Interconnecting PV System Components

This information is from the Stand-Alone PV System Handbook published by the PVSAC at Sandia. The table below allows you quick access to these issues.

It is important to select wire, connectors, and protection components that will last twenty years or more. To obtain this long life, they must be sized correctly, rated for the application, and installed carefully. Connections are particularly prone to failure unless they are made carefully and correctly. Obtain a quality crimp tool and ask an experienced electrician for advice on ways to make and protect long lasting connections. Remember the performance and reliability of the entire system depends on each connection.

Selecting wire for your application may seem confusing because there are so many types of wire and insulation available. However, only a few types are popular with PV system installers. In most cases you don't need special (and therefore expensive) wire. Talk to a local electrician or a wire supplier and describe how and where the wire will be used. Ask for recommendations.

Wire Type and Size

In the United States, the size of wire is categorized by the American Wire Gage (AWG) scale. The AWG scale rates wires from No. 18 (40 mil diameter) to No. 0000 (460 mil diameter). Multiple conductors are commonly enclosed in an insulated sheath for wires smaller than No. 8. The conductor may be solid or stranded. Stranded wire is easier to work with particularly for sizes larger than No. 8. Copper conductors are recommended. Aluminum wire is less expensive, but can cause problems if used incorrectly.* Many different materials are used to make the sheath that covers the conductors. You must select a wire with a covering that will withstand the worst-case conditions. It is mandatory that sunlight resistant wire be specified if the wire is to be exposed to the sun. If the wire is to be buried without conduit it must be rated for direct burial. For applications such as wiring to a submersible pump or for battery inter-connections, ask the component dealer for recommendations. Often the dealer or manufacturer will supply appropriate wire and connectors.

Some wire types commonly used in the United States are listed below.

Ñ Underground Feeder (UF) - may be used for interconnecting balance-of-systems (BOS) but not recommended for use within battery enclosures; single conductor UF wire may be used to interconnect modules in the array but this type of wire is not widely available.

Ñ Tray Cable (TC) - multi-conductor TC wire may be used for interconnecting BOS; TC has good resistance to sunlight but may not be marked as such.

Ñ Service Entrance (SE) - may be used for interconnecting BOS

Ñ Underground Service Entrance (USE) - may be used for interconnecting modules or BOS; may be used within battery enclosures,

Ñ THHN - indicates wire with heat resistant thermoplastic sheathing; it may be used for interconnecting BOS but must be installed in conduit--either buried or above ground. It is resistant to moisture but should not be used in wet locations.

Ñ TW - refers to moisture resistant thermoplastic sheathing; it may be used for interconnecting BOS but must be installed in conduit. May be used in wet locations.

The use of NMB (Romex) is not recommended except for ac circuits as in typical residential wiring. Although commonly available, it will not withstand moisture or sunlight.

More useful information is contained in the NEC. It is recommended that any designer/installer review Article 300 before proceeding. This article contains a discussion of wiring methods and Table 310-13 gives the characteristics and recommended usage of different wire types. Table 310-16 gives temperature derate factors. Another useful reference available from the PVSAC at Sandia National Laboratories is "Photovoltaic Power Systems and the National Electrical Code, Suggested Practices."

Selecting the correct size and type of wire for the system will optimize performance and increase reliability. The size of the wire must be capable of carrying the current at the operating temperature without excessive losses. It is important to derate the current carrying capacity of the wire if high temperature operation is expected. A wire may be rated for high temperature installations (60-90°C) but this only means the insulation of the wire can withstand the rated temperature&emdash;it does not mean that ampacity is unaffected. The current carrying capability (ampacity) depends on the highest temperature to which the wires will be exposed when it is carrying the current. According to Table 310-16 in the NEC, a UF type wire operating at 55°C can safely carry only 40 percent of the current it can carry at 30°C&emdash;a significant derate. If the ampacity of the wire is exceeded, overheating, insulation break-down, and fires may occur. Properly sized fuses are used to protect the conductors and prevent this kind of damage.

Loss in a dc circuit is equal to I²R where I is the current and R is the resistance of the wire. For 100 ampere current this means 10,000 times the loss in the circuit. It is easy to see why resistance must be kept small. Also, the voltage drop in the circuit is equal to IR. Voltage drop can cause problems, particularly in low voltage systems. For a 12-volt system, a one volt drop amounts to over 8 percent of the source voltage. Avoid long wire runs or use larger wire to keep resistance and voltage drop low. For most applications AWG No. 8, No. 10, and No. 12 are used.

