Cables and Currents

John Wiles

©1993 John Wiles

In the early days (before NEC awareness), PV systems were wired with any wire that was at hand. Little attention was paid to the quality of the wire, its current carrying capability, or how it was connected. Experience with 12 years of large and small PV installations and the test of time along with help from the local electrical inspector has shown us better ways. Now, conductor types, ampacities, and terminals are a hot topic in the back rooms of most PV distributors, dealers, and installers. Conductor selection and ratings in various PV applications are the topic of this Code Corner.

Module Wiring

Rigid and flexible nonmetallic and metallic conduit can be used with modules having the appropriate conduit fittings on the junction boxes. If conduit is not required by a local code, Section 690-31 of the 1993 National Electrical Code (NEC) permits the use of single-conductor cable that is identified as sunlight resistant for PV module interconnections. Underground Feeder (Type UF), Service Entrance (Type SE), and Underground Service Entrance (Type USE) cables are allowed for module interconnects. Most UF cables are made with PVC insulation. Problems have been identified with PVC insulation when used in direct-current circuits where moisture is present. Under these conditions, the insulation dissolves. It is unknown whether PV module wiring in wet climates provides the conditions necessary for PVC insulation failure. It might be wise to use USE or SE cables in locations where the cables are in contact with standing water. Furthermore, although passing the Underwriters Laboratories (UL) standards for sunlight resistance, UF cable has shown signs of deterioration after only four years in hot, sunlight-exposed installations.

USE and SE cable are generally not marked sunlight resistant, but they have passed the sunlight resistance tests and most inspectors are familiar with the use of these cables outdoors in exposed locations. If the USE or SE cable has cross-linked polyethylene (marked XLPE or XLP) and is further marked RHW and RHH or RHW-2, it is one of the best, commonly available cables. Standard USE cable has only a 75°C insulation when wet. The RHW designation indicates rubber 75°C insulation for use in wet conditions, and the RHH indicates a rubber insulation, when dry, with a 90°C insulation. The new RHW-2 and USE-2 designations indicate insulation with a 90°C rating even when wet. SE cable has a slight advantage in that it has flame resistant additives that USE does not have. The Underwriters Laboratories label (UL) will ensure that the cable meets the highest quality standards and will be the most durable product.

Section 400-7(a)(10) allows the use of flexible cables to connect moving parts. Tracking flat-plate and concentrating PV modules are moving parts and these cables could be used. Types W and G are recognized by the NEC as flexible cables. Types SEO, SEOO, and the like usually have the necessary sunlight and weather resistance. These flexible cables are not allowed when connecting fixed arrays.

This wiring method using exposed, single-conductor cable is only allowed for module connections. At some point near the modules, the wiring method must be changed to one of the other methods meeting the requirements of the NEC. The exposed, single-conductor cables could be routed to a weather head and into conduit and then into the building and to the PV Disconnect Switch. Another alternative is to route the single-conductor cables to a junction box where the cables can be spliced to a jacketed, multiple-conductor cable like NM (Romex) or UF (Underground Feeder). These jacketed cables would then be installed with the required physical protection, and routed to the disconnect switch. NM cable, of course, can only be installed in indoor locations, while the UF cable has sunlight resistance and, with appropriate protection from physical damage, can be installed in outdoor locations.

Tray Cable (TC) comes in two or more conductor cables and is generally marked sunlight resistant, but some inspectors object to its use based on the NEC requirement in Section 340-4 to have it mechanically supported by a cable tray or other means. Also, Section 340-5 prohibits the use of tray cable as open cable on brackets or cleats. Tray Cable requires special calculations for current-carrying capacity (ampacity); the NEC must be consulted carefully when using this cable.

Temperature Derating

Because the PV modules are in the sunlight, they get significantly hotter than the surrounding air temperatures. Ambient air temperatures in some parts of the country may be as high as 45°C (113°F). The backs of the modules, the module junction boxes, and other nearby areas where the conductors must operate can have temperatures as high as 65°C to 75°C. The ampacity of the cables used to connect the modules must be derated for these higher temperatures.

Most installations should use an ambient temperature of 65°C to derate the conductors. In hot locations, with no ventilation provided for the back of the modules (e.g. mounted directly on a roof), a 75°C temperature should be used in the temperature derating calculations. In less sunny, cooler sections of the country, maximum module temperatures might be lower.

An Example

In a particular installation, it has been decided to use number 10 AWG conductors because of the size of the module terminals. Single conductor number 10 AWG USE-2 cable has been ordered with XLPE, RHW-2, and UL markings which indicate a 90°C temperature rating. The modules are mounted on a rack on a brown shingled roof, but for esthetic reasons, the spacing between the modules and the roof is only two inches. The wiring is to be in free air (not in conduit) so Table 310-17 in the NEC may be used. Since the 90°C module terminal rating matches the USE-2/RHH wire temperature rating of 90°C, the cable can be operated at the maximum temperature for which it was rated. In Table 310-17, Number 10 AWG cable with 90°C insulation has an ampacity (current carrying capacity) of 55 Amps at ambient temperatures of 30°C. A footnote to the table notes that number 10 AWG conductors may not have an overcurrent device rated at more than 30 Amps. Because the modules have little ventilation space and the roof is brown, the area between the modules and the roof and in the module junction boxes can be expected to be as high as 75°C on hot, sunny days. The ampacity of the conductor must be derated for this temperature which is the ambient temperature in which the conductors operate. Ampacity Correction Factors are presented in the lower section of Table 310-17. For conductors rated at 90°C, the derating factor is 0.41 yielding a number 10 AWG cable with a derated ampacity of 22.6 Amps (55 x 0.41).

