normal startup procedures.
It is common for ground-fault protection systems in nonisolated inverters to use what UL refers to as a functional ground, which is an intentional high-impedance connection between the ungrounded circuits that are being monitored and the equipment-grounding system. This connection exists for the sole purpose of fault detection. Since this intentional high-impedance path only exists when the inverter is operating, the PV system is not solidly grounded, according to the definition in Article 100 of the NEC. UL allows this strategy since it recognizes the role that these detection circuits play in reducing the potential for property damage due to stray ground-fault currents. An NRTL evaluates a non-isolated inverter’s ground-fault protection system, including any functional ground, as part of the product testing and listing.
As described previously, once the inverter is online, if it measures ground-fault current above the maximum level allowed or if it measures a sudden increase in fault current, even at very low levels, it will cease operating and indicate the presence of a fault.
Residual-current detector. Rather than using GFDI fuses to identify and interrupt ground-fault current as is typically done in isolated inverters, non-isolated inverters include a residual-current detector to continuously monitor the PV array. This detector circuit is similar to the ac ground-fault circuit interrupter (GFCI) devices with which most electrical professionals are familiar. Like a GFCI, the residual-current detector in a non-isolated PV inverter is an electronic monitoring circuit that identifies ground-fault current before it reaches destructive levels. Unlike a common household GFCI, this device functions to identify fault currents that could cause damage to property and is not specifically set to levels to protect people from electrical shock.
Residual-current detectors constantly monitor the current in an operating PV power circuit, and associated software looks for any imbalance. The outgoing and returning current in the dc circuit should be offsetting—equal in intensity but opposite in direction. In an ideal circuit, the sum of these currents would equal zero. If the residual-current detector indicates that the currents are imbalanced, then the control logic interprets this as a fault. The most likely fault path is to ground through the equipment-grounding system, but stray current in any other parallel circuit path would also be detected. If an imbalance is indicated, then the inverter ceases to operate and indicates that a ground fault has occurred.
In practice, all PV arrays have some small amount of residual leakage current due to a capacitance effect that is dependent on the specific module, the mounting system and the environmental conditions. This means that a residual-current detector system used in non-isolated inverters cannot actually be set at zero, as this would result in nuisance tripping, especially on very large arrays. However, since the residual-current detector is an electronic protection device, its trip points are much lower than the conventional GFDI fuse ratings commonly found in isolated PV inverters. The UL 1741 Standards Technical Panel, which includes manufacturer representatives, determined that the 300 mA ground-fault trip limit for nonisolated inverters up to 30 kW was adequate to prevent groundfault arcs that could ignite fires.
Note that residual-current detection does not provide overcurrent or short-circuit protection. On the ac side of the system, this protection is provided by the overcurrent-protection device required in NEC Section 705.12(D). Overcurrent protection for the PV source or output circuits may be required according to Section 690.9.
NEC Section 690.35 contains specific requirements for ungrounded PV systems. These requirements have implications for what products are used and how, from the inverter upstream to the PV array. It is important that electrical engineers and PV system designers understand these requirements so that they can specify the right components in their plans for ungrounded PV systems. Similarly, electricians and PV system installers need to understand these requirements well enough to verify that the correct components are called out in the plans and that suitable materials are available. The inventory requirements for ungrounded and grounded PV systems are meaningfully different. Many common mistakes can be avoided by ensuring that the components called for in ungrounded PV system designs meet the requirements outlined in NEC Section 690.35.
Design and Deployment Considerations
Installing ungrounded PV systems with non-isolated inverters is very similar to installing grounded PV systems with isolated inverters. The same tools and personnel are used. In many cases, the same PV modules and mounting systems are used. The part numbers for the inverters and some of the BOS components may differ, but the parts will be similar to install. According to SMA’s Smith, “Installers with experience installing grounded PV systems will find their existing skill set well suited to installing ungrounded PV systems.” This is because the important differences between ungrounded and grounded PV systems relate to system design and product specification. Smith explains, “While the actual installation differences are few, there are important design differences.”
