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LEARNING GOALS: • identify the components of a residential utility interactive solar photovoltaic system. • recognize the electrical hazards unique to solar photovoltaic systems. • apply National Electrical Code requirements to the installation of a residential utility-interactive solar photovoltaic system. Electricity from sunlight? Is this possible? This is not only possible, but it has become very practical through the installation of utility-interactive photo voltaic systems. A combination of factors recently has made photovoltaic (PV) systems very popular: • Increased use of electricity throughout the United States • Increased costs for electricity production • Environmental concerns with the use of fossil fuels • Concern over dependence on foreign sources of energy • Increased efficiency of photovoltaic systems Installation of photovoltaic systems is being encouraged by government and electrical utilities through tax incentives and rebates. Some states require utilities to produce defined amounts of electricity through use of renewable resources. Electrical utilities can meet the requirement by construction of large central photovoltaic generating plants or through distributed generation. Generation of electricity by the consumer at the point of use (distributed generation) decreases the need for utility-generated power. Excess electricity will be supplied to the utility grid for use by other customers. Electrical utility companies are required to purchase customer-generated electricity at predetermined rates. Photovoltaic systems such as these are known as utility interactive. Depending on size, a utility-interactive PV system can supply most or all of a home's electrical load. Photovoltaic systems are being retrofitted to existing homes and even installed on new homes as they are constructed. Battery storage of the generated electricity may also be included, but is not as common. Electrical Hazards Electrical work for the installation of photovoltaic systems is not complex, but there are significant differences when compared to typical residential wiring. First and foremost, a utility-interactive PV system is a supply of electricity for the home, not another load. Photovoltaic modules on the roof are generating electricity when exposed to sun light. Although the utility disconnect (main circuit breaker) for a house can be turned off, dangerous levels of electricity will still be present on the dwelling as long as the sun is shining. Modules on the roof of a house are connected in series in strings that operate at up to 600 volts. Electricity generated by the photovoltaic modules is direct current (dc). All conductors and components must be listed for use with dc voltage. The sun shines for more than three hours at a time, so all conductors/components must be sized for continuous currents (three hours or longer of operation). Because string conductors of the correct type are permitted to be exposed on the roof of a dwelling, good workmanship is critical. There are open conductors, exposed to the elements, operating at up to 600 volts of continuous current (Vdc) that cannot be turned off! If a short circuit does occur, it is not always obvious. Module short-circuit current is only slightly higher than normal operating current. Contrast this with the short-circuit current from sources such as a utility or even 12-Vdc vehicle batteries. Shorting out these sources results in extremely high fault currents. High-fault currents are easier to detect. High-fault currents will also cause an overcurrent device (fuse or circuit breaker) to open quickly. The connection of the utility-interactive PV system to the existing service panel is of concern. Service disconnecting means, grounding, and proper labeling must be accomplished to maintain a safe electrical installation. It is obvious that there are some special considerations for the installation of residential photovoltaic systems. Article 690 was added to the National Electrical Code (NEC) in 1984 to establish minimum electrical standards for the installation of photovoltaic systems. Sections 1 through 4 of the NEC, along with the requirements of Article 690, will apply to residential PV installations. Local jurisdictions may have amendments to the NEC for PV installations. Always check with the local Authority Having Jurisdiction (AHJ) before starting the installation in a new area. THE BASIC UTILITY INTERACTIVE PV SYSTEM Several components are required in order to convert sunlight into useful amounts of electricity. A basic utility-interactive system will consist of modules, mounting racks, combiner/transition boxes, inverter(s), and several disconnects. A grounding electrode system and connection to the existing service panel will be required. See Fgrs. 1 and 2 for examples of basic PV system components and arrangements. FGR. 1 Identification of solar photovoltaic system components. (Figure 690.1(A) reprinted with permission of NFPA 70-2011.) FGR. 2 Identification of solar photovoltaic system components in common system configurations. (Figure 690.1(B) reprinted with permission of NFPA 70-2011.) Solar Cells, Modules, and Arrays FGR. 3 Rooftop array of modules. A photovoltaic module is the basic unit of power production in the system. A module is a manufactured unit made up of many semi- conductor PV cells encased in a protective covering and mounted to an aluminum frame. All modules are required by the NEC to be listed to nationally recognized standards (ANSI/UL 1703). Individual photovoltaic modules are mounted to a support rack which is connected to roof members, or in some cases, to ground mounted structural