Furthermore, Section 690-8 requires that a 25% safety factor be used when sizing the conductors so that they will not be operated continuously at more than 80% of the rated ampacity. This calculation indicates that the maximum short-circuit current that this conductor can handle is 18.1 Amps (22.6/1.25). The sum of all short-circuit currents for all of the modules connected in parallel on this number 10 AWG USE-2 cable should not exceed 18.1 Amps.

If the modules were spaced six or more inches from the roof, the maximum operating temperature would drop to about 65°C on hot, sunny days. In this case, a derating factor of 0.58 is given which, when multiplied by the 55 Amp rating of the cable at 30°C, gives a derated ampacity of 31.9 Amps (55 x 0.58). After the 25% safety factor is applied, the maximum short-circuit current that can be carried by this cable is 25.5 Amps (31.9/1.25).

Interior Wiring

All interior wiring of DC PV source circuits and DC and ac load circuits must comply with all aspects of the NEC. The cables for DC circuits are similar in most cases to that required for ac circuits. In some cases a larger size conductor is used to reduce voltage drop in DC circuits, but the installer must ascertain that switches and outlets have terminals that will take the larger conductors.

Battery and Inverter Cables

Large conductors such as the 2/0-4/0 AWG cables used to connect batteries and inverters are very stiff if made with building wire such as THHN or USE with 19 strands of copper. The inspector may require the use of such cable because the NEC requires it to be used in fixed installations and the inspector frequently sees electricians using these stiff cables in standard ac power installations. The NEC also requires that space be allocated for wire bending and connection areas when installing equipment using these large cables. Use of these cables requires the proper tools, available from electrical supply houses, to deal with the stiffness.

Most PV installers use either battery cable (controlled by SAE Standards) or welding cable for the larger cables. These cables have numerous small strands that provide a degree of flexibility not found in the more rigid building cables. Stand-alone inverters and large battery cells are being manufactured with flexible cables attached, but these products are generally designed for mobile applications or industrial applications which do not fall under the NEC. The flexibility makes for ease of installation, but the NEC does not make definite provisions for their use in fixed installations. If the flexible cables are used, they should be UL Listed, acid resistant, and installed in conduit. Flexible, Type W single-conductor cables are available and identified for extra hard usage. UL Listed, flexible welding cable is also available, but is not recognized in the NEC for this use.

Code Corner

There are restrictions in Section 400-8 that prohibit these flexible cables from being run through walls or being attached to building surfaces. Section 400-10 of the NEC also requires that strain relief be used wherever flexible cables are connected. This would indicate that if the inspector approves their use, it will most likely be for short runs to a nearby junction box where the flexible cables are connected to a standard, stiff cable. A proposal will be submitted for the 1996 NEC that permits this particular use of flexible cables in an otherwise fixed installation.

Manufacturers of inverters are starting to deliver products with the necessary conduit fittings that will allow the use of the more rigid standard building cables. Underwriters Laboratories is addressing the cable and cable termination requirements as they develop standards for the inverters and battery systems used in residential and commercial PV systems falling under the NEC.

High-Current Cables

The inverter-to-battery cables should be sized based on the inverter continuous power rating at the lowest battery voltage. More and more systems are being installed with large inverters, backup generators, and auxiliary battery chargers. Deep-well pumps filling large storage tanks present a significant load especially when other DC and ac loads are being used simultaneously. If the inverter has the ability to deliver continuous power, and a generator, micro hydro, or the PV array can hold the batteries above the low voltage disconnect point, then that exact situation can and will occur. For example, the 85% efficient inverter is rated at 2000 Watts on a 12 Volt system with a low voltage disconnect of 10.5 Volts, the input current under full power is 2000/.85/10.5 = 224 Amps. The 25% safety factor increases this to 280 Amps and Table 310-16 of the NEC indicates that 250 MCM cable (one size larger than 4/0) in conduit should be used.

Summary

Cables and equipment that meet the requirements established by the NEC are available and can be used for PV installations. Ampacity calculations that are related to high-temperature PV installations are required. In some cases, waivers by the electrical inspector may be required. In other instances, new (to the PV installer) installation techniques may have to be used to deal with the existing, required cables.

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Author: John C. Wiles, Southwest Technology Development Institute, POB 30001/Dept 3SOLAR, Las Cruces, NM 88003 • 505-646-6105

National Electrical Code - 1993, National Fire Protection Association, Batterymarch Park, Quincy, MA 02269

Underwriters Laboratories, 333 Pfingsten Road, Northbrook, IL 60062 M-

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