Inverter. The inverters used in ungrounded PV systems must be tested and listed for the application; see Section 690.35(G). While UL refers to these as non-isolated inverters, it is far more common for manufacturers to refer to these products as transformerless inverters. In the “2012 Grid-Direct String Inverter Specifications” table (pp. 50–62), several manufacturers— including, Advanced Energy, Chint, Growatt, Ingeteam and SMA—use the letters “TL” in the inverter model number to identify their non-isolated products. Other transformerless inverters in the table include Eaton’s PV240, PV250, PV260 and PV270; Exeltech’s XLG18A60; KACO new energy’s blueplanet 6400xi and 7600xi; Power-One’s PVI-3.0, PVI-3.6, PVI-4.2, PVI- 5000 and PVI-6000; and REFUsol’s 012K-UL, 016K-UL, 020K-UL and 024K-UL.
While all inverters for PV applications are listed to UL 1741 and need to conform with the power quality and safety tests found in IEEE 1547.1, the NRTL evaluates non-isolated inverters to a slightly different set of conditions than isolated inverters. Because of the inherent design differences between these products, installers and inspectors need to ensure that inverters used in ungrounded PV systems are listed and labeled as non-isolated inverters. An installer cannot field-configure an isolated inverter to operate on an ungrounded PV system. Similarly, a non-isolated inverter may not be field-modified for any purpose except as indicated in the manufacturer’s installation manual. Always refer to the specific manufacturer’s documentation for installation requirements and allowances.
Inverter input and PV output conductors. One of the easiest mistakes for installers to make is to instinctively grab white wire off the truck for use in an ungrounded PV array. System designers can make the same mistake and specify white wire in their drawings, especially if they start working from existing plans for a grounded PV array. Ungrounded PV power circuits may not use conductors with white insulation.
Conductors with white insulation are commonly used in positively or negatively grounded PV systems to identify the grounded conductor. However, both of the current-carrying conductors in an ungrounded PV system are floating with reference to ground, so there is no grounded conductor. Therefore, according to Section 200.7 of the Code, neither polarity in an ungrounded PV system can be wired using conductors with a continuous white insulation. This section also excludes the use of conductors with continuous gray insulation or with three continuous white stripes for ungrounded PV array circuits. These colors are reserved for grounded circuits. Insulation that is green or green with yellow stripes can be used to identify grounding conductors only; see Section 250.119.
Aside from these restrictions, the NEC does not dictate what color code to use for conductors in an ungrounded PV system. Just be sure to use different colors for PV positive and PV negative conductors to make it easy to maintain the correct polarity throughout the system. Many dc circuits in the US use red for positive and black for negative. Since these colors are probably already in your inventory, this is a logical convention to follow. However, the Code provides installers with a great deal of leeway to do otherwise. In the event that specific color codes do apply, the local AHJ dictates them.
DC disconnects. The brevity with which NEC Section 690.35(A) describes the requirements for dc disconnects in ungrounded PV systems is somewhat misleading: “All photovoltaic source and output circuit conductors shall have disconnects complying with 690, Part III.” In practice, this is one of the most significant changes to how ungrounded PV circuits are wired when compared to grounded systems.
To understand the implications of Section 690.35(A), look at Section 690.13, which is the first section under Part III of Article 690. As a rule, Section 690.13 requires that a means be provided to disconnect the current-carrying conductors in a PV array from other conductors in a building or structure. However, this does not apply to the grounded conductor if opening it with a switch or a circuit breaker would leave “the marked, grounded conductor in an ungrounded and energized state.” This is why disconnects in grounded PV array circuits open ungrounded currentcarrying conductors only, and the grounded conductors are unbroken (except as allowed in Section 690.13 Exceptions No. 1 and No. 2).
In an ungrounded PV system, both the PV positive and the PV negative conductors are ungrounded current-carrying conductors. Since NEC Section 690.13 requires a disconnecting means for all ungrounded current-carrying dc conductors, both the PV positive and the PV negative conductors need to be switched in an ungrounded PV system. Double-pole disconnecting must be used to switch both conductors.
