Photovoltaic Electrical Integration

Learning Goals:

• Identify the electrical codes, regulations, and practices applicable to PV systems.

• Calculate the voltage and current limits for various circuits of a PV system.

• Determine appropriate conductor ampacities and overcurrent protection ratings for various circuits.

• Identify the appropriate types of conductors for PV system circuits based on application and environment.

• Describe the required types of disconnects and their locations.

• Identify acceptable PV system grounding methods.

• Describe the functions and requirements of electrical balance-of-system (BOS) components.

NATIONAL ELECTRICAL CODE

Electrical integration is arguably the most important, and misunderstood, part of installing a PV system. Since there are many different ways to configure a PV system, the electrical integration requirements can be very different between systems. These requirements involve calculating circuit parameters, such as volt age and current, and using this information to size, specify, and locate various components. The majority of the regulations governing electrical installations, including PV systems, are found in NFPA 70: National Electrical Code (NEC).

The National Electrical Code is a nation ally recognized standard on safe electrical installation practice and is used as the governing electrical code in most jurisdictions in the United States. The primary intent of the NEC is to safeguard persons and property from electrical hazards. It is not intended as an instruction manual for untrained or unqualified persons.

[Most PV electrical installations must be completed by a licensed electrician or contractor and inspected by the authority having jurisdiction (AHJ) for code compliance Code compliance can be ensured through the plans review permitting inspection, and approval process of electrical installations.]

[Electrical integration includes the installation of all wiring, overcurrent protection, disconnects, grounding, and other electrical equipment in a PV system.]

Nearly all types of electrical installations are covered by the NEC, including PV systems installed on public and private premises, commercial and residential buildings, as well as mobile homes and RVs. It also applies to the installation of conductors and equipment that connect to an electricity supply, such as a PV system or any other distributed power system.

Unfortunately, the evolving nature of PV systems is such that it sometimes fosters improperly installed systems. Because PV systems are still relatively uncommon, prospective PV system operators may not fully understand all that is involved with installing a PV system, believing that it is a simple do- it-yourself project or that there is no danger in working with DC power. It may also be difficult to source DC-rated components that are adequately tested and listed, leading some installers to use inappropriate or unsafe equipment.

Improper electrical integration can have significant consequences, including poor performance, component damage, and electrical shock hazards. PV installers must understand all applicable electrical codes, regulations, and recommendations, use only listed and appropriate equipment, follow safe working and installation practices, and educate their customers and others about the importance of code-compliant electrical integration.

Article 690

Article 690, "Solar Photovoltaic Systems," appeared for the first time in the 1984 NEC. Since then, major revisions and additions have been made to this article based on industry input and developments. Article 690 addresses requirements for all PV installations covered under the scope of the NEC. Article 690 is divided into nine parts: General, Circuit Requirements, Disconnecting Means, Wiring Methods, Grounding, Marking, Connection to Other Sources, Storage Batteries, and Systems Over 600 Volts. Each covers a certain aspect of PV systems.

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 Ill. 1. Whenever the requirements of Article 690 and other articles differ, the requirements of Article 690 apply.

Selected Applicable NEC Articles

11 0 Requirements for Electrical Installations 200 Use and Identification of Grounded Conductors 210* Branch Circuits 220 Branch-Circuit, Feeder, and Service Calculations 230* Services 240* Overcurrent Protection 250* Grounding and Bonding 280 Surge Arrestors, Over 1 kV 285 Surge-Protective Devices (SPD5), 1 kV or Less 300 Wiring Methods 310* Conductors for General Wiring 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS 338 Service-Entrance Cable: Types SE and USE 400* Flexible Cords and Cables 422 Appliances 445 Generators 450* Transformers and Transformer Vaults 480* Storage Batteries 490* Equipment, Over 600 Volts, Nominal 690 Solar Photovoltaic Systems 702 Optional Standby Systems 705* Interconnected Electric Power Production Sources 720 Circuits and Equipment Operating at Less Than 50 Volts Articles directly referenced in Article 690

Ill. 1. Many articles in the NEC are applicable to the electrical integration of a PV system, particularly Article 690.

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 Ill. 2.