sup ports. Individual modules are wired together in a series circuit. Factory installed leads are provided by the manufacturer for this purpose. Type USE-2 or PV conductors can be spliced to module leads to facilitate circuiting. A separate equipment grounding conductor is used to ground each module. Most strings will consist of eight to fifteen modules. The quantity of modules in a series circuit will be limited by the combined open circuit voltage of the string. A typical residential array of roof mounted modules is shown in Fgr. 3. Multiple strings are combined together (in parallel) at the combiner box. This allows a single pair of conductors to deliver current to an inverter. Combiner boxes may be fused or non-fused. Most residential PV systems are made up of three to six strings that can be combined in a single box. String fusing is generally not required for three strings or fewer. Module ratings will determine this, but some designers specify fused combiner boxes even when not required. Many inverters have the ability to combine several strings at the input terminals. With an inverter like this, the string conductors are routed to the inverter without being combined first. The combiner box would not be required. A junction box, known as a transition box, is used to splice the open string conductors to a wiring method (usually conduit and THWN conductors) for connection of the inverter. The entire assembly of racks, modules and combiner/junction boxes is known as a photovoltaic array. Available space, sunlight, and the ultimate amount of electricity desired will determine the number of modules in the array. Obviously, the larger the system, the more expensive the installation, so the budget must be considered. A disconnect for the ungrounded dc string conductor(s) is required before the conductors enter the dwelling unless a metallic raceway is used as the wiring method. Use of a metallic wiring method such as EMT or FMC is common so that string conductors can be routed through a dwelling unit attic. Metallic raceways provide more protection for the 400-600 Vdc string conductors, which remain energized until the sun goes down. Firefighters responding to a house fire will open the service disconnecting means on arrival. This does not de energize the PV string conductors if the sun is still shining. Metallic raceways will provide a level of protection for the firefighters who may encounter the dc wiring method when responding to a fire. Use of a metallic raceway eliminates the requirement for an array disconnect on the roof but not at the inverter. The inverter will be energized from two different sources: dc from the array and ac from the utility connection. Disconnecting means must be provided for both sources. Some inverters are manufactured with integral disconnects. A circuit breaker in an adjacent electrical panel can serve as the ac disconnect, but all safety disconnects must be within sight of the inverter and may need to be grouped. The dc grounded conductor is not permitted to be opened by any disconnecting means. The inverter in Fgr. 4 has an integrated dc disconnect. FGR. 4 A residential utility-interactive inverter. The Inverter The inverter will need to be installed in a location with sufficient working space per NEC 110.26. Larger systems may use two or more inverters with output combined in a separate ac panelboard before supplying the service. Grounding/bonding of the utility-interactive PV system is required and is usually accomplished at the inverter location. Both the ac and the dc systems contain grounded current carrying conductors. Grounding of the dc conductor is accomplished at the inverter. A new grounding electrode must be installed (and bonded to the existing premises' grounding electrode) or the existing grounding electrode for the dwelling must be accessed for connection. Correct termination of the grounding electrode conductor, equipment grounding conductors, and grounded ac/dc conductors at the inverter is critical. Utility-interactive inverters are required to provide ground fault protection for the array. Proper operation of the ground fault protection system is dependent on the correct terminations of the grounding and grounded conductors. Inverter design will dictate whether the positive or the negative dc conductor is the grounded current carrying conductor for the array. Inverter size is based on the capacity of the array. Most residential inverters are in the 2 kW to 10 kW range. There are advantages to installing an inverter indoors. Cooler temperatures result in better operating efficiency, but outdoor installation is also common. FGR. 5 Building-integrated photovoltaic roof tiles. Safety Features of Inverters Just as with modules and combiner boxes, inverters are required to be manufactured to nationally recognized standards. Inverters that are listed by a Nationally Recognized Testing Laboratory (NRTL) to ANSI/UL 1741 have met this standard. One requirement of UL 1741 is known as anti-islanding. Anti-islanding means that an inverter must automatically turn off ac output when utility power is lost. This feature prevents an inverter from supplying electricity into the utility grid when there is an outage. Electrical utility workers risk being electrocuted by inverter-supplied electricity without this. Connection of Inverters The two options for connection of the inverter to the electrical service panel are known as a supply side or a load-side connection. A connection on the utility side of the main disconnect is known as a supply-side point of connection. A load-side connection will supply electricity on the customer side of the main disconnect. Supply-side connection