According to a survey of transformerless inverter manufacturer representatives, it is not unheard of for installers used to working with grounded PV systems to make the mistake of switching just one of the dc conductors when wiring a PV disconnect external to the inverter in an ungrounded PV system. Fortunately, this is often an easy fix. In some cases, no additional equipment is required. As Berdner at SolarEdge points out: “Most switches and breakers used today are wired with multiple poles in series, allowing some of the poles to be used for the other conductor. If no additional poles are necessary, then the cost impact of installing a PV disconnect in an ungrounded system versus a grounded system is minimal.”
Designers should be aware that many of the off-the-shelf disconnects commonly used to disconnect multiple PV circuits in grounded systems are not suitable for the same application in ungrounded systems. For example, a three-pole disconnect that is rated to disconnect two or three source circuits in a grounded PV array is able to disconnect only a single circuit in an ungrounded system. Because there is a disparity between ungrounded PV system requirements and commonly available electrical parts, many transformerless inverter manufacturers have proactively taken steps to close this gap.
Many UL-listed string inverters and some central inverters incorporate an integrated dc disconnecting means intended to meet the requirements in NEC Section 690.13. This is also true of non-isolated utility-interactive inverters. Residential system integrators may not need to go to the trouble and additional expense of sourcing and carrying listed multi-pole dc disconnects for use in ungrounded PV systems. Simply use the inverter-integrated PV disconnect as indicated in the manufacturer’s installation instructions, which an NRTL evaluates as part of the product testing and listing process. For larger systems with combiner boxes external to the inverters, appropriate switches need to be sourced.
Overcurrent protection. NEC Section 690.35(B) applies to overcurrent-protection requirements for ungrounded PV systems. The language used in this section parallels that found in Section 690.35(A) but refers back to Section 690.9, which in turn refers back to the general overcurrent-protection requirements found in Article 240. This means that the dc fuses or circuit breakers used in an ungrounded PV system are sized and located precisely as they would be in a grounded PV system. (See QA “Series String OCPD Requirements for Grid-Direct Inverter Applications,” December/January 2009, SolarPro magazine, and “DC Combiners Revisited,” February/ March 2011, SolarPro magazine.)
There is, however, a critical difference in the way that the general overcurrent-protection requirements are applied to ungrounded PV systems. If overcurrent protection is required, then both the PV positive and the PV negative conductor need to be protected. The rationale is similar to the disconnect requirements described previously. In this case, Article 240.21 requires that overcurrent protection “be provided in each ungrounded circuit conductor.” Since both PV positive and PV negative conductors are ungrounded current-carrying conductors, both require overcurrent protection.
In practice, this means that source-circuit combiners in ungrounded PV systems have twice as many fuse holders as they would in grounded applications. One set of fuses is dedicated to the PV positive conductors and the other set is dedicated to the PV negative conductors. The same is true of subarray combiners. It does not matter whether these dc combiners are external or internal to the inverter. If overcurrent protection is required, it is required in both polarities of the circuit. If a servicing disconnect is required for these fuses, then that device needs to switch both conductors. When it comes to disconnects and fuses in ungrounded PV arrays, make sure you are prepared for double the fun.
Source-circuit conductors. For many years, PV manufacturers and system integrators have used sunlight-resistant, wetrated single conductors for PV source circuits. The cable type most commonly used in these applications is underground service entrance cable (USE) marked USE-2. While the insulation for both USE and USE-2 cables is rated for use in wet locations and exposure to sunlight, USE-2 cable has a higher temperature rating when wet (90°C) than does USE (75°C). However, despite its suitability for use in grounded PV systems, the NEC does not allow the use of type USE-2 cable as an exposed single-conductor wiring method for source circuits in ungrounded PV systems.
NEC Section 690.35(D) specifies the requirements for PV source-circuit conductors in ungrounded PV systems. Nonmetallic, jacketed multi-conductor cables are allowed, as are conductors installed in raceways. However, to install exposed single conductors for the PV source circuits in an ungrounded PV system, Section 690.35(D)(3) requires the use of “conductors listed and identified as Photovoltaic (PV) Wire.”
