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Performance/Cost Trade-offs in PV Systems Wiring/Cabling Selection

As the solar industry develops, we find many variations in design and specification requirements. Along with these variations, there is a great deal of misinformation and misunderstanding of the distinctions among various cable types and the characteristics of different cable constructions.

Additionally, with the great mission for the industry being to bring down balance of system (BoS) costs, clarity of proper applications, cable characteristics and code compliance will enable engineers to make better decisions in their choices and the performance cost trade-offs involved with different choices.

With these issues in mind we will attempt to provide some simple clarification on some of the most common issues we encounter in supplying owners, developers and contractors with the appropriate wire and cable components.

USE-2 vs PV Wire

NEC 2008, section 690.31B restricts exposed single conductor wire used to connect to PV modules and circuits to USE-2 and PV wire. Because PV modules operate at elevated temperatures and are exposed to a variety of environmental conditions, the insulation of the module interconnection must be sunlight resistant and rated for wet locations at 90oC. But what are the differences and when should each be chosen? The primary physical difference is simply a thicker insulation required on PV wire.

USE-2 is a standard wire designated for underground service entrance, typically used to connect terminals of a variety of service equipment. PV wire is specifically designated for use in PV module interconnections. Both cables are rated for 90oC wet or dry. PV wire, however, has thicker insulation to withstand the harsh environments frequently found in PV installations. USE-2 carries a 600V rating, whereas PV wire can be rated at 600V, 1000V or 2000V. There are separate ratings with increased insulation thickness for each cable.

Both USE-2 and PV wire can be used with grounded systems, but only PV wire may be used with ungrounded systems per NEC 2008. Since USE-2 has typically been installed underground or in environments where flame is unlikely, it usually does not require a flame retardant rating. The PV wire designation does include a VW-1 flame test requirement. Both ratings are sunlight resistant, but the PV wire rating applies a more stringent test requirement of 720 hours in weatherometer vs. 300 hour weatherometer test for USE-2.

In summary, PV wire provides, through thicker insulation, greater protection and a wider range of suitable applications.

Differentiating Cable Constructions

One of the biggest areas of confusion is with regard to the terms double insulation, double-jacket and dual-layer. This is an important area to clarify as a number of these terms have been used by marketing departments to enhance the perceived performance and applications for certain products. We have found that engineers using the word "double," are looking for two separate layers, which would mean an insulation layer with a separate protective jacket.

Dual-layer (also called double-pass) should be two independent layers usually consisting of different compounds such as ethylene-propylene-rubber (EPR) insulation and a chlorinated polyethylene (CPE) jacket. This combination of EPR/CPE provides the highest level of weather/sunlight resistance and electrical properties. Single-pass comprises one insulation layer, such as an XLPE or EPR compound.

The primary compounds used for PV wire include cross-linked polyethylene (XLPE), cross-linked polyolefin (XLPO), which is also a low smoke zero-halogen product; ethylene-propylene-rubber (EPR), and chlorinated polyethylene (CPE). XLPO has been used extensively in the European market. However, it's main benefit, low smoke zero-halogen, is really designed for human safety in enclosed environments in the case of fire, such as in a tunnel. Since PV installations are outdoors in open spaces, there is no reason to pay the substantial increase in price for an attribute that will realize no benefit in PV installations. Polyvinyl chloride (PVC) is not included in the list because as a thermoplastic compound it is subject to softening or melting when exposed to high temperatures.

 Table. Jacket and insulation properties. SOURCE: General Cable Industrial Catalog, Jacket and Insulation Properties, January 2008, SPEC010(2).

Single pass XLPE and a dual layer EPR/CPE are the two most common constructions in the North American market. An EPR/CPE combination provides better protection against UV, ozone, heat, abrasion and oxidation (see Table). Additionally, separate layers of insulation and jacket means that if the cable is subject to a nick or abrasion, the insulating layer can remain uncompromised, thus retaining the integrity of electrical insulation.

Fine-strand vs Class B-strand Conductors

Many module manufacturers, as well as many plant installations, particularly in Europe, use a fine-stranded copper conductor rather than the standard class B 7 strand wire. The fine stranding provides a more flexible cable, which is easier to pull through conduit and has tighter bend radius. Most US installations specify 7-strand class B conductor. The questions come mostly from installers or engineers who are picking up a European design.

John Wiles, of the Southwest Technology Development Institute, New Mexico State University, wrote a fine article addressing these issues [1].

First of all, class B 7 strand is the standard conductor stocked by most manufacturers and distrbutors in North America, so it tends to be more readily available and less expensive. Beyond that, the most compelling reason for specifying 7 strand over fine strand is the integrity of terminations at the combiner box.

Wiles points out that there have been reports of failed connections when fine-stranded conductors are used with mechanical terminals or lugs that use a set screw to hold the wire in the terminal. The problem is threefold. First, the screw has a tendency to break some of the fine strands of the conductor. Second, some of the fine strands can fray and not fully connect. Third, the fine-stranded conductors will continue to compress after the initial torque setting, eventually resulting in a loose connection. Subsequent re-torqueing can result in a higher-than-normal resistance, which heats the copper and contact, causing expansion and further loosening. Ultimately, there is an increased risk of failure with fine strand if not used with appropriate terminations.

UL engineers have said that few ( if any) of the normal screw type mechanical terminals that the PV industry commonly uses have been listed for use with fine-stranded wires.

Even if the modules comes with a fine-stranded wire, the end of string or "home-run" cable that connects the modules or string to a combiner box, typically will not require fine-stranding.

Direct Burial Cables

PV wire with direct burial rating or USE-2 cable, is typically not required to be installed in conduit. The slight increase in cost for these cable types vs. using the less expensive THHN/THWN-2 in conduit provides far greater savings vs. the additional costs of conduit materials and additional labor required to assemble and install conduit systems. In the interest of bringing down BoS costs, more consideration should be given to developing designs with direct burial cable.

References

1. J. Wiles, "Flexible Cables and Proper Termination, " Home Power, Jan. 2005, pp. 2-4.

2. General Cable Industrial Catalog, Jacket and Insulation Properties, Jan. 2008, SPEC010.

Hugh Robertson received his BBA from The U. of Texas, MBA from Pepperdine U,. and is VP of Sales and Marketing at USA Wire & Cable, Inc., 6301 E. Stassney Ln., Bldg. 6-100, Austin, TX 78744 USA; ph.: 512-443-9473; email hrobertson@usawire-cable.com.

FOOTNOTE: Statements and opinions expressed herein are those of the author. While every care has been taken in the compilation of this information and every attempt made to present up-to-date and accurate information, we cannot guarantee that inaccuracies will not occur. USA Wire & Cable, Inc. will not be held responsible for any claim, loss, damage or inconvenience caused as a result of any information within these pages. Always consult your local code enforcement, licensed engineers and manufacturers cable specifications before making any decisions.

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