size is not limited by the NEC, but a second electrical service is created so the requirements of NEC Article 230 (Services) will apply. A load-side connection will have limitations. A dedicated circuit breaker (suitable for backfeed) or fusible disconnect is required. The total supply of current to a panelboard is limited to 120% of the busbar rating. The service panel is supplied by both the utility (through the main circuit breaker) and the inverter (through a back-fed circuit breaker). The sum of the ampere ratings of the main circuit breaker and the inverter circuit breaker cannot exceed the rating of the panelboard bus multiplied by 1.2 (120%). A circuit breaker used for connection of the inverter will have to be located at the opposite end of the bus from the main circuit breaker, to avoid over loading of the bus. Building-Integrated Photovoltaic Modules Photovoltaic modules that also serve as an outer protective finish for a building are known as building-integrated photovoltaic (BIPV) modules. This type of module is often installed in the form of roofing tiles intended to blend in with surrounding non PV roof tiles. See Fgr. 5. BIPV modules are investigated through the listing process for conformance to appropriate fire-resistance and waterproofing standards, which apply to roofing tiles, along with the electrical standards of UL/ANSI 1703. A module the size of a roofing tile is obviously much smaller than the standard photovoltaic module. Many more of the smaller modules are required to create strings. Micro-Inverters and ac Photovoltaic Modules A single small inverter connected to each photovoltaic module is known as a micro-inverter. Instead of connecting multiple modules to a single inverter, each module will have its own attached inverter. The output of each module is connected directly to the micro-inverter with the existing module leads. Multiple micro-inverters are connected in parallel on a single circuit, which then supplies the service panel of the home. Micro-inverters are required to be listed to ANSI/UL 1741 just as with the larger string inverters. Ground-fault protection, anti-islanding, and the other requirements of utility- interactive inverters are applicable. Output current for a single micro-inverter is approximately 0.8 amps. This would permit up to 15 inverters on a single 15-amp circuit (allowing for the continuous current multiplier). The inverter shown in Fgr. 6 is a micro-inverter. An ac photovoltaic module is essentially a normal dc module with the micro-inverter installed at the factory. The dc conductors are covered and inaccessible on an ac module. The absence of field-installed dc string conductors is a big advantage with both micro-inverters and ac modules. Hazards associated with the dc string conductors are minimized with the use of micro-inverters and eliminated with ac modules. NEC requirements for ac circuits connecting micro-inverters/ ac modules to service panels are similar to normal branch-circuit rules. NATIONAL ELECTRICAL CODE REQUIREMENTS Article 690, Solar Photovoltaic Systems, of the NEC provides the specific requirements for installation of utility-interactive PV systems. In addition, the general requirements of NEC Sections 1 through 4 apply, except as modified by Article 690. A utility-interactive PV system operates in parallel with the utility (primary source); portions of Article 705, Interconnected Electric Power Production Sources, will apply as well. We will look at some of the requirements of Article 690 in the following section. Part I. General Article 690 begins by providing the scope or what is covered by the article and defining many terms used in the article and PV industry. Specific applicable sections of Article 705 are referenced in NEC 690.3. An exception to NEC 690.3 makes it clear that PV systems installed in hazardous (classified) locations must comply with the applicable portions of Articles 500 through 516. Many of the basic rules are covered in 690.4. Separation of PV system conductors from other systems, arrangement of module connections, and the requirement for listing of equipment are all found in NEC 690.4. Ground fault protection for grounded dc photovoltaic arrays is required, NEC 690.5. Part I ends with NEC 690.6, the rules for the installation of alternating current modules. FGR. 6 A utility-interactive micro-inverter. (Enphase Energy) Part II. Circuit Requirements Requirements for PV source and output circuits are found in this section. Voltage and current parameters for design of PV circuits are defined. Maximum dc voltage for a series string of modules is defined as the sum of the rated open-circuit volt age (Voc) of the modules, corrected for the lowest expected ambient temperature. Part I must be used if the manufacturer does not provide temperature correction coefficients. This table provides multipliers that correspond to the lowest expected ambient temperature. Multiply the sum of the module Voc by the factor from Table 690.7 to find maximum voltage. The size of PV system conductors and components is determined by the amount of current that flows through them. NEC 690.8 provides the method for determining the circuit size and cur rent. This is a two-step process. First, a multiplier is used to calculate the maximum source circuit current, 690.8(A); then a second multiplier is applied to determine the size of system conductors and overcurrent devices, 690.8(B). The rated module short-circuit current (I_sc) is the starting point for both of these calculations. Sunlight may be more intense in the field than at the testing lab, so 690.8(A) requires the module short-circuit cur rent to be multiplied by 1.25 (125%) to