As the name suggests, PV Wire is designed for making PV module interconnections, specifically where single conductors are run outside raceways or conduits and exposed to a unique set of physical and environmental abuses. PV Wire differs from USE-2 in that it uses thicker insulation or a jacket to provide additional mechanical protection. As a result, PV Wire has greater sunlight resistance and can better withstand extreme cold than USE-2. Because of these improved physical properties, PV Wire can be rated for use at higher voltages than USE-2, such as 1,000 volts versus 600 volts. PV Wire can also be used in applications where USE-2 cannot, such as in ungrounded PV array source circuits.
PV modules. The PV Wire requirement in NEC Section 690.35(D)(3) has implications beyond the cable type used to connect a string of modules to a junction or combiner box. The listed PV modules themselves must be constructed using PV Wire for the module interconnection cables. This means that not all PV modules sold in North America can be used in ungrounded PV systems—only modules constructed with listed PV Wire.
System designers need to carefully study product datasheets to qualify module models for use in ungrounded PV systems. When in doubt, contact the module manufacturer. Some suppliers and distributors have a blanket policy to carry only modules with PV Wire whips. It is also increasingly common for manufacturers to supply modules with PV Wire, since this product is now more readily available and is rated for use on modules sold internationally. Designers can also narrow the field by referring to desktop reference guides with module specifications published in SolarPro magazine, such as the “2011 c-Si PV Module Specifications” table (October/November 2011).
Labeling. In addition to the general labeling requirements common to all PV systems, NEC Section 690.35(F) requires that “each junction box, combiner box, disconnect, and device where energized, ungrounded circuits may be exposed during service” have a label that reads as follows:
ELECTRIC SHOCK HAZARD. THE DC CONDUCTORS
OF THIS PHOTOVOLTAIC SYSTEM ARE
UNGROUNDED AND MAY BE ENERGIZED.
The intent of this language is to alert anyone servicing equipment in an ungrounded PV system that no grounded conductor exists. Therefore, all current-carrying conductors need to be treated as “hot” conductors. This is the reason why you do not use white or gray insulation on any dc conductors in ungrounded PV circuits.
Grounding. The most important thing to remember about working with non-isolated inverters and ungrounded PV systems is that all of the PV array circuits are floating with reference to ground. This means that neither of the dc current-carrying circuit conductors is bonded to ground or to any grounded equipment. The practice of intentionally connecting a normally energized conductor is what we refer to as system grounding. Ungrounded PV systems do not have this intentional system-ground connection.
Paul Mync is the technical sales manager at KACO new energy, a manufacturer that developed its first transformerless solar inverters in 1999. Mync points out: “When the term ungrounded is used, it can lead to the mistaken impression that the PV array has no ground. That is not the case. It is only the dc conductors that have no electrical connection to ground.”
In addition to system grounding considerations, all electrical systems have equipment-grounding considerations. While NEC Section 690.35 allows PV systems to operate with ungrounded PV source and output circuits, it says nothing about changes to equipment-grounding requirements. Therefore, all of the usual equipment-grounding requirements apply to an ungrounded PV system just as they do to a conventional grounded system.
Grounding requirements for PV systems are described in Part V of Article 690. Notable equipment-grounding provisions found in Section 690.43 state that equipment grounding is required for PV systems; an equipment-grounding conductor (EGC) is required; metal structural components (racking or mounting systems) may be used as the EGC, provided that they are listed and identified for the purpose; and PV-mounting hardware used for grounding must be identified for that use.
System designers, installers and the AHJ should have a good understanding of these equipment-grounding requirements. Installers do not need to do anything different to provide equipment grounding in an ungrounded PV system as compared to a grounded system. Similarly, inspectors do not need to look for any additional equipment. They just need to verify that normally non–current-carrying metal components are connected together and that this conductive path is continuous back to the inverter and ultimately to the ac grounding system.