An abbreviated wire sizing table for a 12 Volt dc system is shown below. The table indicates the minimum wire size that should be used if the voltage drop is to be limited to 3 percent for any branch circuit. (This table can be adjusted to reflect different voltage drop percentages or different system voltages by using simple ratios. For example, a 2 percent loss can be calculated by multiplying the values in the table by 2/3. For a 24 Volt dc system the values can be multiplied by two.) The calculations show one-way distance, taking into account that two wires, positive and negative, are used in an electrical circuit. As an example, assume the array is 30 feet from the controller and the maximum current is 10 amperes. The table shows that a No. 8-size wire can be used up to a one-way distance of 30 feet (no temperature derate included). While the general rule is to limit the voltage drop for any branch circuit to 3 percent, there may be some applications, particularly those operating at or below 12 Volts, where the loss should be limited to 1 percent or less. For the total wire run on any path from source to load, the loss should be no greater than 5 percent.

Switches and Fuses

There is a specification sheet provided in Appendix B that can be used to size and record the switches, diodes, and fuses for the system. Switches, circuit breakers, and fuses are used to protect personnel and equipment. The switches provide the capability to manually interrupt power in case of emergency or for scheduled maintenance. The fuses provide overcurrent protection of the conductors in case of system shorting or ground faults. Diodes are used to control the direction of current flow in the system.

These protection components should be located throughout the stand-alone PV system. The designer should ask "What might happen?" and try to guard against reasonable failure scenarios. The largest current source in the system is the battery. A typical battery can provide over 6,000 amperes for a few milliseconds if faults occur and the battery is short-circuited. These levels of current can destroy components and injure personnel so an in-line fuse should be installed in all battery circuits. The fuses must be rated for dc operation and have an amperage interrupt capability (AIC) sufficient for these high currents. The NEC requires that there must be a method of disconnecting power from both sides of any installed fuse. This may require additional switches to be installed. Any switch used in a dc circuit should be specifically rated for dc operation. An ac switch may operate properly a few times, but it will probably fail when it is needed most. Dc components are rated for voltage and current. Common voltage levels are 48, 125, 250, and 600 volts dc. Current ratings of 15, 30, 60, 100, and 200 amperes are common. The switch or breaker must be sized to handle the maximum possible current. This is the same current level used to specify the fuses. Fused disconnect switches with both devices incorporated into one assembly may be available. Using these will save on installation costs. DC rated circuit breakers can be used to replace both switches and fuses. They may be more difficult to find but the reliability is high and they are preferred by many system designers.

The current produced by the PV array is limited, but the array short-circuit current, multiplied by a safety factor of 1.56, is commonly used to specify the size of a slow-blow fuse in the array output circuit. Should a ground fault occur in the array while the controller is engaged, this fuse will protect the array modules and the conductors from high battery current. In the load circuits a fuse or circuit breaker is usually installed for each significant load.

Switches, fuses, blocking diodes, movistors, and any sensors used for data acquisition are normally installed in a centrally located weather-proof junction box (J-box). The controller is often installed in the same J-box which may be referred to as the control center of the system. All negative wires should be attached to the negative buss and a solid copper wire used to connect this buss to the ground lug in the J-box. (The ground lug is connected to the common ground rod of the system). The positive leads are usually connected through a fuse to the positive buss. A surge protection device such as a movistor can be connected from each positive lead to ground. (See the wiring diagrams for the system design examples in this manual.)

Connections

Poorly made connections are the biggest cause of problems in stand-alone PV systems. Making a good connection requires the correct tools and connectors. Do the following:

Ñ Use connectors--do not try to wrap bare wire around a terminal. Make sure the connector size and wire size are compatible.

Ñ Strip 3/8 to 1/2 inch of insulation from the wire and clean.

Ñ Use a good quality crimp tool to attach the connector to the wire. A ring-type connector is superior to a spade-type connector because there is no possibility of the wire falling off the terminal.

Ñ Solder the crimped connection. This is particularly important if the installation is in a marine environment or exposed to the weather. However, soldering makes a wire brittle and subject to breaking if the wire is repeatedly flexed near the connection.

Ñ Use weather resistant boxes to make connections between subsystems. Do not try to make more than two connections to the same terminal. Make sure the terminals and connectors are clean and of the same type of metal. Tighten firmly. Split bolt connectors should be used instead of terminal strips if the wire size is greater than No. 8. If disassembly is not required, soldered connections may also be used but only if the connection is electrically and mechanically sound before the soldering.

Ñ Allow plenty of wire for entry and exit of the boxes. Use boxes with strain relief entrances and tighten the clamps firmly around the wires. After making the connection to the terminal, check each wire for strain relief.

Ñ Test thoroughly after installation. Check the connector attachment--give it a pull test. Look for places where the connections or bare wire might touch the metal box or other metal equipment. Make sure the wires to the terminal strip are neatly aligned and do not overlap. Check entry and exit points for nicks or cuts in the wire insulation.

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