determine maximum circuit current. Module current is continuous (3 hours or more), which is why the maxi mum circuit current is required to be multiplied by 1.25 (125%) again for sizing of conductors and overcurrent devices, 690.8(B). An Informational Note states that applying both 690.8(A)(1) and (B)(1) results in a multiplication factor of 1.56 (1.25 3 1.25 5 1.56). Multiplying module short circuit current by 1.56 will provide the current value required to size the dc system conductors and overcurrent devices. Inverter current output is continuous, so the rated output of the inverter must be multiplied by 1.25 (125%) for sizing of conductors and overcurrent devices. See 690.8(A)(3) and 690.8(B)(1). Requirements for photovoltaic system over current protection are found in 690.9. The general requirement is that all conductors and equipment are to be protected in accordance with Article 240, Overcurrent Protection. There is an exception to this general requirement, which is often used. If circuit conductors were sized using the requirements of 690.8(B) and there are no external sources of parallel currents that exceed the ampacity of the conductors, then overcurrent protection is not required. This exception permits parallel strings of modules to be combined without fuse protection of the individual strings. As long as the sum of the short-circuit currents from parallel strings does not exceed the rating of a faulted module or conductor, no overcurrent protection is required. Consider the following example: an array contains three parallel strings of modules, and short-circuit current from each module/string is 7 amperes. If a short circuit occurs in one of the modules/strings, the maximum fault current would be the sum of the short-circuit currents from the other two strings. In this example the sum is 14 amperes (7 1 7 5 14). As long as the modules have a rating greater than 14 amperes, the exception to 690.9 permits combination of the strings without fuse protection. A fused combiner box would still be allowed, of course. Many designers specify fused combiner boxes even if not required. Part III. Disconnecting Means Means shall be provided to disconnect all current-carrying conductors of a PV source from all other conductors in a building or structure per NEC 690.13. This does not apply to grounded dc conductors. Energized grounded current-carrying conductors are not to be interrupted by the disconnect(s), just as with grounded ac conductors (neutrals). Several disconnects may be required for a typical residential installation. The direct-current circuit generated by the array must have a disconnecting means installed at a readily accessible location out side the building, or if inside, closest to the point of entrance. The disconnect is not permitted to be located in bathrooms. An exception to 690.14(C)(1) permits the direct-current photovoltaic conductors to enter the building without a disconnecting means if contained in metal raceways or enclosures. See 690.31(E). Photovoltaic equipment, such as an inverter, is required to have a disconnecting means. The disconnects are required to be grouped and identified if the equipment is energized from more than one source. A utility-interactive inverter is connected to two sources of electricity. The array on the roof generates direct current anytime the sun is shining, and the conductors that connect the inverter to the ser vice are energized by the electric utility. Interruption of the array circuit to the inverter will prevent the inverter from supplying alternating current to the service panel, but the conductors are still energized by the utility supply. If the inverter is located within sight of the circuit breaker connection in the ser vice panel, then the circuit breaker can serve as the disconnecting means. Always check with the local Authority Having Jurisdiction (AHJ) and electrical utility company for policies regarding disconnecting means. Some utility companies will require a dedicated utility disconnect. Part IV. Wiring Methods Exposed single-conductor cable is permitted to be installed for array interconnection. Only types USE-2 and listed PV wire are permitted, and there are other limitations. If the circuit operates at over 30 volts and is in a readily accessible location, a raceway must be used. Direct-current array conductors that enter the structure are required to be installed in metal raceways or enclosures from the point of penetration to the first readily accessible disconnecting means, per 690.31(E). Wiring methods for the inverter output circuit are only limited by the general requirements of Sections 1 through 4 of the NEC. Inverter wiring to the service is essentially a branch circuit if a load-side connection is used. A supply-side connection will involve the service wiring method limitations of Article 230. Junction, pull, and outlet boxes are permitted to be located behind removable modules that are connected with flexible wiring methods. See 690.34. Ungrounded photovoltaic power systems are permitted if the installation complies with 690.35(A) through (G). Part V. Grounding According to 690.41, one conductor of a 2-wire PV system over 50 volts shall be grounded. The exception to 690.42 allows the grounded conductor bond to be made by the ground-fault detection device required by 690.5. The grounded conductor of the dc system can be either the positive or the negative conductor. Inverter design will determine which conductor is grounded, but any PV system over 50 volts is required to be a grounded system unless it complies with 690.35. Exposed non-current-carrying metal parts of module frames and other equipment are required to be grounded. Methods