Choosing the right wire for your photovoltaic system

By Tim Clancy

Photovoltaic installations continue to dominate the share of newly installed generation capacity. This growth is driving a need for designers and installers to be more competitive and design savvy. Guidelines have been established to enable the photovoltaic system designer to build safe and effective systems wiring circuits. However, knowing how to interpret these guidelines, and where to look for additional guidance, is not always intuitive.

Many lessons have been learned as more knowledge is gained from the field related to electrical safety. Better understanding of the proper wire selection and sizing can reduce the risk of overheating the system and its components. The National Electrical Code (NEC), specifically Article 690 Solar Photovoltaic Systems, provides the first step. Within this article, there are key areas to consider relative to wiring sizing. Further information on these components is found in the following: Underwriters Laboratories (UL) component standards specific to Photovoltaic Installations, UL 4703 Standard for Photovoltaic Wire, UL 6703 Connectors for Use in Photovoltaic Systems, and UL 9703 Distributed Generation Wiring Harnesses.

The NEC shows us several things to consider when determining the system size requirements. First, the system voltage determines string size and length. Commercial buildings are restricted to a maximum system voltage of 1000 volts while residential installations can be no greater than 600 volts.

There are also many PV modules to choose from so understanding their specifications and ratings is important. Each of these modules are certified and qualified for use under STC (Standard Test Conditions), typically 25°C. For this reason, the PV open-circuit voltage may need to be corrected for a lower temperature if the system is expected to see lower operating temperatures. Lower ambient temperatures reduce resistance and positively affect the voltage.

Next, the circuit size and maximum current need to be established. This includes examining the PV system source and output sources starting with the modules and including any power electronic converters. There are two methods to determine the maximum current of the PV source circuit:

• The first requires the short circuit current of PV modules connected in parallel to be summed together then multiplied by a factor of 125 percent. At times, PV modules can exceed the short circuit current outputs under conditions of high solar incidence, which justifies the additional factor used to allow proper sizing of conductors.
• Alternatively, for systems with an inverter output capacity of greater than 100kW, a stamped and signed PV system design can be used from a licensed professional electrical engineer to establish the highest current for that system.

Another consideration for circuit sizing is where a circuit containing a fuse rated lower than the conductor ampacity is connected to an electronic power converter. In this case, the rated input current of the power converter can be established as the maximum current of that circuit.

Now that the maximum current is established, it is time to determine the conductor ampacity. For this process, the greater of two ampacity values calculated from two methods is used to establish the minimum conductor size:

• Comparison A multiplies the maximum system current by 125 percent without applying any additional adjustments. Recall, the maximum system current took into consideration the source and output currents of the PV modules in parallel, the source and output currents from any dc-to-dc converters, and the output currents from any inverters.
• Comparison B uses the maximum system current as the basis for selecting the appropriate conductor size after adjusting for max ambient temperature and number of current-carrying conductors. The temperature correction can be significant due to the irradiance effect on exposed surfaces often being much higher than the ambient temperature in the height of summer. Ampacity tables are based on either single conductor installed in air or up to three current-carrying conductors installed in a raceway or cable. Many PV installation designs on the dc side run single PV Wires back to either a collection box, to the power inverter in a tray, or suspended from a messenger. Routing these circuits often requires bundling or grouping more than three PV circuit wires without spacing. In this instance, the conductor ampacity must be properly de-rated.

Finally, conductor size is determined from the larger of the two Comparisons A (fixed 125 percent factor applied only) and B (corrected and adjusted conductor, i.e. de-rated conductor).

As can be seen, both voltage and ampacity are influenced by ambient temperature extremes and must be accounted for in the system design. Voltage needs to be corrected for the lower end of the temperature range, while current requires correction for the upper end of the temperature range. Other issues can arise from grouping cables and not properly de-rating the conductor size for the system ampacity. Proper design and sizing of circuits is about heat management and keeping your system operating within safe design limits for years to come.

Tim Clancy is a Technical Sales Manager for Prysmian, providing sales support for the company’s Industrial and Specialties businesses. He has held previous roles in R&D and Engineering within wire and cable for over 33 years. Prysmian (www.prysmian.com) manufactures a full line of wire and cable products for the photovoltaic industry, from module level interconnection to high voltage transmission.

Q3 2024