Solar Photovoltaic Systems and the 2014 NEC
Although understanding of solar photovoltaic systems in the 2014 National Electrical Code (NEC) requires an understanding of the basic principles first, including a clear knowledge of definitions as used in Article 690. An apprentice or a journeyman electrician cannot begin to comprehend and deal with the intricacies of a photovoltaic (PV) installation without knowledge of the intent, purpose, and design requirements involved with these systems. A brief explanation on some of the major sections or subsections may help with the understanding of these systems.
Solar PV Definitions
The scope of Article 690 covers the provisions of this article that apply to solar PV electrical energy systems. These systems originate with solar cells installed in a series configuration in what are called modules. A module is defined as a complete, environmentally protected unit consisting of solar photocells, where sunlight is turned into direct current (dc) energy, optics that help focus and direct the sunlight into the photocell, as well other components of the PV system, designed to generate and process dc power when exposed to sunlight.
The dc output from a module can be connected to other modules connected in series to increase the voltage output and then in parallel to increase the current, which are then used to power dc loads, charge batteries, or be converted into alternating current through a dc to alternating current (ac) inverter or converter. Modules can be combined into panels, and panels can be combined into subarrays or arrays. A panel is defined as a collection of modules mechanically fastened together, wired, and designed to provide a field-installable unit. A subarray is an electrical subset of a PV array, and an array is a mechanical integrated assembly of modules or panels with a support structure and foundation, tracker, and other components, as required, to form a dc power-producing unit. Field people often mistakenly call a "module" by the term "panel" causing confusion, especially with apprentices. Then both journeymen and apprentices have problems understanding the requirements in the National Electrical Code. Care should be taken to always use the proper terminology to avoid confusion.
An inverter is equipment used to change voltage level or wave form, or both, of electrical energy. An inverter can be called a power conditioning unit (PCU) where it is used to match the voltage levels, phase angles, and other utility company power characteristics. An inverter may also be called a power conversion unit where it is used to change dc to ac, with the additional function of power conditioning. Inverters may also function as battery chargers by converting ac power into dc and then acting as a charge controller or regulator so the batteries are not overcharged.
There is also a new definition in 690.2 of the 2014 NEC that covers “dc to dc converters,” a device installed in the PV source circuit (the output of the modules) or the PV output circuit (usually from a combiner box to the inverter) that can provide a dc output voltage and current at a higher or lower value than the input dc voltage and current into the converter.
There are three different types of inverters: The standalone inverter, the utility interactive inverter, and the multimode inverter. The standalone inverter is a solar PV system that supplies power independently of an electrical production and distribution system, such as the utility power grid. A utility interactive inverter is an inverter that operates in parallel with and may deliver power to an electrical production and distribution system. A multimode inverter has the capabilities of both the interactive inverter and the standalone inverter, since there can be a branch circuit or feeder at the inverter that supplies loads not interconnected with the utility, plus feeders that are interactive with the utility supplied loads.
There are some additional definitions that are critical to understanding PV systems and the requirements in the NEC. A PV power source is an array or aggregate of arrays that generate dc power at system voltage and current. PV source circuits are the dc circuits from module to module and from the output of the modules to the common connection point or points, such as the combiner boxes, of the dc system. The PV output circuits are the circuit conductors between the PV source circuits (output of the modules) and the inverter or dc utilization loads, where the dc is not being converted into ac power. PV output circuits are usually the output conductors from the combiner boxes to the inverter. A combiner box is a special box that often contains a busbar, as well as individual overcurrent protective devices, and is used to combine the output of the modules, panels, or arrays so that a single feeder can be used to supply the inverter, rather than installing individual conductors from each series set of modules to the inverter.
General requirements for PV installations are located in 690.4 and contain provisions dealing with listing of equipment, qualified personnel, and installation of multiple inverters. This general requirement section states that photovoltaic systems are permitted to supply power to a single building, multiple buildings, and other structures, such as pole for security lighting, telephone kiosks, recreational vehicles, boats, and as well as countless other similar applications. This general section also requires inverters, motor generators used for PV applications, PV modules, PV panels, ac PV modules (these devices are a combination of modules and inverters used to supply an ac output to a particular load and are listed from the factory as an integral unit), dc combiner boxes, dc to dc converters, and charge controllers to be listed for PV application and use.
Subsection 690.4(C) requires the installation of PV equipment, all associated wiring, and all PV interconnections to be performed by a qualified person, such as an electrician or someone else equally qualified for these installations. A qualified person is defined in Article 100 of the NEC as a person who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved. The Informational Note immediately following this definition refers the user to NFPA 70E-2012, the Standard for Safety in the Workplace, for electrical safety training.
The final subsection, 690.4(D), states that a PV system is permitted to have multiple inverters installed in or on a single building or other structure. Where these multiple inverters are installed remote from each other, a directory must be installed at each dc PV disconnecting means, at each ac disconnecting means, and at the main service disconnecting means showing the location of all ac and dc disconnecting means for the building. However, there is an exception that states a directory is not required where all inverters and PV disconnecting means are grouped at the main service disconnecting means, since the disconnects would all be located in close proximity to each other.