A PV power source is an array or collection of arrays that generates DC power. A PV source circuit is the circuit connecting a group of modules together and to the common connection point of the DC system. PV source circuits are usually (but not always) a string of series- connected modules. For small systems, the PV power source may be only one source circuit. For larger systems, the PV power source is usually composed of several paralleled PV source circuits. A PV output circuit is the circuit connecting the PV power source to the rest of the system. Distinctions such as these are very important for understanding code requirements.

Other circuits defined in Article 690 are named logically as an input or output of a sys tem component. However, this can cause con fusion with some configurations. For example, in an interactive system, the PV output circuit and the inverter input circuit are essentially the same circuit. Since a disconnect is required in this circuit, some may adopt the convention of naming the array side of the disconnect as the PV output circuit and the inverter side as the inverter input circuit.

VOLTAGE and CURRENTREQUIREMENTS

Article 690, Part II, addresses circuit requirements for PV systems, such as maximum system voltages and currents. These values must be determined because they affect the sizing and ratings for conductors, overcurrent protection de vices, disconnects, and grounding equipment.

Maximum PV Circuit Voltage

According to Section 690.7, the maximum DC voltage of a PV source circuit or output circuit must be less than the maximum voltage limits of all components on the DC side of the system, including the modules, inverter, charge controller, disconnects, and conductors. Also, for one- and two-family homes, the maximum output-circuit voltage can't be greater than 600 V. Systems over 600 V are allowed for commercial or utility-scale PV systems, but must follow different code regulations, particularly those in Article 490, "Equipment, Over 600 V. Nominal."

Electrical integration

Ill. 2. The NEU defines the various circuits and components in PV systems and specifies their requirements. NOTE: MANY SYSTEMS WILL BE CONFIGURED DIFFERENTLY, OFTEN WITH FEWER and /OR COMBINED COMPONENTS

Since temperature affects the voltage output of PV devices, the maximum possible volt age is the array's open-circuit voltage at the lowest expected temperature for the location. Low temperatures for certain locations are included in the solar radiation data sets and other sources. The voltage is then calculated using the module manufacturer's temperature coefficient for voltage.

If temperature coefficients are not pro vided by the manufacturer, the NEC provides a temperature correction factor calculation for estimating the maximum voltage.

See Ill. 3. The maximum PV source-circuit or output-circuit voltage is estimated with the following formula:

V_max =V_oc n_m C_T

where V = maximum PV circuit voltage (in V)

= module rated open-circuit voltage at 25°C (in V)

= number of series-connected modules

C_T = low-temperature voltage correction factor

Voltage Correction Factors for low Temps:

24 to 10 19 to 15 14 to 10 9 to 5 4 to 0

-1 to-5

-6 to-10

-11 to-15

-16 to -20

-21 to-25

-26 to -30

-31 to-35

-36 to -40

Ill. 3. Array open-circuit voltage is corrected for low temperatures to yield the maximum possible PV circuit voltage.

NEC Table 690.7. Reprinted with permission from NFPA 70-2008, the National Electrical Code Copyright 2007, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the official position of the NFPA on the referenced subject which is represented solely by the standard in its entirety.

For example, consider an array of 22 series- connected modules, each with an open-circuit voltage of 21.7 V. The lowest expected temperature of the array is -25°C, which corresponds to a low-temperature correction factor of 1.20. What is the maximum PV circuit voltage? V =V xn xC ,nax oc m T V =21.7x22x1.20 max V = 573 V Since the maximum possible voltage is less than 600 V, this output-circuit voltage is acceptable, provided that it does not exceed the rating of any connected component.

System voltage and current calculations, along with the application and environment, affect the wiring methods and conductors chosen for PV systems.

Maximum PV Circuit Currents

The NEC, Section 690.8, distinguishes between the maximum DC currents of the PV source circuits and the PV output circuit. Each has specific requirements that affect the sizing of conductors and components in the circuit separately.

For PV source circuits, the maximum cur rent is 125% of the sum of the short-circuit current ratings of parallel-connected modules. This 125% factor accounts for the fact that PV devices can deliver currents higher than their rated short-circuit current under enhanced irradiance. Since a source circuit usually consists of a single series string of modules, the maximum current is simply 125% of the module's short-circuit current.

