From kW to MW: System Design Considerations

Many solar contractors have experience installing residential PV systems that are typically less than 10 kW and are connected to single-phase utility services. While the jump to larger commercial systems can seem intimidating, many concepts and design issues are common to both small residential systems and MW scale commercial projects. This article is the first in a series that will demystify the design, installation and optimization of large, commercial scale PV systems.

Photovoltaic systems are highly modular in nature, so many of the components used in large commercial systems are identical to—or scaled up versions of—those used in small residential applications. At a fundamental level both residential and commercial PV systems consist of the same basic set of components: PV modules, a support structure, combiner boxes, ac and dc disconnects, inverters and a connection to the utility grid. Other than scale, the primary difference between the two systems lies in the choice of inverter and how it is interconnected to the utility grid. Small single-phase residential systems are typically connected to the utility grid with minimal protective equipment. Large commercial systems are three-phase and subject to many more utility interconnection and protective equipment requirements. Before installing a commercial system, it is essential to contact the utility to determine the impact of any additional requirements.

Diagram 1 is representative of a utility-interactive residential PV system with a single inverter. Much of the design process used in this residential system example, while smaller in scale, is identical to that used for a large, three-phase commercial system.

Both residential and commercial PV arrays consist of one or more strings of five to twenty modules wired in series. The exact number of modules, including the number in series and the number of strings, depends on the electrical characteristics of the modules, the input voltage and current range of the inverter, and the expected high and low ambient temperatures of the site. In a well designed system the array and the inverter will be chosen so that the array’s operating voltage, current and power output will be within the inverter’s operating range at all times throughout the year. Inverter manufacturers typically provide sizing guidelines or programs to assist in matching a particular array configuration to a specific inverter.

RESIDENTIAL INTERCONNECTION

NEC® Article 690.64(B)(1) requires a dedicated overcurrent and disconnect device for each inverter in the system. The size of the overcurrent device is specified by the manufacturer and the NEC. The minimum size of the overcurrent device is defined by code and calculated by dividing the power output rating of the inverter by the nominal service voltage. Article 690.8(A)(3) requires inverter output circuit currents be considered continuous. As a result the ampacity of the wiring and the rating of the overcurrent devices must be sized to carry 125% of the output current in accordance with Article 690.8(B)(1). For the system in Diagram 1, the minimum ampacity of the wiring and the minimum size of the overcurrent device rating are computed using the inverter output current calculated as:

Inverter Output Current
≥ (inverter output rating / nominal voltage) x 125%
≥ (7000 VA / 240 Vac) x 125%
≥ 36.45 A

Article 240.6(A) lists standard size overcurrent devices with ratings of 35 and 40 A, so the minimum overcurrent device rating approved for use is 40 A. Note that UL 1741 requires the manufacturer to specify the maximum size of the overcurrent device to be used with the inverter. Use of overcurrent devices larger than the one specified by the manufacturer violates the conditions of the listing and therefore must be avoided. Since the wiring between the overcurrent device and the inverter is protected by the 40 A overcurrent device, the minimum ampacity of the wiring must be 40 A or more. Also, it is important to note that any additional code-required ampacity correction factors for ambient temperature and conduit fill are applied to the inverter output current of 36.45 A not to the breaker size of 40 A. The wire ampacity sets the minimum wire size required to meet NEC requirements. Larger wire sizes may be required to reduce voltage drop and system losses.

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