CODE CORNER: 2017 Circuit Requirements

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Under the 2017 Code, an alternative maximum system voltage calculation can be used for large-scale PV systems, resulting in significant cost-savings.
PV string maximum voltages can now be defined by the manufacturers’ specifications on DC-to-DC converters.

In the 2017 National Electrical Code (NEC), Article 690, Part II (“Circuit Requirements”) was subject to a handful of significant changes—especially to the sections PV designers are accustomed to using in their everyday designs.

Often, Code changes help clarify the requirements. In some of these sections, though, new methods are introduced. This article focuses on some of the biggest changes.

Article 690.7

Article 690.7, “Maximum System Voltage,” was changed in several ways. For PV system DC circuits for one- or two-family dwellings the maximum system voltage allowed is 600 VDC (unchanged from previous Code cycles, but moved up in this section). For PV system circuits located on buildings that are not one- or two-family dwellings, system voltages up to 1,000 VDC are allowed. For circuits not on or in buildings, there’s a maximum system voltage of 1,500 VDC and the restrictive high-voltage rules of Article 490 Parts II and III do not apply to the PV system circuits. Once the exact circuit limitations have been determined, 2017 Code then outlines the methods for calculating the maximum voltages.

Article 690.7(A) provides three options for calculating the maximum circuit voltage for PV source and output circuits. The first two still require that the sum of the voltages for series-connected modules be adjusted for the lowest expected ambient temperature, either through calculations or the supplied table. Prior to 2017, the table applied only to crystalline and multicrystalline modules for when the manufacturers didn’t supply temperature correction factors. Using values in the table results in a more conservative result than the module-manufacturers’ values.

A third method for arrays with a generating capacity of 100 kW or more, uses a documented and engineered industry-standard method for calculating maximum system voltages. The definition of “generating capacity” was added to Article 690.2 and is essentially the AC power output of the system (i.e., the sum of AC inverter outputs). The Sandia paper—SAND 2004-3535, Photovoltaic Array Performance Model—is cited in the associated informational note as one such standard method. This calculation may become more common, since it results in longer string lengths and cost savings, especially in large systems, where adding even a single module in a string can have significant cost implications.

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