For example, a source circuit consisting of one string of series-connected modules, each with a short-circuit current of 4.8A, has a maximum current of 6A, regardless of the number of modules in the series string. However, if a source circuit includes multiple strings in parallel, the maximum current is 125% of the module short-circuit current multiplied by the number of parallel strings.

For PV output circuits, the maximum cur rent is the sum of the maximum currents of the parallel-connected source circuits. For example, a PV output circuit combining three parallel strings of modules, each with a maximum source circuit current of 6A, has a maximum PV output circuit current of 18A (3x6A= 18A).

These maximum current numbers are involved in further calculations when determining the appropriate conductor size and required overcurrent protection ratings. This applies to PV source circuits and the PV output circuit.

Maximum Inverter Input Current

For an interactive inverter with the PV output circuit connected directly to the inverter, the inverter input circuit is the same as the PV output circuit and , therefore, has the same maximum current.

For stand-alone systems with batteries, the inverter input current depends on battery voltage. As battery voltage decreases, the inverter input current increases to provide the same power input. At low battery voltages and peak power output, this current can be considerably higher than the inverter input current rating at nominal battery voltages. Thus, the highest possible input current is associated with the lowest inverter operating voltage. Maximum inverter input current is calculated with the following formula:

...where I = maximum inverter input current max (in A) AC = rated inverter maximum AC power output (in W) V_min = minimum inverter operating voltage (in V) = inverter efficiency For example, consider a 5500 W stand alone inverter that is 85% efficient and can operate at input voltages down to 44 V. What is the maximum inverter input current?

Maximum Inverter Output Current

Since inverters are limited-power devices, their AC output circuits are sized based on the maximum inverter output rather than load calculations. The maximum current for the inverter output circuit is equal to the inverter's continuous output current rating. This information is provided on inverter labeling and specifications. For example, a 2500 W inverter outputting 240 VAC will have a listed continuous output rating of approximately 10.4 A (2500W - 240 VAC = 10.4 A).

CONDUCTORS and WIRING METHODS

Any conductors and wiring methods allowed by NEC Chapter 2, "Wiring and Protection," and Chapter 3, "Wiring Methods and Materials," may be used with PV systems, in addition to any wiring equipment specifically identified for use with PV systems. Part IV of Article 690 further covers the applications and conditions for conductors used in PV systems, and special equipment and practices allowed.

Generally, only copper conductors of various types and sizes are used in PV systems. They may be exposed to a variety of conditions, including extreme temperatures, mechanical stress, UV exposure, and moisture. These factors determine the type of conductor permitted by the NEC for the application, while the maximum circuit current dictates the appropriate conductor size. For any application, the conditions of use must be well understood to ensure that proper types and sizes of conductors are specified and installed.

Ill. 4. 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 Size

Conductor sizes used in most electrical systems are expressed in American Wire Gauge (AWG) numbers. See Ill. 4. 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. 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.

Conductor size is chosen to be the smallest size that safely conducts the maximum continuous current of a circuit with the appropriate margin of safety. For PV source circuits, conductors must be sized to handle at least 125% of the maximum current. Note: This 125% is in addition to the 125% used to estimate the maximum current under high irradiance. For example, if a string of series-connected modules has a short-circuit current of 4.8 A, the maxi mum current is 6 A (4.8 Ax 125% = 6 A) and the minimum ampacity for conductor sizing is 7.5A (6A x 125% = 7.5 A).

Conductor Ampacity. 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 (cop per or aluminum), size, insulation type, and application (direct burial, conduit, or free air). See Ill. 5.

Since temperature affects a conductor's ampacity, this nominal ampacity is derated (reduced) for ambient application temperatures higher than the nominal 30°C. The derating of conductor ampacities is covered in NEC Section 310.15.

Large conductors are nearly always stranded while small conductors may be solid or stranded.

Ill. 5. Ampacity is the current-carrying capacity of a conductor, which depends on the conductor's type, size, and application.

Ambient-temperature-based ampacity correction factors are given in a table. The correction factor is multiplied by 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 de-ratings.

Since conduits exposed to direct sunlight on or above rooftops experience higher temperatures, an extra value is added to the ambient temperature before determining the temperature-based correction factor. This adder increases the effective ambient temperature by an amount dictated by the distance of the conduit from the rooftop.

