2 Electrical Integration Chapter 11Electrical IntegrationNational Electrical CodeÒ • Voltage and Current Requirements • Conductors and Wiring Methods • Overcurrent Protection • Disconnects • Grounding • Battery Systems
3 Many articles in the NEC® are applicable to the electrical integration of a PV system, particularly Article 690.Many other articles of the NEC® are referenced in Article 690 or otherwise apply to PV installations. Applicable articles depend on the type and configuration of the system. See Figure Whenever the requirements of Article 690 and other articles differ, the requirements of Article 690 apply.
4 The NEC® defines the various circuits and components in PV systems and specifies their requirements. The beginning of Article 690 defines specific terminology for PV system components and circuits. Most are easily distinguishable, though a few may require special attention when referencing the code. For example, the NEC® differentiates between a PV power source and a PV source circuit. See Figure 11-2.
5 Array open-circuit voltage is corrected for low temperatures to yield the maximum possible PV circuit voltage.Voltage-temperature corrections for crystalline silicon PV modules must use the factors provided in Table unless lowest temperatures are below –40°C (–40°F), crystalline PV modules are used, or the voltage-temperature coefficients are provided with the listed module instructions by the manufacturer. If the open-circuit voltage temperature coefficients are supplied in the manufacturer’s instructions for listed PV modules, they must be used to calculate the maximum PV system voltage instead of using Table in accordance with Section 110.3(B). See Figure 11-3.
6 Conductor sizes typically used in PV systems range from 18 AWG to 4/0 AWG. Conductors may be solid or stranded. Larger conductors have lower resistance for a given length.Conductor sizes used in most electrical systems are expressed in American Wire Gauge (AWG) numbers. See Figure Larger diameter conductors have smaller AWG numbers. Larger conductors have greater current-carrying capacity and less resistance. However, solid (single wire) conductors can be stiff and difficult to work with especially for larger sizes. Therefore, conductors are also available stranded (made up of multiple smaller wires), which makes them more flexible. Solid and stranded conductors of the same AWG size have the same cross-sectional area, though stranding makes the diameter slighter larger. At size 6 AWG and larger, conductors are generally only available in stranded versions.
7 Ampacity is the current-carrying capacity of a conductor, which depends on the conductor’s type, size, and application.Conductor sizing is based on a conductor’s ampacity. Ampacity is the current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Nominal conductor ampacity at 30°C is determined by the conductor material (copper or aluminum), size, insulation type, and application (direct burial, conduit, or free air). See Figure 11-5.
8 Conductor ampacity must be derated for high operating temperatures. Ambient-temperature-based ampacity correction factors are given in Table (B)(2)(a). See Figure The correction factor is multipliedby the nominal ampacity to calculate the derated ampacity. Therefore, for a certain current-carrying capacity rating, the size of the required conductor must be increased to account for deratings.
9 For conduits installed on rooftops, an extra temperature adder is needed to account for the extreme ambient temperatures of the environment. The adjusted ambient temperature is then used to determine the temperature-based ampacity correction factor.Since conduits exposed to direct sunlight on or above rooftops experience higher temperatures, a temperature adder adjusts the ambient temperature before determining the temperature-based correction factor. See Figure This adder increases the effective ambient temperature by an amount dictated by the distance of the conduit from the rooftop.
10 Conductor ampacity must be derated for more than three current-carrying conductors together in a conduit or cable.When more than three current-carrying conductors are installed together in a conduit or raceway longer than 24′, conductor ampacities must be further derated. This situation can occur in PV systems with arrays having multiple source circuits. Because bundling several current-carrying conductors together affects their ability to dissipate heat, an additional correction factor is applied to the temperature-corrected ampacity. See Figure For example, if USE-2 conductors are used for three source circuits run through a conduit, each with positive and negative conductors, the total is six current-carrying conductors. The correction factor is 0.80 and the ampacity of each conductor is further reduced to 18.6 A (23.2 A × 0.80 = 18.6 A).
11 Information printed on the outer jacket of a conductor includes size, insulation type, environmental exposure, temperature ratings, and listing approval.Insulation protects a bare conductor from coming into contact with personnel or equipment. The insulating material defines three critical properties for a conductor: its maximum operating temperature, its application and environmental resistance (such as to sunlight, oil, or moisture), and its permissible installation locations (such as direct burial, in conduit, or exposed). This information is marked on the outer insulation or jacket of the conductor, sometimes in abbreviations or codes, along with other information, such as maximum voltage, manufacturer, and size. See Figure 11-9.