PV System Voltage
One of the key issues with any photovoltaic system is the maximum and minimum voltage for the system. Part II of Article 690 starts with 690.7(A) requiring that a dc PV source circuit (the output of the modules) or the PV output circuit (the output from the combiner box) must have a maximum voltage for that circuit calculated as the sum of the rated open circuit voltage (before any current flows in the circuit) of the series-connected PV system modules corrected for the lowest expected ambient temperature. The open circuit voltage of a module will always be highest at the time when the ambient temperature of the module is at its lowest temperature and before any current flows in the circuit. The lowest temperature of a module is usually at first daylight before the sun has had a chance to warm the modules.
An Informational Note has been provided immediately following 690.7(A) that states one source for statistically valid, lowest expected, ambient temperature design data for various locations is the Extreme Mean Minimum Design Dry Bulb Temperature found in the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Handbook—Fundamentals. This temperature data can be used to calculate maximum voltage using the manufacturer’s temperature coefficients relative to the rating temperature of 25° C (this is the temperature at which the module has been tested and certified for its voltage level). The voltage calculation, at the modules, panels, or arrays lowest temperature, should not exceed the voltage rating of the inverter, but the voltage should be high enough during the hottest part of the day to still be able to turn the inverter on in case of a shutdown for any reason. Most inverters have a minimum trigger voltage necessary for the inverter to start up and operate.
Sizing the dc conductors from module to module, from the modules to the combiner box, and from the combiner box to the inverter is covered in 690.8. Section 690.8(A) provides the calculation for the maximum current for each specific part of the system. In 690.8(A)(1), the PV source circuit current (current output of the modules) is the sum of parallel module rated short circuit current multiplied by 125 percent. The PV output circuit current (the output of the combiner box or the individual module conductors installed to the inverter) is the sum of the parallel source circuit maximum currents already calculated in 690.8(A)(1) above. At the ac output of the inverter, the maximum current is the inverter nameplate continuous output current rating (the inverter ac output can never exceed the continuous ampere rating of the inverter). The inverter ac output and the dc to dc converter output current is always considered to be continuous since it will be operating for three hours or more whenever there is enough light and enough trigger voltage for the inverter to be operating. Section 690.8(B) also states that PV system currents are always considered to be continuous currents.
All PV circuit conductors are sized to carry not less than the larger of 690.8(B)(1) or (B)(2). In other words, both (B)(1) and (B)(2) must be calculated and then the larger size conductors between the two calculations must be used. In 690.8(B) (1), the conductors must be calculated at 125 percent of the currents calculated in 690.8(A) before the application of any conductor adjustment factors [based on the number of current carrying conductors in a cable or raceway in 310.15(B)(3)] and conductor (ambient temperature) correction factors as covered in 310.15(B)(2). An example of this calculation would be for the conductor ampacity to be figured at 125 percent times 125 percent (which equals 156 percent) times the sum of the parallel source circuit rated short circuit current (for module conductors) or for the PV output circuit conductors (the conductors from the combiner box to the inverter), the sum of the parallel source circuit rated short circuit current. For example, if there were 6 parallel sets of modules (8 modules in each string) and a short circuit current (Isc) of 8.33 amps per set, the following calculation will provide the ampacity necessary for the conductors from the combiner to the inverter. The short circuit current of each string of 8 modules would be 125 percent times 125 percent times the number of parallel sets where there are 6 parallel sets = 8.33 amps per string times 156 percent times 6 parallel sets = 77.97 amps or 78 amps. Based on the 75°C column in Table 310.15(B)(16), the conductor required would be a 4 AWG THWN copper good for 85 amps without any correction factors.
For a calculation based on 690.8(B)(2), the maximum current would be calculated in accordance with 690.8(A) without the extra 125 percent, however, now adjustment and correction factors must be applied. For example, short circuit current of one string times 125 percent times the number of parallel sets plus the adjustment and correction factors = ampacity for conductors, such as 8.33 amps times 125 percent times 6 sets = 62 amps of current in a wet location at 40°C (104° F). Using the ambient temperature correction factors in Table 310.15(B)(2)(a) for the above temperature the correction factor in the 75°C column is 88 percent or 0.88 so 62 amps divided by .88 = 70.45 or rounded off to 70 amps. Based on the 75°C column in Table 310.15(B)(16), the conductor required would be a 4 AWG THWN copper good for 85 amps with correction factors.
Careful study of Article 690 will provide both an apprentice and a journeyman electrician with the appropriate amount of information to become proficient with sizing of PV conductors and the installation requirements outlined in Article 690.
Mark C. Ode, Lead Engineering Instructor at the Underwriters Laboratories Inc. in Research Triangle Park, North Carolina, has worked at UL University and Knowledge Services for five years and was a member of UL’s Regulatory Services Department for 10 years. Ode worked for over 27 years as a licensed electrician and a licensed electrical contractor. He was a principal member for Panel 20 for the 1990 NEC. He was a principal member of Panel 13 for the 2011 NEC and is principal for the 2014 NEC. He was an alternate member of Panel 3 for the 2002, 2005, 2008, and 2011, and he is an alternate member for 2014. He was an alternate on the NEC Technical Correlating Committee (TCC) for the 2005, 2008, and 2011 NEC and is an alternate member of the NEC TCC for the 2014 NEC.