For example, 10 AWG USE-2 conductors in a conduit have a 90°C rating and nominal ampacity of 40 A. However, this nominal ampacity is for 30°C. Since the conductors will experience higher temperatures, their ampacity must be derated. The highest expected ambient temperature for the location is 43°C. Since the conduit will be installed about 1" above the roof surface, an additional 22°C is added to the ambient temperature. The correction factor for an adjusted ambient temperature of 65°C (43°C + 22°C = 65°C) is 0.58. Therefore, their ampacity is reduced to 23.2 A (40 A x 0.58 = 23.2A) for operating under those temperatures.

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 Ill. 8. For ex ample, 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 (23.2A x 0.80= 18.6A).

More often, however, these calculations are done together and in reverse. In such a case, a minimum conductor size must be determined from a maximum circuit current, taking into account both types of ampacity de-ratings. The resulting nominal ampacity is calculated with the following formula:

where

I_wm = conductor nominal ampacity (in A)

I_max = maximum circuit current (in A)

CF = correction factor for temperature temp

CF = correction factor for number of conduit current-carrying conductors in a conduit or cable

Ill. 6. Conductor ampacity must be derated for high temperatures.

Ill. 7. 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.

Ambient Temperature Adjustments for Conduits Exposed to Sunlight On or Above Rooftops

Ill. 8. Conductor ampacity must be derated for more than three current-carrying conductors together in a conduit or cable.

For example, four source-circuit conductors must each carry 15 A of current at 40°C ambient. The conductors are installed together in a conduit that is secured 4" above a rooftop, which adds another 17°C to the ambient temperature. If an XHHW-2 conductor is used, the temperature correction factor for 57°C is 0.71. Since there are four current-carrying conductors, the multiple-conductor correction factor is 0.80. What is the minimum nominal ampacity, and therefore minimum size, for an XHHW-2 conductor in this situation?

I_nom = 26.4A

Therefore, the nominal conductor ampacity must be at least 26.4A in order to have sufficient ampacity after derating for the installation conditions. From the table of nominal ampacities, this means that the 12 AWG conductor is of the appropriate minimum size.

These circumstances illustrate the significant effect temperature has on conductor ampacity. Over 40% of this conductor's nominal ampacity can't be utilized in this application.

Voltage Drop. Voltage drop can be an issue in any electrical system, but it is particularly important to address in PV systems. The PV output circuit may have a relatively low voltage and the conductor runs can be long, increasing voltage drop. Excessive voltage drop can also affect charge controllers, batteries, inverters, loads, and other devices that require certain voltages to operate properly. Because voltage drop is not considered a safety concern, the NEC does not establish specific requirements. However, it does recommend a maximum voltage drop of 3%.

Voltage drop and the associated percentage of voltage drop of conductors are calculated with the following formulas:

V =1 x R x L drop op C

where V_drop = voltage drop (in V)

= operating current (in A)

R = conductor resistance (in 2/kft)

L = total conductor length (in kft)

V_drop_% = voltage drop

V = operating voltage (in V) op

Conductor resistance is determined by the conductor material, size, and ambient temperature. Also, the total conductor length is the total (round-trip) distance that current travels in a circuit. Therefore, the total length used in calculations is twice the length of the conductor run.

For example, consider the voltage drop in an 80V output circuit from the array to a disconnect, a distance of 100'. The circuit operating current is 15 A and uses 10 AWG stranded copper conductors. The DC resistance of this conductor is 1.24 ohm/kft and the total length is 200' (0.2 kft). What is the voltage drop in this circuit? V =1 xR xL drop op C V =15x1.24x0.2 drop V =3.7V drop

Vd% = 0.046 or 4.6%

This voltage drop is higher than the recommended maximum, but there are a number of potential options to reduce the voltage drop in the circuit.

First, shortening the conductor length will reduce the voltage drop, but this may not be feasible. The distance between the array and the rest of the system is usually not change able, since the array location is chosen based on factors that are more significant.

More commonly, circuit resistance is reduced by increasing the conductor size, as larger conductors have lower resistance than smaller conductors. (This will likely be beyond the minimum size determined from ampacity calculations.). If the example array used larger 8 AWG conductors instead, the voltage drop would be an acceptable 2.9%. Also, since conductor connections can affect circuit resistance, and therefore voltage drop, circuits should be designed with as few splices and connections as possible.