12 Conductors in different parts of a PV system have different application requirements. Conductor insulation types used in PV systems must be compatible with the environmental conditions and ratings of the associated equipment, connectors, or terminals. PV systems can include many of the common conductor types used in buildings, but they must be chosen separately for different parts of the system. See Figure Certain requirements apply to the identification, grouping, and labeling of PV circuits and conductors to distinguish them from other electrical systems. The requirements for interconnecting PV systems to other electrical systems, such as the utility grid, are covered in Article 705.
13 Source-circuit conductors for PV module interconnections within the PV array are permitted to be exposed if the conductor insulation has the required environmental resistances.PV module electrical wiring is usually installed with full exposure to the elements, including temperature extremes, sunlight (UV) exposure, and precipitation. Consequently, any conductors used for these circuits may need to be rated for outdoor applications with high temperature, moisture, and sunlight resistance. See Figure
14 Modules are typically connected together in PV source circuits with external, exposed connectors. Modules are typically connected together in PV source circuits with external, exposed connectors. See Figure These connectors must be listed for the conditions of use and have ratings at least equal to those of the rest of the wiring system.
15 When tightened and torqued properly, screw terminals produce secure and low-resistance connections. Screw terminals use the compressive force of a screw to secure a conductor to a terminal and are highly reliable connections. See Figure A screwdriver or Allen wrench is used to tighten the screw terminal to a certain torque. These mechanical connections are common on many types of electrical components, including disconnects, overcurrent protection devices, PV modules, charge controllers, and inverters.
16 Lugs are crimped conductor terminations in ring, fork, spade, or pin shapes. Lugs are used to terminate conductors with special connectors. In most cases, screw terminals are preferable to lugs because they are easier to install correctly and involve one fewer connection. Poorly installed lugs are problematic in electrical installations. However, some PV equipment may require lugs. Lugs are specified by the conductor size, number of conductors, temperature rating, insulation, lug type, and lug size. See Figure
17 Splicing devices, such as screw terminal blocks, are used in PV systems to connect or extend conductors or to parallel array source circuits. This type of terminal would not be used for a tap.Splicing devices include screw-terminal, split-bolt, crimped-barrel, and twist-on connectors. See Figure Splicing devices for direct burial must be listed for that use. With the exception of direct-burial splices, all splices must be made in an approved junction box or enclosure.
18 Several NEMA plug-and-receptacle configurations are acceptable for use with DC branch circuits. Only one NEMA plug-and-receptacle configuration is specifically designed for DC only, and a few two-pole, three-wire designs are specifically for AC only. Most others can be used for either type of power. For most residential and light commercial applications, several NEMA styles satisfy the other two requirements, since they have separate grounding terminals and are not commonly used otherwise in these locations. See Figure
19 Module junction boxes contain and protect the module terminal connections and diodes in the source circuit. Some are field-accessible.Most PV modules include a plastic junction box attached to the back surface. The junction box contains the module’s electrical terminals and often the bypass diodes. See Figure Early module junction boxes had multiple knockouts sized for conduit or individual conductors.
20 Multiple PV source circuits are combined into the PV output circuit within the combiner box. A source-circuit combiner box facilitates and protects array wiring connections and provides a central point to test and troubleshoot arrays. See Figure It also allows one source circuit to be disconnected without interrupting the other source circuits. Combiner boxes may be located indoors or outdoors, near or inside an inverter, or as a separate enclosure near the array.
21 Blocking diodes may be used in the source circuit whereas bypass diodes are installed within a module or its junction box. These diodes help preserve system output and protect PV modules from damage.A blocking diode is a diode used in PV source circuits to prevent reverse current flow. For example, in a simple self-regulated PV system, a fully charged battery may begin to discharge through the array when the array voltage falls below the battery voltage at nighttime. A blocking diode is used to prevent this undesirable discharge. Similarly, in an array with multiple source circuits, a blocking diode prevents one source circuit from feeding another source circuit, such as when the second circuit is shaded and consumes power. A blocking diode is installed in the ungrounded conductor in series with the module string to prevent these types of power losses. See Figure
22 Bypass diodes may be installed in the module junction box. Bypass diodes can be factory-installed by the module manufacturer or field-installed in existing systems by the installer. They may be accessible in module junction boxes, or they may be encapsulated in a junction box or module laminate. See Figure
23 A number of different types of conduit may be used in PV systems if they have the appropriate ratings.Conduit options for throughout PV systems may include electrical metallic tubing (EMT), rigid nonmetallic conduit (electrical PVC, schedule 40 or 80), and electrical nonmetallic tubing (ENT), assuming their use does not exceed their ratings. Rigid metal conduit (RMC) and intermediate metal conduit (IMC) may be used when extra protection is needed. See Figure Like outdoor conductors, outdoor conduit must be rated for high temperatures, sunlight resistance, and moisture resistance. Metallic conduit and raceways are often used for a grounding path.