One thing that confuses inspectors is the absence of dc system-grounding components. For example, Wiles published an article in IAEI magazine (see Resources) that addresses a question from an electrical inspector who asked whether the NEC requires a grounding electrode conductor (GEC) and grounding rod for non-isolated inverters. In his response, Wiles notes that while Section 690.47 of NEC 2011 does not exactly address this question, future Code cycles should clarify the issue.
Wiles explains: “Transformerless inverters do not connect one of the dc circuit conductors in the PV array to ground (as allowed by NEC 690.35) and have no internal bonding jumper. Therefore, there will normally be no terminal to connect the GEC, and the NEC does not require a dc GEC.”
A GEC is not required for a non-isolated inverter, and if the inverter adheres to the UL 1741 requirements, it will not even have a GEC terminal. According to Wiles, “The UL standard requires a grounding electrode conductor terminal … only when there is a bonding jumper in the direct current (dc) side of the inverter.”
Note that auxiliary dc-grounding electrodes and conductors may be installed for other reasons, such as protection against lightning surges. In addition, an ac grounding-electrode system is still required for the utility supply to which the ungrounded PV system is connected.
Permitting and Inspecting
Ungrounded PV systems are more similar to grounded PV systems than they are different from them. The specific differences in equipment selection and usage are based on fundamental distinctions found in the NEC and UL 1741. Provided that system designers and installers understand these technical distinctions and can discuss them with the AHJ, permitting and inspecting ungrounded PV systems can go smoothly.
“Most AHJs will have questions once they see that the inverter is transformerless and that the PV system is ungrounded,” observes Mync at KACO. He recommends preparing for those questions: “Educate yourself on the NEC sections that relate to the installation of transformerless inverters, and be prepared to discuss these in an intelligent manner with the AHJ.” System integrators also need to be proactive and contact the AHJ before installing an ungrounded system within its jurisdiction for the first time. By definition, the AHJ approves equipment, materials, installations and procedures; see NEC Article 100.
Berdner at SolarEdge notes: “Some AHJs and plan checkers are unfamiliar with the Code provisions for use of ungrounded arrays.” He recommends that engineers and system designers make specific reference to Section 690.35 in their plans. He adds, “I would also include a datasheet or manual for the inverter that indicates the presence of a listed GFDI device.”
“The main concern inspectors have is to ensure the safety and Code compliance of any electrical installation,” reminds Advanced Energy’s Ruberti. He encourages integrators to provide the AHJ with copies of applicable listings, such as the UL 1741 certificate of compliance. “The best thing you can do is be patient and prepared to educate them,” he says. “Explain why the new transformerless technology is not just safe, but safer than what they have approved in the past.”
In addition to educating plan checkers and inspectors, system integrators also need to educate their designers and installers. “Most installation errors can be avoided with team meetings and proper training,” KACO’s Mync says. “A change in technology can slow things down at first, but these impacts are minimized when installers know what they are supposed to do and why.”
Jason Fisher / SunPower / Charlottesville, VA / us.sunpowercorp.com
David Brearley / SolarPro magazine / Ashland, OR / solarprofessional.com
Mead, John, “Ungrounded DC Systems,” SolarWorld Engineering Bulletin 1002–2010, solarworld-usa.com
Wiles, John, “More Questions from Inspectors: Numerous PV Systems Pose Issues,” IAEI magazine, March/April 2012, International Association of Electrical Inspectors, iaei.org
Wiles, John, “Ungrounded PV Systems,” Home Power magazine, October/November 2010), homepower.com
LISTED NON-ISOLATED INVERTER MANUFACTURERS
Advanced Energy / 800.446.9167 / advanced-energy.com
Chint Power Systems / 972.761.3992 / chintpower.com/en
Eaton / 877.386.2273 / eaton.com
Exeltech / 800.886.4683 / exeltech.com
Growatt New Energy / 832.615.5047 / growattusa.com
Ingeteam / 262.240.9850 / ingeteam.com
KACO new energy / 415.931.2046 / kaco-newenergy.com
Power-One / 805.987.8741 / power-one.com
REFUsol / 408.775.7744 / refusol.com
SMA America / 916.625.0870 / sma-america.com
SolarEdge / 530.273.3096 / solaredge.com