Conductor Insulation

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 Ill. 9.

[As current flows through a conductor the resistance of the material, though very small causes power loss in the form of heat The amount of heat is proportional to the square of the current, as in the formula P = FR An ampacity rating is determined by how much heat the conductor can withstand without damage, which depends on the insulation material and how well the heat can be dissipated]

{{ Conductor Insulation Codes

Conductor insulation is typically identified and marked with letter codes. Some codes are only abbreviations of conductor names, such as "SE" for service-entrance conductors. Others give information about the actual material type and ratings of the insulation. Each letter or pair of letters describes something about the conductor type or ratings. Common insulation codes include the following:

Insulation Materials:

E = elastomer R = thermoset S = silicone

I = thermoplastic

X = cross-linked synthetic polymer

Outer Covering:

N = nylon jacket Heat Ratings:

H = 75°C HH 90°C

Special Conditions:

o = oil resistant jacket

oo = oil resistant jacket and conductors U = underground (direct burial) applications W = moisture resistant (usually for 60°C)

W-A = outdoor applications

-2 high temperature (90°C) and moisture resistance. For example, a conductor marked 'THWN" has a thermoplastic insulation material covered by a nylon jacket, which offers 75°C heat resistance and moisture resistance.

Insulation letter codes are intended as a reminder only and can easily be misinterpreted. For example, the "5" in "SE" does not stand for silicone. In this case, the abbreviation refers to the conductor's application (service entrance) rather than specific materials or ratings. However, "USE" does indicate a service-entrance conductor that is suitable for underground installation Therefore, if a conductor type is unfamiliar from previous experience, its specifications should be checked to ensure that it has the proper insulation and ratings for the intended application. Some information may be printed separately on the conductor more explicitly, such as "90°C:' or the information may be found in manufacturer literature. Also, tables in NEC Article 310 give more information about the ratings and uses of various conductor and insulation types.}}

Ill. 9. Size, insulation type, resistances, and other in formation are printed on the outer jacket of conductors.

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 Ill. 10.

PV Source-Circuit Wiring. PV module electrical connections are 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. Single-conductor USE-2 is widely used because it has high temperature, moisture-resistance, and sunlight-resistance ratings, and is readily avail able. Also, single-conductor cable listed and labeled as "photovoltaic wire" or "PV wire" was added in the 2008 NEC as approved wiring.

Ill. 11. Source-circuit conductors are per mitted to be exposed if the conductor insulation has the required environmental resistances.

PV modules and arrays operating on hot days under full sunlight can reach very high temperatures. Any conductors, conduits, junction boxes, connectors, or terminals located close to arrays or installed on rooftops in direct sunlight may experience these high temperatures as well. For this reason, conductors and other components used for PV source circuits may need to be rated for at least 90°C (194°F).

Ill. 10. Conductors in different parts of a PV system have different application requirements.

Conductors in array tracking systems have special considerations. These conductors must be especially flexible to withstand repeated movement, which can weaken or crack regular insulation and break conductor strands. The cables must be rated as "hard-service" flexible cables, in addition to being listed for outdoor use. Requirements and ampacity derating regulations are detailed in Section 690.31 and Article 400, "Flexible Cords and Cables." PV Output-Circuit Wiring. The conductors in the PV source circuits do not need to be the same as the conductors for the PV output circuit. Since USE-2 and PV wire are more expensive than most other types of conductors, it may be advisable to transition to a different conductor type for the rest of the exterior wiring. This transition is usually done at the source-circuit combiner box.

Conductors between the combiner box and the DC disconnect are usually enclosed in conduit for protection, so they do not need to be sunlight resistant, but they must still be rated for at least 90°C and wet conditions. (Even though the conductors are protected within conduit, the conduit is exposed and may trap water inside. Therefore, the conductors require a wet rating.) Single- conductor RHW-2, THW-2, THWN-2, and XHHW-2 types are commonly used for the PV output circuit.