24 Current-limiting overcurrent protection devices open a short circuit before current reaches its highest value.An overcurrent protection device is a component that protects conductors for higher-than-rated currents, which prevents damage from high temperatures. High temperatures can damage conductor insulation, causing electrical shock and fire hazards. The overcurrent protection device opens the circuit before damage can occur. See Figure An overcurrent condition can be the result of an overload, ground fault, or short circuit.
25 Overcurrent protection devices include fuses and circuit breakers of various types and ratings. Overcurrent protection devices include fuses and circuit breakers. See Figure A fuse is a metallic link that melts when heated by current greater than its rating, opening the circuit and providing overcurrent protection. Requirements for overcurrent protection in PV systems are covered in Section and Article 240. Overcurrent protection devices must be listed and specifically rated for their intended use, such as for DC circuits. Automotive fuses are not listed electrical equipment and may not be used in PV systems.
26 Array source circuits are fused individually within the source-circuit combiner box. This source-circuit overcurrent protection may be provided by supplementary overcurrent protection devices. See Figure A supplementary overcurrent protection device is an overcurrent protection device intended to protect an individual component and is used in addition to a current-limiting overcurrent protection device. In this case, a series string of modules is treated as a single component. The use of such devices enables the source-circuit protection to be closer to the specified ratings required on module labels. These devices must be accessible, but are not required to be readily accessible, so they may be installed in junction boxes or other enclosures on rooftops or in attics.
27 Overcurrent protection for the inverter output circuit depends on the system type. Overcurrent protection and disconnecting means can be accomplished by using circuit breakers or fused disconnects.AC output overcurrent protection for interactive systems depends on the location of the utility interconnection. See Figure Some systems may use a separate fused disconnect. Alternatively, the inverter output may be connected to the AC distribution panel through a back-fed circuit breaker.
28 Connecting a 120 V inverter to a 120/240 V panelboard with multiwire branch circuits may cause dangerous overloading in the grounded (neutral) branch circuit conductor and must be avoided.A particular overcurrent problem arises when one stand-alone inverter with a 120 V output supplies a 120/240 V distribution panel where the L1 and L2 bus have been connected together to create a single 120 V supply. If the system includes a multiwire branch circuit with two 120 V circuits sharing the neutral conductor, the neutral conductor can become dangerously overloaded. See Figure Since current on the two ungrounded conductors will be in-phase, instead of out-of-phase as with a normal 240 V split-phase supply, currents in a multiwire branch circuit will add when they return on the shared neutral conductor. Therefore, the neutral conductor may carry twice its rated circuit current. (A similar problem can occur with interactive systems when PV systems are added to buildings that are already wired for standard 120/240 V service.)
29 The array disconnect opens all current-carrying conductors in the PV output circuit. A disconnect must be provided to isolate all current-carrying conductors of a PV power source from all other conductors in a building or structure. See Figure This disconnect is also known as the DC disconnect or PV disconnect.
30 The AC disconnect of an interactive PV system may be located close to the main utility service disconnect, which can satisfy utility requirements for an external, visible-break, and lockable PV system disconnect.A disconnect must also be installed on the AC side of a PV system to isolate the system from the rest of a building’s electrical system. In an interactive system, the PV system’s AC disconnect is often located near the main utility disconnect for convenience. See Figure Although this is not an NEC® requirement, this location can satisfy utility interconnection requirements for an accessible, visible-break, lockable PV system disconnect.
31 Switches or circuit breakers are required to isolate and disconnect all major components in a PV system from all ungrounded conductors of all power sources.Disconnects must be provided to open all ungrounded conductors to every power source and each piece of PV system equipment, including inverters, battery banks, charge controllers, and other major components. If equipment is connected to more than one power source, each power source must have disconnecting means. See Figure These disconnects must be circuit breakers or switches, though circuit breakers are more common because they are less expensive and easier to source for DC circuits.