of grounding module frames will vary among different manufacturers. Installation instructions for the module must be referenced. The size of the equipment grounding conductor is determined by NEC Table 250.122. When there is no overcurrent device in the circuit, the module/string short-circuit current shall be the assumed overcurrent device size for reference to Table 250.122 per 690.45(A). A grounding electrode system is required for both dc and ac photovoltaic systems. Systems with ac and dc grounding requirements, such as utility interactive systems, must meet the requirements of 690.47(C). Optional methods of installing a grounding electrode system are permitted. All methods include a bond to the existing premises' grounding system. A conductor that serves as an equipment grounding conductor and the bond between ac and dc systems is permitted by 690.47(C)(3) for an inverter with ground-fault protection. Ground mounted arrays require a local grounding electrode. The structure of a ground-mounted array is permit ted to serve as the grounding electrode if the requirements of 250.52(A) are met. An array installed within 6 feet (1.8 m) of the premises' wiring grounding electrode will not require an additional grounding electrode according to Exception No. 2 of 690.47(D). Part VI. Marking Photovoltaic modules will have labels marked with specific information as required by 690.51 (dc modules) and 690.52 (ac modules). This information is required so that system designers and installers are able to size the balance of system components. The completed system will have operating parameters (voltage and current) unique to the design. Field labels marked with the specific dc- and ac-operating parameters must be installed per the rules of 690.53 and 690.54. Buildings with stand-alone or utility interactive PV systems are required to have a plaque or directory indicating the presence of the system and location(s) of disconnecting means. See 690.56. Installers of PV systems should be aware that there are marking requirements for specific components throughout Article 690. Part VII. Connection to Other Sources The rules for connection of the utility-interactive PV system to the electrical service are found in this part. An inverter or ac module is required to automatically de-energize its output when utility power is lost. An inverter must stay in this de-energized state until utility power is restored. This is known as "anti-islanding" and is a requirement of 690.61 as well as ANSI/UL 1741. A neutral conductor smaller than the ungrounded (phase) conductors is permitted per 690.62. A smaller neutral for the inverter is only allowed when used for instrumentation or detection purposes. Installation instructions for an inverter will provide the minimum neutral conductor size. The National Electrical Code permits two methods for connection of a utility-interactive PV system to the electrical service. Requirements for both methods are found in 690.64 Point of Connection. A connection on the line side of the service disconnecting means is known as a supply side connection, 690.64(A). Size of the photovoltaic system is virtually unlimited, but the rules of NEC Article 230 will apply to these service conductors. Requirements for load-side connections are found in 690.64(B). The photovoltaic system size is limited by the panel bus rating and main circuit breaker size. See 690.64(B)(2). A dedicated PV system circuit breaker, suitable for back-feed and positioned at the opposite end of the bus from the main circuit breaker, is a requirement of 690.64(B). See Fgrs.7 and 8 for a representation of the two connection methods. === METER UTILITY Main circuit breaker Branch circuits Back-fed circuit breaker UTILITY INTERACTIVE INVERTER Load-Side Connection FGR. 7 Load-side connection. (--) === Supply-Side Connection METER UTILITY Main circuit breaker UTILITY INTERACTIVE INVERTER FUSED DISCONNECT Branch circuits FGR. 8 Supply-side connection. (--)
=== Part VIII. Storage Batteries and Part IX. Systems over 600 Volts Requirements for photovoltaic storage battery systems are found in Part VIII of Article 690. The provisions of Article 480, Storage Batteries, will apply as well. Specific rules for dwelling units are found in 690.71(B). Part IX of Article 690 requires photovoltaic systems with a maximum dc voltage over 600 volts dc to comply with Article 490, Equipment, over 600 Volts, Nominal. Note that 690.7(C) limits photovoltaic source and output circuits to 600 volts or less for one- and two-family dwellings. = = = ============== QUIZ Refer to the National Electrical Code when necessary. Where applicable, responses should be written in complete sentences. 1. Article of the National Electrical Code contains most of the requirements for installation of photovoltaic systems. 2. NEC Sections through will apply to PV installations except as modified by Article ___. 3. Name four electrical hazards associated with photovoltaic systems: ___ 4. Name five components of a utility-interactive photovoltaic system: ___ 5. Which conductor types are permitted to be installed exposed for array circuits? 6. To size conductors and overcurrent devices for source circuits, you must multiply the module string short-circuit current by ___. 7. When is a metallic raceway required for photovoltaic source circuits? 8. Photovoltaic modules that also serve as an outer protective finish for a building are known as. 9. Grounding electrode system requirements for a utility-interactive photovoltaic system are found in section of the NEC. 10. The two methods permitted for connection of a utility-interactive photovoltaic system to a service are known as and ___. Prev: |