Interior Wiring. When DC conductors enter a building, they no longer require high temperature ratings (except when routed through attics), sunlight resistance, or moisture resistance. However, they must be contained within metal conduit or enclosures (up to the first readily accessible disconnect) and rated for fire resistance. Single conductors in conduit, or multi-conductor sheathed cables may be used, except where the cables are subject to damage from exposure or where otherwise required to be in conduit. For residential applications, types NM and NM-B (both known as "Romex") and UF are common accept able multi-conductor sheathed cables. USE-2 conductors that also include RHH or RHW-2 designations may be used inside buildings, but only when installed in conduit.

Factory-installed wiring inside components such as inverters may not follow North American color codes for field wiring. These conductors should not require modification, but the manufacturer's installation instructions should be carefully consulted to avoid any confusion with this wiring.

Battery Wiring. Battery conductors include those between batteries or cells within the battery bank, and those connecting the battery bank to the nearest circuit device, such as a junction box or disconnect. Such conductors must be listed for hard-service use and moisture resistance. It is recommended that the conductors be flexible, as identified in Article 400, since they may be sized at 2/0 or larger. Flexible cables put less stress on battery terminal connections. Finely stranded building-wiring conductors such as USE, RHW, and THW are commonly used. The connectors permitted on these types of conductors must be specifically identified for such use, but certain types of ring lugs are generally compatible. Conductors intended for welding, automotive, or marine applications are not permitted.

Conductor Insulator Color Codes: The color of a conductor's insulation is used to identify the purpose of the conductor in the circuit and to match the ends of a conductor. The insulation may be a solid color or may consist of mostly one color with one or two stripes of another color. Most conductors are available in many colors. For conductors larger than 6 AWG, which are usually only black, colored tape is wrapped around the ends for identification.

There are separate color designations for grounded, grounding, and ungrounded conductors and they apply to both AC and DC circuits. Grounded conductors are white or gray. Grounding conductors can be bare or can have insulation that must be either solid green or green with yellow stripes. Ungrounded conductors may be of various colors, but must be distinguishable from the grounded or grounding conductors, so they can't be white, gray, or green.

The common convention in AC systems is that the first three ungrounded conductors are colored black, red, and blue. The ungrounded conductor for DC circuits (usually the positive conductor) may be black, red, or any other color except green, white, or gray. These color conventions do not always apply to factory- installed wiring inside of components.

Wiring Connections

Quality wiring connections are critical to the safe and reliable operation of PV systems. Poor connections add circuit resistance, in creasing voltage drop, and power loss. In the worst cases, loose or corroded connections cause hot spots and thermal expansion at the connection, leading to failure of the connection or the entire system. To minimize the chances of wiring failure, installers should use high-quality connectors and be careful to install them properly. In humid environments, terminations should be coated with an anti-oxidation compound to retard corrosion. It is also important to use as few connections as possible.

A variety of connectors, terminals, splices, and lugs may be used for field- installed PV system wiring, as long as they are appropriate and rated for the application, such as for conductor material, size, and environment. All wiring connections other than module connections must be made within junction boxes or equipment enclosures using approved methods and listed devices. This protects the connections from the environment but allows access for inspections and service.

Module Connectors. Modules are typically connected together in PV source circuits with external, exposed connectors. See Ill. 12. 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.

Additionally, these connectors must meet the following five requirements from Section 690.33:

• The connectors must be polarized and non-interchangeable with other electrical equipment on the premises. This prevents accidentally reversing the array connections or connecting the incorrect circuits.

• The connectors must protect the live parts from accidental contact with persons during assembly and disassembly.

• The connectors must have a latching or locking mechanism to prevent accidental disconnection. When readily accessible and used in circuits over 30 V, a tool is required for opening.

• When multiple connections are made, the grounding conductor must be the first conductor to make contact and the last to break contact.

• The connectors must be either capable of safely interrupting the circuit current or require a tool to open and are marked "Do Not Disconnect Under Load" or "Not for Circuit Interrupting." Ill. 12. Modules are typically connected together in PV source circuits with external, ex posed connectors.

Connectors meeting these requirements facilitate quick, yet secure, safe, and reliable array connections. Some modules may not include the conductors and /or connectors, and require special crimping and installation tools to attach the connectors. However, many module manufacturers now factory-install the conductors and connectors, eliminating many preassembly requirements. The module junction boxes are often then sealed since the installer no longer needs access.

Screw Terminals. Screw terminals use the compressive force of a screw to secure a conductor to a terminal and are highly reliable. See Ill. 13. 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.