32 The DC grounding system and the AC grounding system must be connected together with a bonding conductor. The array may also require a separate grounding electrode system.Most PV systems involve both AC and DC circuits. Since the DC grounded conductor is not directly connected to the AC grounded conductor, they are considered separate systems. These two grounding systems must be bonded together. See Figure There are three acceptable methods of meeting this requirement.
33 Some inverters include fuses as array ground-fault protection in their DC input circuits. Ground-fault protection is often built into the inverter, which includes a serviceable fuse that is either internal or external to the inverter. See Figure Inverters are designed to immediately shut down and disconnect the ungrounded conductor if the fuse is opened.
34 Circuit breakers can be used for array ground-fault protection when the inverter does not already provide this protection.For low-voltage PV systems, a pair of circuit breakers can be used to provide array ground-fault protection. See Figure A lower-rated circuit breaker is mechanically tied to a higher-rated array circuit breaker, which acts only as a switch. The ground-fault circuit breaker trips when current between the grounded and grounding conductors exceeds its rating and forces the other circuit breaker to open the ungrounded conductor.
35 A ground-fault circuit interrupter (GFCI) senses differences between the current in the grounded and ungrounded conductors, indicating a ground fault, and opens the circuit in response.A ground-fault circuit interrupter (GFCI) is a device that opens the ungrounded and grounded conductors when a ground fault exceeds a certain amount. It does this by sensing a difference between the current flowing out through the ungrounded conductor and returning through the grounded conductor. See Figure For GFCI devices to function properly, the grounded conductor must be properly bonded to the equipment grounding conductor, typically at the service equipment.
36 Modules should be connected to each other and the mounting structure with grounding conductors to ensure a continuous grounding connection.The integrity of the electrical contact between the module frames and a grounded mounting structure cannot always be assured with typical fasteners. This is because the thin anodized layer of aluminum frames and structures, or the corrosion of inappropriate materials, may prevent a good electrical connection. Specially listed and identified devices that provide a secure electrical connection can be used to bond module frames to grounded mounting structures or module frames. Alternatively, equipment grounding can be accomplished with continuous runs of bare conductor that are secured to each module with a special connector. See Figure
37 Equipment grounding conductors are sized based on the rating of the overcurrent protection device in the circuit.When ground-fault protection is used, PV circuit equipment grounding conductors are sized in accordance with Article 250. The article establishes the minimum size for equipment grounding conductors based on the overcurrent protection rating for the circuit (regardless of whether an overcurrent protection device is actually used). See Figure For example, if the PV output circuit overcurrent protection device is 60 A, then a 10 AWG equipment grounding conductor is required. The size of the equipment grounding conductor may need to be increased for voltage drop considerations.
38 Lightning protection is especially important in the southeastern states, which have the highest lightning-strike density in the United States.Lightning strikes can cause dangerous and damaging voltage transients. Because PV arrays are often mounted on elevated structures, such as rooftops, many PV systems must be protected from potential lightning damage. Lightning protection is especially important in the southeastern United States, which experiences the highest rates of lighting strikes in the country. See Figure Lightning protection system requirements are covered briefly in Article 250 and more extensively in NFPA 780, Standard for the Installation of Lightning Protection Systems.
39 A lightning protection system includes a network of air terminals, a grounding electrode (down) conductor, and a set of grounding electrodes.Lightning protection systems consist of a low-impedance network of air terminals (lightning rods) connected to a special grounding electrode system. This does not violate the rule mandating only one ground connection for the DC system since the lightning grounding electrode system is not connected to the grounded conductor. The system conducts any surges induced by direct or indirect lightning strikes to ground, safely away from the building and equipment. See Figure
40 Surge arrestors may be incorporated into equipment or can be installed on circuits as separate devices.Surge arrestors must be listed and marked with their ratings, and because they rely on a connection to ground to dissipate surges, they cannot be used in ungrounded systems. For maximum protection, surge arrestors should be located as close to the protected equipment as possible. Some components may already include surge arrestors, but they can be field-installed as separate devices. See Figure
41 Connectors may be used for disconnecting high-voltage battery banks into lower voltage segments for servicing.Battery systems of greater than 48 V must include a disconnect to divide the system into segments of 48 V or less for maintenance. Plug-type or bolted connections are permitted for this purpose and are not required to be load-break-rated. See Figure A disconnect means accessible only to qualified persons is required for battery systems greater than 48 V to disconnect only the grounded battery circuit conductor for maintenance per Section (E). Live battery parts must be guarded for any battery installations 50 V or greater by insulating terminal protectors, elevation, barriers, or location in rooms accessible to only qualified persons and under lock and key per Sections and