Special terminals can be used as junction points combining several smaller conductors in parallel to connect with a single larger conductor. Screw terminals should not be used with finely stranded or multiple conductors unless listed for such use.

Ill. 13. When tightened properly, screw terminals produce secure and low-resistance connections.

[Some electrical circuits use aluminum or copper clad aluminum conductors. These conductors have tower ampacity than all copper conductors for the same sizes but are significantly tighter However they require special connectors and terminating methods and are not generally used in PV systems]

Lugs. 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 Ill. 14.

Lug types include fork, ring, and disconnect (spade or pin) lugs. Fork and ring lugs are used for connections made by threaded screws or nuts and the lug is sized according to screw diameter designations, such as 1/4" and #8. Fork terminals can be easily removed by loosening the terminal screw or nut, while ring terminals require complete removal of the fastener. For this reason, ring lugs are usually used for large conductors, so they can't accidentally slip out from under the screw or nut due to their weight or movement. Spade- or pin-type lugs have both male and female elements that connect only to each other and can be disconnected without tools.

Ill. 14. Lugs are crimped conductor terminations in ring, fork, spade, or pin shapes.

Most types of lugs require crimping. Crimping compresses the conductive lug material around the exposed end of a conductor, making secure mechanical and electrical connections. Heavy-duty crimpers are required for conductor sizes greater than 8 AWG. Secure crimping is vital, so only quality crimping tools should be used. Every connection should be tugged and checked after crimping to ensure that a quality connection has been made. Some battery cables are available with preinstalled lugs that are both crimped and soldered for extra strength.

Lugs may or may not include insulation around the crimped portion. To ensure quality crimping without interference from the plastic insulation, uninsulated lugs are preferred. If insulation is required, it can be added with tape or heat-shrink tubing after the crimp connection is made and checked.

Splices. Splices are used in PV systems to connect or extend conductors, parallel array source circuits, or tap service-entrance conductors for supply- side interconnections. The splice and any exposed areas on the conductor must be insulated (covered with appropriate tape or heat-shrink tubing) with a level of protection and rating equivalent to the conductor insulation.

Splicing devices include screw-terminal, split-bolt, crimped-barrel, and twist-on connectors. See Ill. 15. 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.

Twist-on splicing devices, called "wire nuts," are commonly used in many electrical applications. Some types are rated for wet locations or direct burial and are prefilled with anti-oxidation compounds. However, these connectors can easily produce poor splices and are not commonly used in PV systems.

DC Plugs and Receptacles. Branch circuits installed for powering DC loads may require the use of plugs and receptacles for making quick or temporary connections. The National Electrical Manufacturers Association (NEMA) standardizes the configurations of dozens of plug and receptacle designs. The NEC does not require a specific design for use in DC branch circuits, but allows any that conform to three requirements: the plugs and receptacles must be listed for DC power with a rating of at least 15 A, must have a separate equipment-grounding terminal, and must be different from any other receptacles used on the premises.

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 Ill. 16.

Two-conductor plugs and receptacles, such as automotive "cigarette lighter" styles, can't be used. It is also not acceptable to use the third grounding conductor of a three- conductor plug or receptacle to carry common negative return currents on a combined 12/24 V system.

Ill. 15. Splicing devices, such as screw terminal blocks, are used in PV systems to connect or extend conductors, parallel array source circuits, or tap service-entrance conductors for supply-side interconnections.

Junction Boxes

A junction box is a protective enclosure used to terminate, combine, and connect various circuits or components together. Junction boxes may be empty enclosures, within which conductors are routed or spliced together, or they may contain terminal strips for making connections.

Junction boxes can be either a box permanently attached to the outside of a major component for making connections to terminals of that component, or a separate box for combining circuits. Junction boxes located behind modules or panels must be accessible by the removal of module or support fasteners. Thus, modules may not be permanently fastened or welded to a support structure.

The common types of junction boxes used in PV systems are module junction boxes and source-circuit combiner boxes. Separate junction boxes must be listed for the application.

Module Junction Boxes. 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 by pass diodes. See Ill. 17. Some module junction boxes have multiple knockouts sized for conduit or individual conductors. For the junction box to be used with conduit, the module nameplate must include a CR rating.

Exposed conductors must use a method of mechanically securing the conductor to reduce any pulling forces on the connection. Junction boxes may include strain-relief posts for this purpose, or the installer may use strain-relief clamps at the entrance to the junction box.

Some junction box terminals allow limited reconfiguration of the cell circuit arrangements within the module. For example, by changing terminal connections, a module consisting of two series strings of cells in parallel can be reconfigured into one long series string.

Permanently sealed module junction boxes are becoming increasingly common. The module's primary electrical connections are encapsulated in a sealed enclosure on the back of the module. Short lengths of conductors extend from the box and terminate with positive and negative connectors. This makes field installation faster, safer, and more reliable.

Plug and Receptacle Configurations for DC Branch Circuits: Ill. 16. Several NEMA plug-and-receptacle configurations are acceptable for use with DC branch circuits.

PV Source-Circuit Combiner Boxes. A combiner box is a junction box used as the parallel connection point for two or more circuits. Combiner boxes are used to combine parallel array source circuits into the PV output circuit. These boxes typically include terminal blocks, fuses, and fuse holders.

A source-circuit combiner box facilitates and protects array wiring connections and provides a central point to test and troubleshoot arrays. See Ill. 18. 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.

Ill. 17. Module junction boxes contain and protect the module terminal connections and diodes in the source circuit. Some are field- accessible.

Combiner boxes often have circuit boards that include built-in provisions for PV source-circuit fuses and wiring connections.

Source-Circuit Combiner Boxes

Ill. 18. Multiple PV source circuits are combined into the PV output circuit within the combiner box.

Protection Diodes

In PV systems, diodes are used in the array circuits as either blocking diodes or bypass diodes. These diodes are typically built into the PV modules, installed inside the module junction boxes, or built into components that interface with the array output circuit. When field-installed, these diodes must be properly sized for the maximum expected voltages and currents.

Blocking Diodes. A blocking diode is a diode used in PV source circuits to prevent re verse 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.

Protection Diodes: BLOCKING DIODES; BYPASS DIODES: Ill. 19. Blocking diodes are installed in the source circuit and bypass diodes are installed within a module or its junction box. These diodes prevent power loss due to reverse current or high-resistance conditions.

Blocking diodes are rarely installed today as separate components. If used separately, they are usually installed in the PV source-circuit combiner box or the module junction box at the head of each series string. More commonly, they are incorporated into components that control battery charge and discharge current, such as charge controllers and bimodal inverters. In shunt-type charge controllers, a blocking diode also prevents the controller from shorting the battery bank during charge regulation.

Bypass Diodes. A bypass diode is a diode used to pass current around, rather than through, a group of PV cells. In contrast to blocking diodes, bypass diodes are installed in parallel with modules or groups of cells. Each module may include one to three bypass diode(s).

Under normal operating conditions, current does not pass through bypass diodes. How ever, if cells develop an open-circuit or high- resistance condition because of shading or module damage, bypass diodes minimize power output losses by routing current around the strings of affected cells.

Bypass diodes can be factory-installed by the module manufacturer or field-installed by the installer. They may be accessible in module junction boxes, or they may be encapsulated in a junction box or module laminate. See Ill. 20.

Ill. 20. Bypass diodes may be field-in stalled in the module junction box.

Conduit Selection

Most PV system conductors are installed in conduit. However, source-circuit and output- circuit conductors are required to be in conduit only when installed in a readily accessible location and their maximum circuit voltages are greater than 30 V. Otherwise, the extra lengths of exposed conductors should be neatly gathered and secured to the mounting structure to prevent abrasion damage from wind or ice.

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. Like outdoor conductors, outdoor conduit must be rated for high temperatures, sunlight resistance, and moisture resistance.

When DC conductors from a PV power source are run inside a building, they must be contained in metallic conduits or enclosures up to the point of the first readily accessible disconnect. In the case of building-integrated arrays, the entire array circuit falls under this requirement. Conduit provides physical protection for these conductors. Metallic conduit also provides fire resistance and a ground-fault path for ground-fault protection devices.

The conductors of PV source and output circuits must not be contained in the same conduit, junction box, or raceway with conductors of any other electrical system. However, DC circuits can be run together with AC circuits if they are both directly related to a specific PV system.

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